5 dollar voltmeter with 5 digit (0.1mv) resolution - Page 1
Forum section: electronics / projects

author: paulie
author_rank: Frequent Contributor
timestamp: January 06, 2015, 12:54:58 pm
content: A while back I purchased one of these:http://www.ebay.com/itm/310601671625?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AITHowever the new 0-4.3000v-33.000 version has a 0.1mv spec when used with isolated supply. A fellow amateur radio club member swears he gets close to that and I'm wondering if anybody here also has direct experience with one.http://www.ebay.com/itm/370966721056?_trksid=p2059210.m2749.l2649&var=640165395235&ssPageName=STRK%3AMEBIDX%3AITUpdate: I ordered one of the 4.3000v meters and it arrived so time to put together a set of voltage references. Included are 0.1% LM4040a-2.5, 6.9v precision zener circuit, LM336-2.5, and 0.1% 4.1v LM4040a (see photo). Out of the box the meter seems to be quite close to expected accuracy. What I would like to eventually do is get some more accurate readings of these and compare those with the meter. Hopefully get the accuracy in the same ballpark as the resolution.Pinout for the references: black dot=ground, white=Vin (+5v or +9v), third pin=Vout.Update: Thanks to nikifena we have a schematic from a Russian web site that was very close to the Ebay meter. I made a couple minor corrections and attached below.Update: In response to an offline request for more info on the calibration circuit mod a photo of that is attached below. A slight improvement over the first version. Basically a 10meg resistor in parallel with the 330k then 9.5k to 10.5k variable resistor (fixed 9.5k + 1k trimpot) in series with the input. These components were attached to the unpopulated pads near the edge so soldering directly to existing resistors was avoided. Didn't want to risk changing original component values by overheating. End result was a small fraction of a percent range which provided very comfortable calibration adjustment.Ignore the dual header. That is for a multichannel input expander and PC datalog interface and is unrelated to the calibration mod.Update: Another meter modification. Recently needed to measure geiger tube voltage with more precision than my normal 3 1/2 digit meter so thought it would be nice to take advantage of the 5 digit Ebay unit. Problem is voltages in the area of 650v so decided to build a 100x range expander. Fortunately it's as simple as adding 30 some meg in series which gives up to 3300v measurement capability. It took 3x 10meg, 1.5meg, 1meg, and a pot for fine adjust.A side benefit is increased input impedance which is always a good thing. Now going from 330k to over 30meg.One caveat is insuring dangerous HV breakdown won't damage the meter. Spark-gap rating for 3300v is 0.15" so to leave a little margin I kept the hot end at least 0.2" away from everything else. And no 0805 here, only 1/4w or bigger through hole. Naturally great care should be taken with potentials this high so safety measures like enclosure and insulation are required. In this case a little epoxy potting and shrink. However I thought it might be a good idea to show components more clearly before wrapping it up so before doing that some preliminary photos during the test stage.Works like a charm. Shown below measuring one of the dollar store USB chargers modified for access to rectified mains. Beware, there are dangerous voltage and current levels so take all necessary precautions in your own tests. Good idea to start by checking simple 12v dc or 9v battery which should give some indication of accuracy.

author: electr_peter
author_rank: Unknown
timestamp: January 06, 2015, 01:54:46 pm
content: There are 0-33.000V and newer 0-4.3000-33.000V versions. Some versions have 3 wires, some 4 (these should be powered by isolated supply).I had older 0-33V type with 3 wires - worked OK when compared to accurate multimeters. Display digits are slightly smaller than typical 3-4 digit panel meters.As for 0.1mV - keep in mind that it is resolution only, accuracy is claimed at ~0.3%+2d. I would not trust it too much on low voltages.These meters are designed to display fixed +4-30V full range, accuracy and resolution at lower end (<1V)is not critical to typical applications anyway. If low range is important, use more sensitive equipment.

author: paulie
author_rank: Frequent Contributor
timestamp: January 07, 2015, 03:12:30 pm
content: Thank you for that input. I was hoping someone had experience with the new hi-res version but it looks like I'll have to order one and evaluate myself.My goal here might be to calibrate one of these so accuracy can match resolution or as close as possible at room temperature. Unfortunately I no longer have access to a precision lab voltmeter so the plan is to send a home built voltage reference to a friend who does and compare with readings from one of these 4.3000v meters.What about basing it on something like lm4040a which has 0.1.% initial accuracy spec? Any opinions?

author: Alex30
author_rank: Regular Contributor
timestamp: January 07, 2015, 03:27:35 pm
content: Quote from: paulie on January 07, 2015, 03:12:30 pmHaha I think you may have got that idea from my power supply project. That's exactly what I was going to try and do. Although even if you can calibrate it to the lm4040 you will still be within +/-0 2.5 mV. I will wait for my voltmeters to arrive and let you know how it goes. Should take about a month to arrive and I will post my findings then or two weeks later as I am going on holiday around that time.

author: paulie
author_rank: Frequent Contributor
timestamp: January 08, 2015, 01:10:11 pm
content: Yes, I've been following your thread since the start. It's one of the more interesting ones along with the LM399 and LTZ1001. Like Peter I've had a 33v version for some time now and curious if these new hi-res units are even close to what they claim. It would make for a huge advance in this area if so.Quote from: Alex30 on January 07, 2015, 03:27:35 pmOr much better (~0.1mv) if calibrated to a LM4040 that was measured with a high quality lab meter. Not too much beyond that as I don't want to get into spending thousands on standards or worry about sensor orientation or humidity at this stage.I've just ordered one myself and very interested in how yours works out. Do you own one of those high end lab voltmeters or other means to check things like linearity, stability, or accuracy?

author: Alex30
author_rank: Regular Contributor
timestamp: January 08, 2015, 01:39:15 pm
content: Quote from: paulie on January 08, 2015, 01:10:11 pmNo but I was thinking of seeing if I could get friendly with some of the EE staff at my uni (even though I am doing civil eng, which is also one of my other interests) and seeing if I could do some calibrations. For now all I can do is calibrate the voltmeters to the LM4040 alone. Just takes a while for the voltmeters to arrive on the slow boat from china.

author: electr_peter
author_rank: Unknown
timestamp: January 08, 2015, 03:15:11 pm
content: I did a quick check of 33.000V panel meter accuracy. Measurements were done with known very accurate references and 0.05% DMM. Actual references values are very close (much less than <1mV) to that of DMM.ReferenceDMM 0.05%Panel meter 33.0002.5V2.4982.4975V ref14.9994.9985V ref25.0015.0007.5V7.5007.49810V10.00310.001Results are surprising - panel meter is off at most by 2mV from accurate reading. I think that this is very good result for a panel meter. 0-4.3000-33.000V meter has intermediate lower range - I expect it to perform similarly well because the panel PCB design seems almost identical.I plan to use 0-4.3-33V panel meters for PSU - seems like a good choice for metering so far.

author: Kalvin
author_rank: Super Contributor
timestamp: January 08, 2015, 03:53:15 pm
content: Thanks electr_peter. I ordered two panel meters for my power supply project and this is excellent news.

author: DVDRW
author_rank: Newbie
timestamp: January 08, 2015, 05:41:06 pm
content: Yes good stuff. Also using these in my diy PSU.There is also 5-digit 0-3A ammeter 100uA accuracyhttp://www.ebay.com/itm/Digital-Red-Ammeter-DC-0-3-0000A-5-Digit-LED-Current-Meter-High-Precise-/271560868818?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3f3a4a03d2

author: electr_peter
author_rank: Unknown
timestamp: January 08, 2015, 06:14:59 pm
content: In case you are measuring DC current of something, that current panel meter is a good choice (when compared to 3 or 4 digit cheap panel meters).Would it be better to use [two voltage panel meters (1st for current, 2nd for voltage)] instead of [current+voltage panel meters] for a typical lab PSU?Current signal converted to voltage can be compared to set current, calibrated via op-amp gain+shunt resistor and controlled remotely if needed. Having direct current reading meter does not allow to show set current easily on the same meter (unless you short output terminals).

author: paulie
author_rank: Frequent Contributor
timestamp: January 08, 2015, 06:43:06 pm
content: I'm using one of the new chinese $1 XL6009 buck-boost converters with a discarded laptop brick as basis for my lab supply. 0.9-36v 4amp output. It uses just one panel meter (0-33v ATM) for both current and voltage because personally there's rarely a need for both at the same time. No op amp though, just using PCB traces as a shunt. The meter is sensitive enough for that.

author: MrAl
author_rank: Super Contributor
timestamp: January 08, 2015, 11:51:02 pm
content: Hi,I have one of the 00.000 to 33.000 volt models and i am happy with it.The model i have reads with only 3 decimal digits to the right, so for example it might read any of:12.54310.48327.123and for voltages less than 10v it reads for example:7.1232.3451.0030.001note that it does not shift down to a higher resolution, so i dont get readings like:0.00011.1234the readings are limited to 3 digits to the right of the decimal point.The accuracy is acceptable but even the resolution of 1mv on these models is much better than the resolution on my power supply built in meter which is only 0.1 volt which obviously really blows. The 0.001 resolution really helps a lot so i use one of these meters.They can be used as a two wire meter for voltage over 3.3 volts by connecting the power supply lead to the signal lead and using the ground as a common ground. The current is surprisingly only 10ma and it is bright enough to see in normal room lighting.I meant to install one into my power supply case as it is much better than the on that came with it, but havent gotten around to doing that yet :-)Keep in mind that it will not read negative voltages unless you reverse the leads manually. That's probably the only drawback i have found with these things and for regular power supplies that doesnt hurt really because their output is usually unipolar anyway.I dont know if they are reverse polarity protected.

author: paulie
author_rank: Frequent Contributor
timestamp: January 09, 2015, 04:28:59 pm
content: Those original models do seem to be quite a good deal for precision meters. The new ones (where IS that slow boat?) are even more impressive. Specially if the low range has similar accuracy and linearity as hinted by Peters tests of the original. Not long ago we had to pay half grand or so to get this kind of performance out of products like Fluke. This wouldn't be the first time cheap chinese "copycats" trump pricey western technology.ps. I must admit though that for most of my DVM applications the even cheaper 3 digit units fit the bill. I've got several of the $1 Ebay DC converters scattered around with one of those strapped on with rubber band so that I don't have to drag out the DMM every time to adjust the output.

author: paulie
author_rank: Frequent Contributor
timestamp: January 14, 2015, 02:28:52 pm
content: Amazingly the high resolution version of the meter I ordered showed up in one week. Not a record but pretty good for China. So a set of cheap DIY voltage references were thrown together and surprisingly the readings were very close to what might be expected. This is a good sign. To really check this out I need to get more accurate readings for the references from a friend who has access to a lab grade voltmeter. Stay tuned.

author: Kalvin
author_rank: Super Contributor
timestamp: January 15, 2015, 06:02:12 pm
content: Quote from: electr_peter on January 08, 2015, 06:14:59 pmI thought about this "setting the current limit". You have two displays, one for the output voltage and the other for the output current. Use the output voltage display also for the current limit setting: For example, you may have a push button which will display the set current limit when the button is being pressed, and normally it will display output voltage.

author: Sparc
author_rank: Contributor
timestamp: January 16, 2015, 04:21:58 am
content: Maybe I missed it, but what is the ADC and reference chip on these meters? Is it just an ADC built into a microcontroller? A 33000 count meter would need 15-16 bits. Some micros have a 16 bit ADC. Or do they do something cheap and oversample a 10-bit ADC? Just curious, HOW they get 5 digits for $5.

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2015, 05:48:33 am
content: Quote from: Sparc on January 16, 2015, 04:21:58 amLook up "dual slope converter".

author: Alex30
author_rank: Regular Contributor
timestamp: January 16, 2015, 06:02:26 am
content: Quote from: paulie on January 14, 2015, 02:28:52 pmExcellent news! Keep us updated! I'm still waiting for mine

author: Psi
author_rank: Super Contributor
timestamp: January 16, 2015, 06:15:15 am
content: Whats the update rate like?I really want to replace the voltage displays in my gwinstek lab PSU which is like 1Hz

author: Sparc
author_rank: Contributor
timestamp: January 16, 2015, 07:27:17 am
content: Quote from: coppice on January 16, 2015, 05:48:33 amBut, what is the part?

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2015, 07:59:12 am
content: Quote from: Sparc on January 16, 2015, 07:27:17 amHuh? The part is typically a multi-meter chip from one of a couple of suppliers in Taiwan. That doesn't tell you how they measure so cheaply, though. Look up dual slope conversion, look at the schematic for any multimeter, and you'll find out how its done.

author: bktemp
author_rank: Super Contributor
timestamp: January 16, 2015, 08:23:24 am
content: Quote from: coppice on January 16, 2015, 07:59:12 amDual slope needs at least a high quality integration capacitor. I can't see any on the board, except for some MLCCs but those are probably used for the power supply.Therefore the question for the part number is valid. The board on the first post does not look like a typical dual slope implementation, more like a microcontroller with an ADC. U2 could be something like a MCP3421 or a voltage reference.

author: coppice
author_rank: Super Contributor
timestamp: January 16, 2015, 08:56:09 am
content: Quote from: bktemp on January 16, 2015, 08:23:24 amThese devices almost always use dual slope converters, but a recent design might be using an MCU or custom device with a sigma-delta converter. The electrical energy market has driven the development of some very low cost high accuracy wide dynamic range converter modules for MCUs. They are usually only 13 to 15 bit accurate for a single conversion (ENOB), due to AWGN, but average over a number of samples and you can get 19 or 20 bits of linearity from some of them. The DC accuracy will generally be somewhat lower, as its hard to achieve a DC offset which doesn't vary at least a little with temperature.

author: Neverther
author_rank: Regular Contributor
timestamp: January 16, 2015, 06:03:01 pm
content: I actually have those in my DIY supply, 0-4.3-33 as voltage display and 0-33 as current display with shunt and opamp to output voltage.Updaterate was more than 1Hz. I can confirm it monday as I'm currently away from my "bench".Accuracy was good enough, it's just generic supply, not high precision.

author: dave_j_fan
author_rank: Regular Contributor
timestamp: January 17, 2015, 10:10:57 am
content: those cheapies are sluggish .. i used one like that in bench supply , i have to wait 1-2 sec to see the new voltages Grrr!

author: Kalvin
author_rank: Super Contributor
timestamp: January 17, 2015, 12:49:47 pm
content: Quote from: dave_j_fan on January 17, 2015, 10:10:57 amAccording to this video the response time is pretty good, if we are talking about the similar cheap panel meters:My panel meters are still in mail, so I cannot verify how fast these meters actually are. But according to the video, I am happy with the response time.

author: Neverther
author_rank: Regular Contributor
timestamp: January 17, 2015, 01:04:50 pm
content: Quote from: Kalvin on January 17, 2015, 12:49:47 pmHey, thats my video. I forgot I did that.

author: paulie
author_rank: Frequent Contributor
timestamp: January 17, 2015, 01:33:21 pm
content: That appears to be the old version and not the one discussed here. It does have faster refresh (200ms) unlike the new one which is slow as molasses (3 seconds). IMO neither of these are appropriate for a PSU monitor. What's the point of 0.1mv output resolution when accuracy is literally orders of magnitude below that?I think the little $1 panel meters which have virtually instant response time are better suited. Or those double meters for both current and voltage. At least you only have to cut one hole in the box.Who needs more than a hundredth of a volt resolution for a power supply? Bling artists and those with money to burn, that's who. LOL.

author: Alex30
author_rank: Regular Contributor
timestamp: January 17, 2015, 02:01:35 pm
content: Quote from: paulie on January 17, 2015, 01:33:21 pmBecause it looks cool!!

author: paulie
author_rank: Frequent Contributor
timestamp: January 17, 2015, 02:17:10 pm
content: Touche! I don't have money to burn but must admit to being a bling artist. When I get this new meter calibrated maybe the old one will go into one of my "ghetto" (hi danny) PSU. 3 seconds not useable IMO but 200ms ain't that bad. And who's to know those last couple digits are for show.Of course it would be really nice to have accuracy of these meters match the resolution. Or as close as possible. I can't wait to see how my friends lab meter readings match up.

author: Alex30
author_rank: Regular Contributor
timestamp: January 17, 2015, 02:31:45 pm
content: Yeah. If it's as good as I've heard it should be fairly reasonable given the price. I mean $5 for even 1-5mV resolution ain't bad. I just hope they are fairly linear and easy to calibrate so you can make the most out of them.. And yeah 200ms suits me to be honest. Anything less than that I would be wanting a peak voltage meter anyway because I hate trying to pick out numbers quickly.

author: Alex30
author_rank: Regular Contributor
timestamp: January 19, 2015, 09:43:32 am
content: So my voltmeters arrived and so far im pretty pleased. I haven't been able to set up a proper test as I don't have a benchtop voltmeter so i'm still looking forward to paulies results. But because I got two I figured I could at least compare one to the other and see what kind of tolerance I got between them. I just quickly hooked em up to batteries and used a voltage divider to get a low voltage for the 0.1mV resolution. I used two batteries so the measurement was isolated from the supply and I noticed a difference of between 0.1-0.2mV which is pretty damned good in my opinion. Refresh rate was also quite good, somewhere between 2-3 per second. I noticed that the readings were a lot more stable when measuring something isolated than measuring from the same supply battery. Quite pleased with this.

author: paulie
author_rank: Frequent Contributor
timestamp: January 19, 2015, 10:45:20 pm
content: Nice colors, yellow and red. Cool. Or should I say HOT. Mine is blue because almost half price. Not as visible but I can live with that.Well the lab measured references came back and not good news. Both 4040 read significantly out of spec. 0.23% for one and 0.22% for the other, Both high so I'm beginning to think there are issues with my friends meter calibration or some problem with noise or TC. I know he purchased this last year as part of an on-line deal but not sure if he repaired or calibrated or what. I'll have to get in touch to find out more. On the other hand the 5 digit Ebay meter reads well within Nationals specs.Not likely a $5 Chinese clone could outperform a piece of professional gear that costs thousands but hopefully I'll get to the bottom of this.

author: splin
author_rank: Frequent Contributor
timestamp: January 21, 2015, 04:03:04 pm
content: I tested my .36" 5.5 digit voltmeter (0 - 4.30000 - 33.000V), using an AD584 based reference:http://www.ebay.com/itm/AD584LH-4-Channel-High-Precision-Voltage-Reference-Module-can-5VDC-charging-NEW-/191342226316?pt=LH_DefaultDomain_2&hash=item2c8ce2ab8cIt has a sticker on it showing the measurements made by the manufacturer using a 6.5 digit HP34401AI also measured the AD584 reference with my own 34401A (not calibrated for several years) with these results:AD584 Calibration sticker (V)2.498875.001497.4990210.00129My HP34401A, 10V scale (V)2.49875.001447.4989910.00129Difference (uV)17050300Ebay 5.5 digit meter (V)2.49845.00057.49810AD584 error (mV)0.470.991.021.29Hp34401A error (mV)0.30.940.991.29Avg err (mV)0.3850.9651.0051.29Avg err (%)0.0140.0190.0130.013Well within the .3% spec and not at all bad for the $5.99 I paid last year.Splin

author: splin
author_rank: Frequent Contributor
timestamp: January 22, 2015, 04:00:12 pm
content: Quote from: splin on January 21, 2015, 04:03:04 pmSome points I forgot to mention:1) With the input at 0V it reads a solid 0.00002) The input resistance is approx 347k ohms3) The meter only has 3 digits after the decimal point above 4.3V; the 5.0005 in the table reflects the fact that the least significant digit was flipping between 0 and 1 every few seconds.4) The update rate is quite quick - perhaps 3 or 4 per second?5) The Power supply is not isolated as some have stated here - the supply is separate from the measured signal but has a common ground.6) My reference and both meters could all be wildly inaccurate - by the same amount!EDIT: 7) The meter uses an STM8S003F3P6 and the ADC is a 6 - pin device marked CAJD8. There is no obvious voltage reference - only the regulator marked 7133-1#

author: bktemp
author_rank: Super Contributor
timestamp: January 22, 2015, 04:32:20 pm
content: Quote from: splin on January 22, 2015, 04:00:12 pmThank you for reading the part numbers.My guess was right:CAxx is MCP3421: 18bit I2C ADC with internal 0.05% reference.This explains why it so accurate.

author: electr_peter
author_rank: Unknown
timestamp: January 22, 2015, 05:12:17 pm
content: Quote from: splin on January 22, 2015, 04:00:12 pmNobody said that power supplyinthe panel meter is isolated. The main issue is that power and signal wires are not isolated inside panel meter - this creates issues with powering and isolation.In posts above it was stated that it isrecommendedto use isolated external power supply (w.r.t. to your circuit) with this panel meter. Doing it this way allows to achieve better results. Keep in mind that when you connect isolated power supply to this panel meter and connect panel meter to voltage of interest in your circuit, isolated panel supply is not isolated anymore w.r.t. to your circuit.You can also power these panel meters directly from the same circuit you are making measurements. This has disadvantage of lower precision.Older type of panel meters could only work with isolated supply - that is a bit annoying when you are dealing with DC circuits. Most lower precision type panel meters only provide non-isolated power and signal wiring for convenience (2 wires - GND and V_power/V_signal or 3 wires - GND, V_power, V_signal).

author: paulie
author_rank: Frequent Contributor
timestamp: January 24, 2015, 03:53:09 pm
content: Thanks for that data splin. It is the most interesting and useful to me so far. Unfortunately I'm still unable to locate a reliable reference for my own unit which I'm hoping is just as accurate. I've modified mine to supply serial output and would love to do some long term stabilty and temperature tests.Apparently my friends lab meter was a "repair special" and while he did manage to fix it there has been no calibration since. This is probably why his readings did not correlate well with even basic 4040 specs. Maybe somebody here might be willing to lend a helping hand. I just need to get some lab grade (6 digit+) 4040 readings. As you can see in page one photos easy to mail back and forth as letter postage. I can cover all costs. If anybody can help me with this please PM or post.

author: paulie
author_rank: Frequent Contributor
timestamp: January 29, 2015, 12:25:26 pm
content: I've recently added a couple more low cost voltage references, TL431 and LM385, to my collection. Along with the original LM336 , LM4040, and zener diodes this makes 6 types total. Since they are so easy to make and so cheap (as low as 5-6 cents) I assembled a dozen of each type. This will allow me to do some long term drift experiments and also allow me to help some friends get their meters calibrated to accuracies rivaling more expensive units.Anybody else who might be willing to do some readings for me on some of these please pm. Your help is greatly appreciated.

author: paulie
author_rank: Frequent Contributor
timestamp: January 31, 2015, 06:48:46 pm
content: Someone asked about my setup for measuring voltage references so here's a photo. Black dot is ground, white is +5v or +9v, and last pin is the reference voltage output being measred. I found using batteries for both meter and reference gave most stable readings. If/when I am able to obtain accurate transfer values this will be the same arrangement for calibrating the meter.

author: splin
author_rank: Frequent Contributor
timestamp: January 31, 2015, 07:08:45 pm
content: You can get an AD584 reference module with 2.5V, 5V 7.5V and 10V outputs for as little as $6 off ebay:http://www.ebay.com/sch/i.html?_odkw=Voltage+Reference+Module&_sop=15&LH_BIN=1&_from=R40&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.XAD584+Voltage+Reference+Module&_nkw=AD584+Voltage+Reference+Module&_sacat=0I got a $15 dollar version (those in the acrylic cases) and can recommend them. However, three things to be aware of:1) They use a 3.6V mobile phone type LI-ON battery - due to shipping restrictions the battery may or may not be included (mine was)2) Some have a calibration sticker showing the values as measured by the supplier (in my case with a HP34401A, which agreed quite closely with my measurements with my own HP34401A) but others might not. You should check this if you haven't got access to an accurate meter.3) Don't use long banana plugs as you can short out the circuit underneath the sockets!

author: paulie
author_rank: Frequent Contributor
timestamp: February 02, 2015, 11:49:35 am
content: Thank you for that information on the acrylic case version. A friend of mine bought the PCB type to calibrate his "repair special" meter but it did not come with the precision readings. Maybe only the expensive units supply that custom data.I hesitate to spend 4 times more for the reference than for my meter and even then risk not getting accurate values. The AD584 itself has 7mv uncertainty and I'm hoping to get at least an order of magnitude better to take advantage of the meters resolution. My problem may be solved due to the kindness of one stranger willing to provide me with the DIY voltage reference readings.

author: Alex30
author_rank: Regular Contributor
timestamp: February 02, 2015, 01:14:08 pm
content: Just wanted to drop by to say thanks for your efforts paulie and splin. I have so far been unable to make any progress with getting access to equipment to check out my meters but will let you know if\when i do. Paulie, I was wondering how you plan on creating a calibration circuit for your meters as they do not have any calibration trim as we all know. There are obviously two things to calibrate for, linearity and offset. Surely to do this you need a reference at a high and a low point to ensure linearity? Either that or just an excellent reference at say, 10V and then a really good ohm meter to lower that voltage by a known amount to take your low reading. Thoughts?

author: paulie
author_rank: Frequent Contributor
timestamp: February 03, 2015, 10:55:47 pm
content: Hi Alex. The meter appears to already have an unpopulated calibration circuit onboard. It allowed me to put a 4.7m resistor in parallel with the 330k and a 50k trimpot in series which seems to give a nice 1% adjust. As far as linearity that is not as easy to control so the plan is to live with the MCP3421 specs which ain't half bad as-is. Maybe can be checked with some cheap resistors using flip flop method or even a Hammond network but IDK if it's worth it.One of my more ambitious mods was to tack on a couple CD4051s and OP07 circuit for monitoring several channels to see how these bandgap dealies really perform in the long run. I'm building a TCO too because of temperature extremes in my shop. Lots of threads on the zener based ones but not so much for the cheapies as splin noticed. Maybe 5 digits is not precise enough unless the drift is significant. Anyway I'm so impressed by this product I plan to buy another before the end of the week that I can dedicate just for these experiments.

author: Alex30
author_rank: Regular Contributor
timestamp: February 04, 2015, 07:39:54 am
content: Thanks for the reply. I was just looking at the circuit board (picture attached) and it does indeed have some space there (R7 and R8). The 330k seems to connect to a pad on R8, which then connects to R7 in series, but I can't work out where it connects to after that without probing around a bit. Did you use R7 and R8 for connecting your resistors? Also any idea what the KEY part is for on the left hand side?Keep us updated on your mods! And yes it is quite an impressive product given it's price, definitely my favorite electronics product I have bought on ebay.

author: nikifena
author_rank: Regular Contributor
timestamp: February 04, 2015, 08:38:09 pm
content: I found this link with the schematics over the net:http://eddy-em.livejournal.com/73375.htmlIt's in Russian.

author: electr_peter
author_rank: Unknown
timestamp: February 04, 2015, 10:42:13 pm
content: Quote from: nikifena on February 04, 2015, 08:38:09 pmInteresting article. Voltmeter consists of linear reg, ADC (18 bit, or 17 bit for pos. values) and MCU. Save some calibration constants in MCU, read ADC via I2C - and you have a nice voltmeter.I did not fully understand part about calibration procedure with PD6/UART1_RX. Article implies that calibration values can be written to MCU with UART, but it is very sketchy.By the way, what does the button/key do? It is unpopulated on the PCB. Is it used for calibration?

author: nikifena
author_rank: Regular Contributor
timestamp: February 05, 2015, 09:25:34 am
content: Probably this button footprint exists from old pcb design and it's not used. I have 2 of these voltmeters with different firmwares and they has the same PCB. Anyway the entire project is very clever with just a couple parts. Pretty nice for DIY projects

author: dannyf
author_rank: Super Contributor
timestamp: February 05, 2015, 12:36:03 pm
content: QuoteSTM8S03F, the little bugger that can.Quotehe was wondering about why they didn't use hardware i2c on that chip,.

author: paulie
author_rank: Frequent Contributor
timestamp: February 06, 2015, 12:10:05 am
content: That "key" signal and it's debounce cap are just connected to one of the MCU input pins (PD6) so it's hard to figure out what the original purpose was. Maybe related to calibration or seeing how it's on the programming connector possibly something to do with bootloader function. It's lablelled "BTN" which probably stands for "button".BTW like nikifena says the old 33v model hardware is identical to the new 4.3v. Only firmware is different. Thanks for that Russian link. I scoured the internet (google/bing SUCK!) and couldn't find anything close to that. Strangely they mention the Ebay meter but the drawing is not exactly correct. There were a few minor differences in part numbers, 7 seg display type, and missing components like the "key" button.I edited it to more closely match my board and attached to the first post.ps. Yes, Alex30, I tacked on the parallel resistor and series trimpot to R8 and R7. These meters seem to be very accurate out of the box and I didn't want to risk changing the resistance of the 330k by soldering directly to it.

author: paulie
author_rank: Frequent Contributor
timestamp: February 06, 2015, 09:33:25 pm
content: Well that's interesting. No more $4.70 meters on Ebay. Since this thread started prices have almost doubled. Typical of sellers who probably spend more time trolling the net than tending to business. Fortunately still available on Aliexpress for 5 bucks shipped. Thank god for language barrier.FYI beware Ebay seller bicoins_de. Notice the low rating. Several complaints for shipping the old low res meter in place of the new one. I suspect most didn't even notice. A few settled for $2 partial refund but not happy. AFAIK the other Ebay sellers and Aliexpress are kosher.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 06, 2015, 11:49:35 pm
content: I've got the first of 4 voltage references that Paulie loaned me running now, the 4040a-4.1. The short story is that I don't think it'll work as better than a +\- 1-2mV voltage reference. In other words, this one meets the spec of +\-0.1%. Will post charts later of the runs and what the voltage refs look like on the scope. Right now my just being in the room was adding noise to my power supply output.When I'm done, I'll run the same trial in my bargain voltage standard.If nothing else this was a good opportunity to see what I really have in the way of stable power supplies.

author: paulie
author_rank: Frequent Contributor
timestamp: February 07, 2015, 01:16:44 am
content: Thanks for the favor and for "coming out" here so more people can benefit compared to PM.I think there might be some misunderstanding of the goal like Alex30 on page one. Or maybe it's me. My requirement is to get some high accuracy readings for those references ASAP. Not adjust them to some to some precise voltage or see how far off they are or do long or short term drift studies. Only need to record their current voltage level along with Vin and temperature and get them back here for meter calibration.Or is it the case that these are generating wild amounts of noise and therefor impossible to characterize? The specs for these bandgap type references indicate otherwise and my scope shows very little problems with stability. I was simply using 5v from a usb port an also a 9v battery and saw only a few microvolts of noise.I simply need a transfer reference not an absolute voltage standard like in the other threads. Am I barking up the wrong tree or missing something here?

author: edavid
author_rank: Super Contributor
timestamp: February 07, 2015, 02:20:41 am
content: Quote from: paulie on February 06, 2015, 09:33:25 pmLook again, the one range meters are still that price.QuoteAll the "BuyInCoins" userids are the same seller, and have about the same rating. If they ship the wrong thing, just insist on a full refund.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 07, 2015, 04:56:47 am
content: The issue with these references is that they depend on a specific current for their voltage. And with a 100 ppm/C temperature coefficient for my resistors, I'm basically just measuring the temperature coefficient of the resistor. There's no way to measure these accurately without a current limiting resistor as shown in the data sheet. Otherwise you're just cooking the device.I'm documenting the setup and will send you the reference resistors I used. I'll also put up the chart of the references over time. Some settle in. Some don't. This is all on the 100uV level.This is the 4040A-2.5VI'm using a 3.3K to get the current down to 0.7mA to avoid heating the part. It has stabilized and is a good 4 digit source. The fifth digit just isn't going to happen, unfortunately. It's more than just reading all the digits off the DMM.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 07, 2015, 06:45:39 am
content: This is a $40 reference standard fromwww.voltagestandard.comafter it has a chance to settle down after 30 mins. I have the previous version of the DMM check. It's just a thermometer now. The drift is largely thermal. . Everything is an interferer at this point. Just being in the room causes drift since all the components are exposed. But this is the next step to get another digit.The standard was stated as being 5.0000V and was calibrated in May or June. So, that's what it will take as a reference for the 0.1mV meter. And even that standard really needs an enclosure. Everything becomes an antenna as well.

author: paulie
author_rank: Frequent Contributor
timestamp: February 07, 2015, 12:48:28 pm
content: Well this is proving to be quite an education for me. I see now just hooking up to the 0.1mv meter and watching those last few digits is nowhere near as useful as I thought. And I was way off base thinking 5 digits was free from the kind of influences they go on about in those other threads. So this whole story just gets curiouser and curiouser.I did notice things went down the tubes fast when connecting mains devices or anything but battery sources for both the reference and the meter (post #39). But never suspected complications due to current and medium term drift. How foolish to think hooking up a 2k thin film and watching the meter for a few seconds was all it took. I was in a real rush to get those "simple readings" back but now I'm grateful to take advantage of your experience and equipment to get real information. Thanks again for your generous willingness to spend time on this.It's a little shock to see how touchy things are even in the 5 digit world but also kind of thrilling to find I'll get to deal with similar issues as the "big guys".

author: LabSpokane
author_rank: Super Contributor
timestamp: February 07, 2015, 09:18:13 pm
content: I need to make a clarification. I in no way meant to say that the voltage standard I referenced previously was in any way of poor quality. What I will try to show is how once we get to the 100uV range, procedures matter. And the "test" I ran basically illustrates what happens when one ignores spec sheets and temperature coefficients.The voltagestandard.com reference is a tremendous bargain and a great tool. I have absolutely zero complaints about a 5 digit standard for $40. I was the one who cheaped out by not putting it into an enclosure.What I was trying to say, which came out badly, that getting to 1 mV accuracy isn't too hard for most of us on a non-professional basis. Each dependable digit after that comes at a price and requires a reasonable amount of attention to procedures, data sheets and environmental conditions.This, of course, begs the question of the value of a 0.1 mV or more precise reading for everyday usage.More to come this evening.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 12:44:21 pm
content: 0.1mv precision is critical for some of my applications and the main reason I started this thread. For example it's required in order to get 1ma resolution current readings from the 0.1ohm cheapo 1/4w shunt resistor in my PSU project. I've been using the old style meter with PCB traces and ground wires to get crude current values but time to improve that by taking advantage of the new more sensitive meter. No precision current sense chips or complicated op amp circuits here. I tend toward the most simple and cost effective solutions.That first graph of yours is interesting to me for a few reasons:1. After the first hour it seems to settle down to less than 0.1mv variation which is quite useable for my purpose. Or maybe I'm not interpreting the voltage scale correctly. Not being a great math wiz those oddball volts/div are hard for me to get a handle on.2. Is that initial warmup drift due to self-heating, component aging, or did it just come in from the cold? What happens if it's disconnected and similar test run again?3. I'm guessing Vin is probably 5v but what about temperature? I'm hoping you are taking note because, in view of your comments, this may be even more important than I first realized. I do have a few temperature controlled ovens of various sizes from day job and can accommodate just about any reasonable ambient.4. It would be very interesting to see what difference there is between your 3.3k 100ppm and the penny 2k resistors I started with. I used those because their TC and the delta Vin/Vout of the reference chips did not seem that critical. I do have a few thousand 10ppm Vishays (again left over from day job) but apparently underestimated the importance of TC here so did not use them. Maybe time to assemble another batch of reference modules.Another reason 1ma was chosen for across the board current is that anything less violates the minimum current spec of the TL431. I.E. 0.7ma not good for that part. The zener too. I was hoping to keep self-heating issues constant.Sorry for all the questions but these tests have got me really excited.

author: Kalvin
author_rank: Super Contributor
timestamp: February 08, 2015, 01:33:08 pm
content: How about adding a push button which would increase the shunt resistance temporarily from 0.1 ohm to (let's say) 100 ohm. The resulting voltage drop will be 0.1V at 1 milliampere. If you know that the device will consume only a milliampere or so in certain operation state or mode, this might be a simple & usable solution to increase the resolution and accuracy.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 08, 2015, 01:55:11 pm
content: The resistors have a 100 ppm/C *temperature coefficient*. I only stock generic 1% and 5% penny resistors as well. That 100 ppm/C is a guess based on what I think I ordered from Mouser. I bought on price for general work, not on specs.My "lab" runs between 58 and 61F. I didn't turn on the heat because I didn't want to introduce a thermal cycle into the results. All the components had reached ambient temp before test.The 0.7mA is just from what I had in stock for resistors. The data sheet specified 3.6k and I have 3.3k on the shelf.The settling you see on that one chart is the resistor reaching thermal equilibrium.My opinion at this point is that without ovenizing, trimming, low temp coefficient resistors etc., that these voltage references will not be reliable for 0.1mV. I would strongly recommend going to voltagestandard.com and getting a multi output reference. He uses components that are up to the task and all you have to do is hit the correct ambient temperature and it will work without guessing. The cost is pretty minimal given the performance and will definitely get you the digit you're looking for.Here's the standard I would consider:http://www.voltagestandard.com/PentaRef.htmlIt's $78.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 02:59:24 pm
content: I appreciate your advice but unfortunately another one of the main purposes for this thread is economy so I'm not in a position to spend more for a reference than the cost of these meters. Certainly not 10x more. In fact even if those were available for $5 it would not be a consideration for me at this time.Again looking at your graph it seems the variation once settled is around 0.04mv which is considerably more than my requirement for calibrating these meters. Am I missing something here?As far as resistors I determined from their specs and the reference chips change in Vin/Vout that the cheap thin films I installed in those references were adequate. And the 2k value was chosen so that all of them would have similar current. Not being an expert it's possible this was not the best choice however I don't see a huge difference between 0.7ma and 1.0ma in terms of heating. My goal was not so much to minimize heating because that can be accommodated with warmup but instead to standardize and not violate the minimum for the TL431. 0.7ma does not work for that part.I assembled those with resistor built-in to facilitate quick and easy readings but have no objection to you running your own experiments. Do you think it would be possible to do a run with the 2k built-in resistors? Hopefully with similar scales so we can compare?Also since temperature is so important here it would be very helpful if we knew exactly what that is. Apparently 1 deg can make quite a difference. I can reproduce whatever on my end ±0.5 deg as long as I know the original conditions.

author: janaf
author_rank: Frequent Contributor
timestamp: February 08, 2015, 04:29:49 pm
content: I bought six of these some time ago for a bench supply to be. When reading this thread, I had to pop the package open and put them to the test. Mine are the older type, power and measurement share ground.My "reference" is a 7.5 digit NI meter. Power from an adjustable PS. At room temperature approximately 20C.I'm impressed by these cheapos!

author: LabSpokane
author_rank: Super Contributor
timestamp: February 08, 2015, 05:33:00 pm
content: Here's what I got for the 4040A-2.5:

author: LabSpokane
author_rank: Super Contributor
timestamp: February 08, 2015, 05:35:23 pm
content: 4040A-4.1V:(The spikes in the power supply are *me* being near the test setup)

author: LabSpokane
author_rank: Super Contributor
timestamp: February 08, 2015, 05:37:25 pm
content: LM336-2.5:The spikes in the power supply this time are actually a VHF transmitter nearby.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 08, 2015, 05:38:59 pm
content: And lastly, for comparison, VoltageStandard.com's DMM Check at 5.0000V:The extended drop in the voltage reading is the heat going off at night. If I had run this test at a constant 68F, I'm certain I would have 5.0000V, not 4.9999X.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 06:45:35 pm
content: Thank you for taking time to run those insightful tests. It's obvious the $44 reference is an excellent standard and far better than the 10 cent specials. It would be great if I had access to one, even briefly, and all my troubles would be solved.So I understand now that the first ones are for Vin and the second Vout. One thing looks promising, most seem to settle to a fraction of a millivolt after a while so there's hope I can get some useful results from this. So if we took a reading say an hour after power up and noted the temperature this might just do it for me. At least hopefully a little better than the absolute default spec of the 4040a which is all I have now.Are these done with your 3.3k resistor and in each case the middle pin is left floating?

author: janaf
author_rank: Frequent Contributor
timestamp: February 08, 2015, 06:58:13 pm
content: Some previous posts asked / commented about refresh rate. I estimate mine update at 4-5 hz.Current is around 8mA / module, with spikes about 1hz, maybe some zeroing going on. I didn't try to measure what it looks like.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 07:05:57 pm
content: The older version does seem to have a slightly faster update rate. The 4.3000v one appears to be around 3hz which might be expected considering the higher resolution. Maybe some averaging or oversampling going on.BTW thanks for the readings on your units. It's amazing that even the 33.000v version is so precise. What a miracle of Microchip engineering to get that amazing performance out of such a cheap and simple circuit.

author: janaf
author_rank: Frequent Contributor
timestamp: February 08, 2015, 07:20:22 pm
content: If it's the MCP3421, they are listed at $1.74 ea @5000 pcs. No way they can make a complete meter and sell and make a profit at $5 each.Makes me think of another thread here about fake FTDI chip. Its quite feasible that someone has the production docs for the MCP3421 and making knock-offs, selling cheap.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 07:28:18 pm
content: It is my personal belief that the whole fake craze is an urban myth, wolf under the bed, flying saucer, romantic notion. It happens but rare. Maybe 0.1% of suspected cases actually verified.Note that there is a difference between "fake" and "clone". FTDI for example shot themselves in the foot over clones. Sometimes completely different part numbers that tried to take advantage of FTDI drivers which is where all the real development costs lie.

author: janaf
author_rank: Frequent Contributor
timestamp: February 08, 2015, 07:40:57 pm
content: Yes, agree on the Fake/Clone. My bad.But still, I'm very skeptical if it's possible to produce & sell legally at that price.

author: paulie
author_rank: Frequent Contributor
timestamp: February 08, 2015, 07:53:00 pm
content: Hey, you don't know those Chinese cookie makers. They will produce things like that for 3 cent margin and work for pennies an hour. Unlike western with sense of entitlement who stick their nose up at $20/hr. That's how China won the cold (economic) war.BTW I did order 5 pcs of this ICs for $20 on Ebay (another slow boat from China).. Almost not worth it with the meter only a buck more but I didn't want to risk bending the reference by unsoldering. I still can't believe such a simple circuit actually PERFORMS.

author: edavid
author_rank: Super Contributor
timestamp: February 08, 2015, 08:04:19 pm
content: Quote from: janaf on February 08, 2015, 07:20:22 pmMicrochip's prices may be different in China, but even if not... I just bought some 4 digit DPMs for $1.50 each. Adding $1.74 to the BOM and $3.50 to the price doesn't seem unreasonable.Hmm, looking again, here's a source for the 5 digit single range voltmeters for $3.95, quantity 15:http://www.aliexpress.com/store/product/Red-LED-display-Color-DC-0-33-000V-0-36-Digital-Voltmeter-three-Wires-5-Digit/835957_1320727454.html(They also have the 5 digit dual range voltmeters for $5.20, and 5 digit current meters for $4.48.)

author: LabSpokane
author_rank: Super Contributor
timestamp: February 08, 2015, 11:56:18 pm
content: I don't have a metrology lab, so for temp, you get what you get. These tests were run between 60 and 61F. To really get a voltage standard to consistently run to the 0.1 mV benchmark, one really needs a feedback mechanism to handle temperature compensation.I'll put up a simple schematic later. The 3.3K resistor was placed in line with the zener reference. I put a 1Mohm in front of the DMM measuring the source voltage to try and reduce the loading on the reading. Is that "correct"? Probably not, but its what one has to do without any feedback compensation. It's a dance between keeping enough current flowing through the zener to get it past the knee, not heat it or the resistor significantly, and keep it at as close to one current as possible. See attached diagram. The IV curve to the left of the knee looks like a cliff, but its not. Once you're in the uV neighborhood, the slope of that line matters.This is pretty much all I have time for at this time. I actually need to get back on track on another project.on't have a metrology lab. These tests were run between 60 and 61F. To really get a voltage standard to consistently run to the 0.1 mV benchmark, one really needs a feedback mechanism to handle temperature compensation.Good luck with your project!!

author: paulie
author_rank: Frequent Contributor
timestamp: February 09, 2015, 12:46:59 am
content: Quote from: LabSpokane on February 08, 2015, 11:56:18 pmIt's unfortunate you are not in a position to help me with basic meter and temperature readings but the graphs and lectures were interesting. Would you do me a favor and return the references in the stamped envelop I provided? A friend of mine has a Keithly that is uncalibrated but indicates he may be getting that done sometime in the future. I would like to be prepared just in case.ps. None of the references I sent were zener based. They were bandgap based integrated circuits with somewhat different characteristics. I do have some zener based references here like the one in the page one photo. It was kind of you to reiterate how those work for anybody not familiar with the technology.Thanks again for your offer to help. Sorry it didn't work out. Good luck with your new project.

author: LabSpokane
author_rank: Super Contributor
timestamp: February 09, 2015, 01:54:23 am
content: Apologies. I skimmed the data sheets very very briefly. They do show a zener symbol, so it was an easy mistake to make. I am returning the samples do you tomorrow.And honestly, it didn't work out because you didn't provide a completed device. None of them had a current limiting resistor. I flat couldn't test one because it was an adjustable source and required bias resistors. As provided, the unit was a dead short.

author: paulie
author_rank: Frequent Contributor
timestamp: February 09, 2015, 12:02:43 pm
content: Quote from: LabSpokane on February 09, 2015, 01:54:23 amI think there might be other reasons it didn't work out. As anyone who skimmed even very very very briefly the first post here can see these are complete voltage references. The resistors are clearly visible. Also the value and type of this resistor and how to connect the reference was described in detail in several of my recent replies.There were a few individuals besides myself who hooked these same ones up and found they performed perfectly as stand alone references. No 3.3k, 1meg, feedback circuit, or adjustable source required. Apply power and read. The only problem being no calibrated measurements available to compare.In any case it would be greatly appreciated if you could drop those off at earliest convenience so I'll have a rough idea when to expect them at this end. Thanks again for your efforts.ps. from page one:

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 09, 2015, 01:08:56 pm
content: For testing things I build myself i nice little voltage standard:It uses a TI REF5025A, base accuracy 0.1%

author: janaf
author_rank: Frequent Contributor
timestamp: February 09, 2015, 01:38:13 pm
content: $1.75 for the 5 digit ADC, then processor, PCB, display, housing, a few small components, development, production cost & margin, seller work & profit selling small quantities. If they can do that, fine with me. If they can do it all legally, I'm happier.Quote from: edavid on February 08, 2015, 08:04:19 pm

author: paulie
author_rank: Frequent Contributor
timestamp: February 10, 2015, 11:59:17 am
content: Quote from: JohnnyBerg on February 09, 2015, 01:08:56 pmWay cool! Looks a lot like those $100 internet references. Not set for round absolute voltage but if measured with a lab meter should be just as precise and useful for calibrating DVMs. I see there is resistor and current standard too which is also way cool.ps. That chip seems to be available on Ebay for around $10. I wonder what advantage over the somewhat more common and less expensive 4040a which is speced for same accuracy.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 10, 2015, 12:09:30 pm
content: Quote from: paulie on February 10, 2015, 11:59:17 amThanks.The advantage is drift and stability. The REF5025A (that is what I am using) has a MAX drift of 8ppm/K. Also the long term stability is very good. Have a look at thedatasheet.Actually, I ordered 10 boards, and use only 1 for myself. I build 7 pcs and sell them for around $15. As the chip cost around $8 I am practicly giving them awayI also sell the 3 empty board, for $3

author: paulie
author_rank: Frequent Contributor
timestamp: February 11, 2015, 05:27:19 pm
content: Oh yeah... 8ppm is a lot better than 4040a 100ppm/1000hr. Maybe sometimes you do get what you pay for.BTW poking around with some very low voltages it seems there is no need for "isolated supply". I tried a divider off the same power rail as the meter and also off a separate battery source and in all cases it maintained very stable 0.1mv resolution. No difference isolated or not. Or maybe I don't understand what they mean by isolated.Looking at the schematic I also don't see why there is benefit to isolation either. Ground for power and input is the same. What am I missing?

author: janaf
author_rank: Frequent Contributor
timestamp: February 11, 2015, 05:48:50 pm
content: Take a look at the results I published pictures of a page back. Those are the common ground type, 3-wires only. Results are imo great. Cant see why common ground should not work. Before the Rail-Rail op-amps it was impossible to measure down to zero with common ground. There where even devices (panels like these) that would self-destroy if used with non-floating power. Had to do with nulling circuit. Earlier, the common ground type where usually a bit more expensive than the floating type. Not sure why.With the 5-digit modules it seems the ones with an extra decimal at low voltages require separate supply, while the ones with common ground don't show the extra decimal at low voltage. Don't know if there is a connection (sorry) there.

author: edavid
author_rank: Super Contributor
timestamp: February 11, 2015, 05:56:58 pm
content: Quote from: janaf on February 11, 2015, 05:48:50 pmHere is what the AliExpress seller says about the dual range/4 wire type:"Flexible supply: There is no need to isolate power supply, you can use the system common ground.(Note: If you need 0.1 mV resolution please use isolated power supply. Otherwise the accuracy drop an order of magnitude."I think this is mainly due to the supply current flowing in the common ground lead.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: February 11, 2015, 06:00:34 pm
content: Quote from: paulie on February 11, 2015, 05:27:19 pmActually, 1000hrs drift of the REF5025A is typ. 45 ppm.Looking at thedatasheet of the 4040Aon page 5 I see 100ppm/K (vs 8 of the REF5025) temp. coef. and 120ppm / 1000hrs.When you pay some extra money for the REF2025 High grade version you get 3ppm/K drift. So yes .. you get what you pay for. Prices have a tendency to rise exponentially with accuracy and stability.

author: paulie
author_rank: Frequent Contributor
timestamp: February 11, 2015, 06:25:01 pm
content: Quote from: edavid on February 11, 2015, 05:56:58 pmYes, that makes sense now. So a long thin common ground wire for both voltage source and for power would cause trouble. The meter draws very few milliamps but when you are dealing in the microvolt range even tiny offsets like that can throw readings off.I didn't experience it because my setup had 2 separate ground wires meeting at the meter. So that's why it didn't matter whether I used the 3 or 4 wire connections. The 4 wire and 3 wire boards are identical but separate connectors on the 4 wire forces using separate grounds thereby preventing current offset. So by isolation they really just mean use both connectors. I see said the blind man.

author: paulie
author_rank: Frequent Contributor
timestamp: February 13, 2015, 12:47:40 pm
content: Photo and more details of my calibration mod attached to post #1:

author: splin
author_rank: Frequent Contributor
timestamp: February 13, 2015, 04:33:00 pm
content: Is it possible to reprogram the STM8 on these meters?

author: paulie
author_rank: Frequent Contributor
timestamp: February 14, 2015, 05:04:33 pm
content: Quote from: splin on February 13, 2015, 04:33:00 pmThese can be programmed with the SWD pin seen in upper left corner of the photo. I tested it and it works but since there is no firmware for this I chose to use an external Mega8 for the input expander data log adapter. The circuit and program already existed for a previous ICL7135 project so it only took a few minutes to update the BCD table to 7seg.ps. I ordered two more of these meters on sale at Aliexpress so maybe when they arrive I can afford to play with flashing one. I do love that SDCC no nonsense C compiler.

author: paulie
author_rank: Frequent Contributor
timestamp: February 17, 2015, 01:35:24 pm
content: Couple developments. The references from LabSpokane arrived back and I want to thank him again for all the hard work. It's unfortunate confusion over the circuits resulted in more effort than necessary. He was nice to include the same resistor used in his tests so maybe I can build a jig with that to garner useful info from the graphs.In other news I am now the owner of one of those Ebay AD584 voltage standards. A friend of mine gave it to me because it did not come with precision readings so not of much use for his purpose. I did determine that the Ebay meter is at least 7ms accurate based on the AD584 initial voltage spec. We probably already guessed that from reviews published earlier in this thread. Probably a lot better in fact based on those. Of course I can't be sure unless I can get some reference readings from somebody who owns a precision meter. My buddy is considering getting his calibrated and got quotes from some standards labs but that may be some time off.Meanwhile I'm logging a set of the DIY references using the Ebay meter with my 16 channel input expander. Hopefully once it's calibrated I can adjust the data and get useful info. Even if not at least a chance to check out my logger and oven hardware which hasn't been operational since last year.

author: JBeale
author_rank: Frequent Contributor
timestamp: February 18, 2015, 01:35:33 am
content: I got a few of these ebay 0.1 mV resolution panel meters (they cost $7 for me) and on my samples, the DC accuracy looks like +/- 0.5 mV in the low range, compared with a Keithley 196 DMM and some voltage refs. The behavior of the meter when the input voltage has a step from 2.500 V to 5.000 V is not so nice. The decimal position update lags behind the digit update, so it briefly shows 30V and 40V before settling at 5.000 V.See also:http://youtu.be/9r6liwNnXaYIn the video, the actual voltages (from an AD584KH-based box) are: 2.50047, 4.99957, 7.50225, 10.00065 V.

author: paulie
author_rank: Frequent Contributor
timestamp: February 18, 2015, 09:21:53 pm
content: Yes, the glitches at that transition point are annoying. I also wish the transition was at 9.9999v instead of 4.3000v. What would be really nice would be 0-9.9999v then 0-99.999v like real auto range voltmeters. That would probably require external circuitry though so might be too much to ask. No matter what these are quite a bargain compared to brand name 5 digit meters even if it is just a panel meter.BTW Thanks for that 24 bit ADC project. Building my own version helped me understand why 24 bits is never really 24 bits.

author: electr_peter
author_rank: Unknown
timestamp: February 18, 2015, 09:49:45 pm
content: Customised SW could improve transition in ranges.As ADC is controlled from MCU via I2C bus, ranges transition goes like: ADC->max value in range; MCU-> saturation, change range(possible fail here); ADC-> different reading (possible fail here).4.300 range uses greater amplification in ADC, maybe it over-saturates/fails during this change and MCU SW simply shows erroneous reading.

author: paulie
author_rank: Frequent Contributor
timestamp: February 22, 2015, 02:06:53 pm
content: Recently needed to measure HV so another mod to increase meter range. More photos and info attached to post #1.

author: paulie
author_rank: Frequent Contributor
timestamp: February 25, 2015, 05:36:24 am
content: So here's the question... can we upgrade the 0-33.000v meter to perform like the one that costs twice as much by replacing the 20k input R with higher value? Imagine 5 digit 0.1mv resolution meter for $3 and change. As added bonus increase input impedance to more closely match commercial DMM in the process.ps.I've just ordered a few more of those just to find out.

author: paulie
author_rank: Frequent Contributor
timestamp: February 26, 2015, 04:33:24 pm
content: So the answer is YES! I couldn't wait for the latest meters to arrive (damn chinese new year) so yanked the old 0-33.000v meter from existing equipment and gave it a try.No problem boosting resolution from 1mv to 0.1mv by simply changing that resistor. Apparently it's also possible to increase even further to 0.01mv. However range would only be 0-0.33v which is not that useful for my current applications. I need to measure 2.5v voltage references so 0-3.3v does the job. Either that or get involved with resistive dividers which is a whole other project.When used with my x10 opamp/multiplexer circuit I do end up with 0.01mv resolution but with VERY restricted range (~2.35-2.65v). So effectively a 6 digit budget voltmeter that's more than stable enough to track 1ppm TC and long term drift.BTW another beneficial side effect to this modification effect is no range change artifacts because there is no range change on the cheaper meter.

author: MartinX
author_rank: Regular Contributor
timestamp: February 28, 2015, 08:58:40 pm
content: Those meters are terrific! I saw this thread and thought that would be perfect to put in my boat to monitor the battery voltage, the resolution is really handy, you can see if the battery is discharging really well. I bought some on Ebay and now that I got them the accuracy is better than I thought. I think I will add some reverse polarity protection and over voltage protection to better cope with the automotive environment, I also plan on taking the board out of the holder and put some conformal coating on it.

author: paulie
author_rank: Frequent Contributor
timestamp: March 01, 2015, 07:09:26 pm
content: When I first started working with these meters, specially the new 0.1mv one, I thought it was crazy overkill to use it for PSU or battery monitoring. Now I'm smarter and see big advantages in those applications. For one thing the current monitor for my own PSU design benefits big time because there is no need for fancy op amp circuits and expensive shunts. It's insane how some of these guys chase their tails over technical issues when really there is a much simpler solution. A cheap 0.1ohm 1/4watt resistor in the ground lead will give readings accurate from1ma to 2amps. A DPST Switch from that point to the V+ line allows using the same meter for voltage and current monitoring. Ultimate efficiency and cost effectiveness.I also see that as you suggest it's very helpful in keeping track of battery discharge. Instead of changes every few hours or days a sensitive meter keeps you updated on a time frame of seconds or minutes. Proved very useful in my solar array. Several more of these meters are on order now and will scatter them all over the place.As far as reverse polarity there is already a diode built in for that as can be seen in the post #1 schematic. Two friends are using these for automotive which sounds similar to boat and all they needed were low cost 7805 type regulators and caps for noise suppression. One fellow measured 80v transients which bricked several meters before but no problems since.

author: janaf
author_rank: Frequent Contributor
timestamp: March 14, 2015, 01:17:53 pm
content: Be aware that the input of the MCP3421 is quite low, 2.25M at unity gain.Quote from: paulie on February 25, 2015, 05:36:24 am

author: paulie
author_rank: Frequent Contributor
timestamp: March 15, 2015, 03:52:47 pm
content: True. In fact the stock meter is much worse with 330k and 20k to ground. This 0-33.000v to 0-3.3000v mod helps a little. Even ignoring the 10x resolution boost, the new shunt brings input impedance closer to a meg.What really makes a difference is using an op amp like in my 16 channel multiplexer which can get you gigaohms. This was necessary in my case to mask minimum resistance of the CD4051 analog switches. Not that important playing around with the stock 5 digit resolution but big help when boosting this cheapie to 6 digits.ps. Note the 2.25m is mostly due to a few pf in the sigma delta modulator so capacitive only and not resistive. Not really all that big a deal specially if you tack on a small input cap.

author: Vince Piotet
author_rank: Newbie
timestamp: April 02, 2015, 10:25:49 am
content: Quote from: paulie on February 17, 2015, 01:35:24 pmQuote from: paulie on January 14, 2015, 02:28:52 pmMay I chime in with the cheer ? Amazing accuracy !I bought 2 five dollar voltmeters and one 4.3V/30V eight dollar voltmeter.Out of the box they gave the same reading to the last few millivolts.So I hooked up a 10K trim pot in series and made all readings exactly the same.Then I was amazed by the result of the following experiments:1. When I supplied different voltages anywhere from 5V to 30V.the 3 readings were identical down to the last digit +-1 !!!https://www.eevblog.com/forum/Smileys/default/scared.gif2. The cheap DC-DC converter I used as variable voltage supply happens to be very stable as shown by the next experiment:Yesterday at 18:40, I connected this DC-DCconverter to these 3 voltmeters plus another 5 digit +-200 MV large size voltmeter(which costs about $15 on Ebay) and they have now been hooked up for the last ten hoursThe reading on the 3 $5 voltmeters and the larger size $15 voltmeter were:18:40 13.004 13.005 13.005 13.005V18:48 4 5 5 3 (meaning millivolts over 13V)19:45 4 5 5 13.00020:55 5 5 6 02:54 AM 7 7 7 12.9984:10 AM 7 7 7 12.996I will use them to build my own private voltage standard.There are cheap .1% resistors available on Ebay (nikkohm)

author: paulie
author_rank: Frequent Contributor
timestamp: April 02, 2015, 09:57:21 pm
content: Hey, welcome to the forum. Lotta fun to be had here.Even without thousand dollar references or lab meters I'm coming to the same conclusion: impressive accuracy and stability for very few dollars. All my meters agree within a couple counts too and with an AD584 which is closest thing to a precision standard ATM. Quite a good sign. Looks like the biggest benefit to tech advances and chinese ingenuity is making things that used to cost big bucks available to the poorest hobbyists.Fortunately a while back another Eevblog member agreed to help me by taking readings of my voltage references again. Overseas this time so it's taking a while but they should be showing up here any day now. I already have his readings which are almost dead on with ones taken by me before they were sent out. When they arrive back I'll take another set to see how these fare traveling half way around the planet then post results. This is fun and must admit I learned a lot about voltages, standards, electronics, and physics in general thanks to these cheapo toys.

author: janaf
author_rank: Frequent Contributor
timestamp: April 11, 2015, 03:53:52 pm
content: Apologies if this has been posted, but is it simple / complicated to move the decimal point?I'd want to use these for a small bench supp, 0-33V and 0-330mA would be perfect for me. It would then of course be neat to display XXX.xx mA and XX.xxx VI would change the voltage divider resistor and use a separate current shunt with a matching op-amp gain, for example 0-3.3V for 0-330mA.

author: paulie
author_rank: Frequent Contributor
timestamp: April 12, 2015, 12:08:43 pm
content: Quote from: janaf on April 11, 2015, 03:53:52 pmActually one of the benefits of using this meter for current monitor in a PSU is no op amp required. Only a 0.1 or 1.0 ohm resistor in the ground leg to get one amp to one tenth ma resolution. A simple DPST switch allows changing from voltage to current using the same display and virtually zero burden voltage drop.ps. To change the decimal point you would have to reflash the STM. I have blacked out all my decimal points to avoid confusion.

author: paulie
author_rank: Frequent Contributor
timestamp: April 14, 2015, 10:17:05 am
content: Reading your recent message regarding availability of meter code there may be some misunderstanding. Similar inquiry from another so maybe best to respond here. I don't have an image or any STM8 firmware. I meant one would have to write a program then flash it. Sorry I wasn't more clear. I may give it a shot some day and you will be first on my list to share.Thanks again for the parts you sent and for the favor. If there's anything else I can do to repay just let me know.

author: janaf
author_rank: Frequent Contributor
timestamp: April 14, 2015, 10:23:31 am
content: Quote from: paulie on April 12, 2015, 12:08:43 pmI interpreted this as you had done it in firmware. Of course, I can simply do it with black & white ink on the panel itself, that will surely do. I have no intention of switching the decimal point anyway.

author: ym58
author_rank: Regular Contributor
timestamp: July 17, 2015, 11:13:32 am
content: Quote from: paulie on January 06, 2015, 12:54:58 pmHi Paulie,Have you ever measured or got any clue regarding the internal resistance of the ammeter/voltmeter that you mentioned above ?Otherwise, our discussion continueshere... you're welcome to join us back !

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2015, 06:30:17 pm
content: From the circuit diagramm on the russian page, the input impedance should be about 350 K Ohms from the divider at the input. Other ranges / Versions might have higher or lower values. The ADC itself is not especially high impedance ( 2 M / Gain), so don't expect a really high impedance. Still this is very well suitabe for power supply.

author: ym58
author_rank: Regular Contributor
timestamp: July 18, 2015, 12:14:56 am
content: Hallo Kleinstein,actually, I was more concerned with the impedance of the ammeter than the impedance of the voltmeter.You may find the ammeter that I am talking about underthis link.

author: ez24
author_rank: Super Contributor
timestamp: July 18, 2015, 02:02:56 am
content: A little off subject but since Paulie started this:"Where is Paulie?" There is no "paulie" in the member list. I am there as is "ym58". I see "paulhm81" and the next name is"Pauline"The reason that this caught my eye is, I think a few weeks ago "paulie" got criticized pretty bad on picking a scope by a member (I could be wrong). It was kinda funny (that the member would put so much energy in the post - it was long) but I think Paulie must have gotten hurt. Well anyway there seems to be no paulie in the member list.Paulie if you are still here - answer up (FYI you were the one that got me started on my PSU - so I miss you). If you were the one that got ripped, just ignore it and come back.Hope I am all wrong about this and I am just having a brain fart.

author: eas
author_rank: Frequent Contributor
timestamp: July 18, 2015, 07:41:54 pm
content: Just click hisnameon his posts in this thread. It appears his account still exists, but he hasn't visited since the end of May.

author: ez24
author_rank: Super Contributor
timestamp: July 19, 2015, 01:06:21 am
content: My guess is if you cancel your membership your profile will remain

author: ym58
author_rank: Regular Contributor
timestamp: July 19, 2015, 06:41:21 am
content: Quote from: ez24 on July 18, 2015, 02:02:56 amI'm trying to figure out what you meant by"picking a scope"?As we are on EEV, I first understood it the literal way i.e."picking an oscilloscope", but I guess there must be another meaning ...(being criticized for "picking an oscilloscope" sounds a bit surrealistic to me)

author: ym58
author_rank: Regular Contributor
timestamp: July 19, 2015, 06:45:42 am
content: Quote from: ez24 on July 19, 2015, 01:06:21 amQuote from: eas on July 18, 2015, 07:41:54 pmI tried to PM him last Friday but the system replied :"User 'paulie' can not receive personal messages"

author: edavid
author_rank: Super Contributor
timestamp: July 19, 2015, 04:02:17 pm
content: Quote from: ym58 on July 19, 2015, 06:41:21 amI'm sure the problem had to do with this thread:https://www.eevblog.com/forum/testgear/do-cehap-meters-explode-when-you-abuse-them/

author: ez24
author_rank: Super Contributor
timestamp: July 19, 2015, 04:50:36 pm
content: From Dave:Quoteso that is what happened, I never saw that posting - too bad there is not a note in his profile that he was banned (under the name Paulie)tks

author: ym58
author_rank: Regular Contributor
timestamp: July 19, 2015, 05:42:57 pm
content: Quote from: edavid on July 19, 2015, 04:02:17 pmQuote from: ez24 on July 19, 2015, 04:50:36 pmI've just read the whole thread, the initial intentions were legitimate but it became real fist fighting !Maybe the mod should have simply closed the thread to calm everyone down ...But anyway, soldier Paulie seems to be lost somewhere in the wilderness and I still don't know what theeBay cheap ammeters impedanceis ;-) !!

author: sorin
author_rank: Frequent Contributor
timestamp: July 19, 2015, 06:25:12 pm
content: In my opinion paulie dont do nothin wrong.It's injustice that Dave banned him. This forum is not a democratic place.

author: Len
author_rank: Frequent Contributor
timestamp: July 19, 2015, 06:31:39 pm
content: Quote from: sorin on July 19, 2015, 06:25:12 pmThank goodness for that, if "democracy" means "freedom to be a dick."

author: edavid
author_rank: Super Contributor
timestamp: July 19, 2015, 09:01:52 pm
content: Quote from: ym58 on July 19, 2015, 05:42:57 pmThis AliExpress seller claims to be a manufacturer, so they should be able to tell you:http://www.aliexpress.com/item/Home-automation-module-LED-display-Color-0-36-Digital-Ammeter-DC-0-3-0000A-Four/32275906238.html

author: Rasz
author_rank: Super Contributor
timestamp: July 19, 2015, 10:10:15 pm
content: Quote from: ez24 on July 19, 2015, 04:50:36 pmyou missed :Quote

author: ez24
author_rank: Super Contributor
timestamp: July 19, 2015, 11:00:43 pm
content: QuoteNICE FINDThis seller puts up wiring diagrams

author: ym58
author_rank: Regular Contributor
timestamp: July 20, 2015, 12:40:58 am
content: Quote from: edavid on July 19, 2015, 09:01:52 pmThanks eDave,Actually, starting from your link, I made a few researches on AliExpress and it turns than there must be two different types of 0-3.0000A ammeters out there in China :#1the one you mentionedhere, coming in different LED colors :#2another type, red LED only, "apparently" a bit more sophisticated (and also more expensive) with a stated10mR impedanceand lab bench extra pics :Still hesitating ...

author: splin
author_rank: Frequent Contributor
timestamp: July 20, 2015, 02:08:19 am
content: Quote from: ym58 on July 19, 2015, 05:42:57 pm47.5 milli-Ohms including leads as measured on my 34401A in 4W mode. I bought 4 from this sellor (100% positive feedback) and had no problems:http://www.aliexpress.com/item/0-3-0000A-Five-digit-0-56-Digital-Ammeter-Four-LinesCurrent-Panel-Meter-Display-Color-Yellow/990444797.html#feedbackThe other seller, RD Factory, oddly will sell one of those for $4.94 but if you want more you lose the free shipping! Their feedback score is also slightly less at 99.3% but probably doesn't mean much.I'm delighted with these meters which seem to be reasonably accurate:Meter 1 HP34401A0.0000 0A0.5000 0.500042A1.0000 1.000106A2.0000 1.99602A3.0000 2.99742AMeter 20.0000 0A0.5000 0.500152A1.0000 0.999871A2.0000 1.99904A3.0000 2.99815AHowever, my meter hasn't been calibrated for some time so that doesn't mean much either![EDIT: corrected Meter 1 1.0000A measurment]

author: ym58
author_rank: Regular Contributor
timestamp: July 20, 2015, 02:31:48 am
content: Quote from: splin on July 20, 2015, 02:08:19 am50mR, that's good enough !I was even wondering ifthis modelfrom the same seller would not be a better deal ... coz what IREALLYneed is 1mA resolution (way enough to control Li-Ion charging currents), so I think that a 4-digit 0-3.000A ammeter can really fulfill my needs.On top of that, it's only 3.50€ a piece with free shipping plus a dual V+A display !Have you also tried that one ?

author: splin
author_rank: Frequent Contributor
timestamp: July 20, 2015, 03:24:01 am
content: Sorry - good try! But that dual meter is incredibly good value and perfect for a bench PSU. If I hadn't already got several of the 5 digit meters I'd definitely try them out. Please let us know how they perform if you do go for it.

author: ym58
author_rank: Regular Contributor
timestamp: July 20, 2015, 05:11:37 am
content: Quote from: splin on July 20, 2015, 03:24:01 amyes, maybe I'll even try one to replace the cheap voltmeter that I installed on my "old-time" customized ATX-PSU several years ago ...a bit "crowded" inside but so powerful a PSU indeed !(edit 08:48) just ordered 4 pieces for 14EUR !!!

author: ym58
author_rank: Regular Contributor
timestamp: August 24, 2015, 05:13:06 pm
content: I am back at EEV with good news : I managed to install two of these 3.50EUR Chinese volt-ammeters on both my PSUs : thevintage ATXand the (trickier) dual 30V/3ASL-1731SB!A bit tedious to get all things done (especially the dual PSU) but I guess it will worth it !Nota: the big pot on the dual PSU is a voltage multiturn that I installed a few months back.

author: Behemot
author_rank: Contributor
timestamp: October 20, 2016, 02:48:15 pm
content: I'll resurrect the thread if you won't mind. It has been suggested herehttps://www.eevblog.com/forum/projects/i-keep-on-blowing-up-panel-meters!/there may be some problems running multiple meters from the same power supply. Since these are the meters I'd like to get (the 4wire versions) and we even have a schematic…So my intention is to run 5 meters to measure output voltage of a power supply (between its 5 rails and GND). And than five more meters to measure voltage accross precise current shunts (so not against ground). I would like to use single power supply to power all the meters (plus some fans), insulated from the measured circuit. Can this work, or do I have to use 10 different insulated power supplies? That would be somewhat, eh, impractical if you see my point.Now I may or may not connect GND of both the measured PSU and the PSU for the meters, however, as most SMPS are ground referenced, they will actually share a common earth ground anyway (unless I select class II PSU which is not ground referenced). Does this complicate things further?

author: Kalvin
author_rank: Super Contributor
timestamp: October 20, 2016, 02:57:00 pm
content: Why don't you use those cheap isolated dc-dc-converters to isolate the meters from each other. A small, isolated 1W 5V/12V or 12v/12V dc-dc-converter will cost around $2 - $4 a piece. To play safe, you will need N isolated dc-dc-converters for N volt meters. Just go to eBay and enter the magic keywords: isolated 1W dc-dc converter. If the 1W is too little, there are 2W and 3W models available with a bit higher price tag.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 20, 2016, 03:29:35 pm
content: It depends on the actual circuit in the meters. It may work to have several voltmeters that measure relative to a common ground powered from the same source. But still you have to check the circuit or carefully try out (with extra resistors for protection first).For the current measurement on floating shunts you will need a separate supply for each meter. The DC/DC converters or transformers (e.g. 2 outputs from one transformer is common).

author: Behemot
author_rank: Contributor
timestamp: October 20, 2016, 04:52:05 pm
content: Those converters look nice, there are double converters in single package for few bucks so I'll need just about five pieces. Didn't even know it came in so many variants so cheap…I've just seen the larger uninsulated converters. Haven't been watching this particular market, thanks for the suggestion.Just seems I'll have to shield it properly so it does not induce extra unnecessary EMI as it's going to be lots of switching stuff in the usuall tens of kHz band. I think I'll make use of some old PC case to house the whole thing so I can also use some ditched PSU casing for the DC-DC unit. This way I'll also use the hole in the back of the caseGuess I can use thin coaxial cables to run supply for the meters which should radiate less noise on the way, right? To be extra paranoid maybe even with two conductors+shielding so I can use the first layer as return path and the outter one earth-grounded as EMI shielding. Will be much more cabling than using common supply line, but well…ADD// found the name - triaxial cable

author: 001
author_rank: Super Contributor
timestamp: January 16, 2017, 04:15:39 pm
content: Funny tale beginsLooks like these china panel meters became certified 'laboratory' bench units (0.1%):Any opinions?P.S.: Can I move decimal point on these beauties without custom firmware? (from 33.000V to 330.00mA, 330V, etc)

author: Behemot
author_rank: Contributor
timestamp: January 16, 2017, 04:18:45 pm
content: I think it's more than adequate for like schools etc. If the assembly company guarantees accuracy and some input protection, what's really the problem?

author: 001
author_rank: Super Contributor
timestamp: January 16, 2017, 04:20:49 pm
content: The reference voltage is not a problem?And what about cruel 0.1% at 5 digits?

author: Behemot
author_rank: Contributor
timestamp: January 16, 2017, 04:24:19 pm
content: They may be using different reference etc., what's the problem? As stated, guarantee the accuracy. It's their problem how they do it. It may very well be just a display having nothing in common with these voltmeters, whaddaya know…

author: edavid
author_rank: Super Contributor
timestamp: January 16, 2017, 04:29:58 pm
content: Quote from: 001 on January 16, 2017, 04:15:39 pmWhy do you say that?QuoteNo.

author: 001
author_rank: Super Contributor
timestamp: January 16, 2017, 04:44:28 pm
content: Sorry for my englishBut attached photo ishttps://vk.cc/66tqXsUnit manual guaranted at least 0.1% tolerance

author: edavid
author_rank: Super Contributor
timestamp: January 16, 2017, 04:49:53 pm
content: Quote from: 001 on January 16, 2017, 04:44:28 pmThat is a 4.5 digit meter, so it is obviously not using the particular China DPM module that we are discussing.

author: sperb
author_rank: Contributor
timestamp: October 05, 2021, 08:05:15 pm
content: Sorry for resurrecting this topic, but I purchased a meter similar to this one and decided to make a custom software for it.The functionality is similar to the one existing on the original module: 0-35.000V and 0-4.4000V.Reflashing the unit will allow you to change the gains, ranges, decimal point position and perform a recalibration.Feel free to use, make changes and update as needed.The software was created following the explanation existing here:https://embedded-lab.com/blog/category/stm8/using the STVD and Cosmic C compiller.Best regards.As requested, follow (attached) some pictures from the unit I have.Unit was bought from Ebay. link:https://www.ebay.com/itm/184581349615

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 05, 2021, 11:28:40 pm
content: Quote from: paulie on January 07, 2015, 03:12:30 pmThey are typically calibrated at a factory. All precision 5-digit meters I have bought were well-calibrated. But this calibration flows away with age as in any other meter.0.1% or 0.05% voltage reference is fine not only for those panel meters but for most DMMs too.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2021, 08:26:22 pm
content: The 35 V range and likely also the 4.4 V range would likely need external dividers. So the gain would be set by the reference and the xtra divider. Usually this is done with a software scale factor and not by using 0.1% accurate parts. So even a 2% accurate reference like LM399 is OK for a 6 digit DMM.AFAIK the original low cost panel meter use an MCP3421 ADC with internal reference only. The internal ref. my be of comparable quality as an LM4040.

author: Canonius
author_rank: Newbie
timestamp: January 06, 2022, 01:43:20 am
content: Hello,i think either your project is not ready, or you uploaded something wrong. Because for example this variablesvalue2 = value2_loc; // update display variablevalue = value_loc; // update display variabledp = dp2;will never be applied to the 7 segment display. At least this routine is missing.Please, can you share the final source code with us? I would really appreciate it because i need to replace the analog meters from my old power supply.Can you also tell me which option bits should be set?I am not familiar with the STM8 and i have to use the IAR IDE because i didnt get the damn licence file for the cosmic compiler up till now.Thanks in advance!Quote from: sperb on October 05, 2021, 08:05:15 pm

author: sperb
author_rank: Contributor
timestamp: January 06, 2022, 04:41:29 am
content: Hi Canonius.The version i shared is the latest I have. And it is supposed to work.The variables that you are seeing (value2, value and dp) are used by the interrupt routine to update the display. Please check the file stm8s_it.c and you will find those variables being used to update the 7 segment display.I don't remember if I had to mess with the option bits, but if they are being set, you will find them in the other files of the project. Just keep digging.If you want something ready, I recommend using the platform I used (wait for the cosmic compiler). The code I shared should work then. And you can customize as you want after that. I have no experience with the IAR IDE, so I don't know how hard will be to convert the project to that platform.Hope it helps.

author: Canonius
author_rank: Newbie
timestamp: January 07, 2022, 08:59:07 pm
content: Hi Sperb,please don't get me wrong. I am really happy that people like you sharing these things!Now i got the license file for the Cosmic compiler. I will check your project later that day and give here feedback!Thanks again!RegardsEDIT:Very Nice! it worked out of the box!Thank you very much!Quote from: sperb on January 06, 2022, 04:41:29 am

author: b_force
author_rank: Super Contributor
timestamp: April 09, 2022, 08:33:03 pm
content: Little kick on this threat, mostly because I am actually looking for a 5 digit meter that sits in the mV range, about 900mV max range (or 999.99 I guess).Ideally one that can also be adjusted for calibration, since I have a 0.02% voltage reference here.Most I can find only go up to 200mV, which is a littler awkward, since my range is around 100-400mV or so.Obviously I could cobble something together with a micro and a proper ADC, but I am more looking for an of the shelf affordable solution.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 09, 2022, 09:07:25 pm
content: Many of these low cost versions use a MCP3421 ADC and a µC like STM8. The MCP3421 has a range up to +-2 V and a few internal gain stages to get ranges like 1 V , 500 mV and 250 mV. So the HW should supprot a higher range, just a question of the SW. Similar the calibration / adjustment is a SW question. This may be even supported, just no documentation provided.

author: b_force
author_rank: Super Contributor
timestamp: April 09, 2022, 11:34:33 pm
content: Quote from: Kleinstein on April 09, 2022, 09:07:25 pmYeah, that's also what I realized after this topic.I was more kind of hoping someone would have direct experience, example or like a quick and easy tutorial or something.Another idea is rolling my own PCB or something.Post 141 is already in the right directionI was more looking for a ready made solution.edit: another option would be adding some external gain, but unfortunately that adds some extra error and accuracy.

author: Shock
author_rank: Super Contributor
timestamp: April 09, 2022, 11:47:50 pm
content: Sperb can you link the one you purchased please? If possible take a photo of your pcb as well. Just so I and others can get or check we have the correct one.

author: b_force
author_rank: Super Contributor
timestamp: April 10, 2022, 01:19:50 am
content: Quote from: Shock on April 09, 2022, 11:47:50 pmFound some schematic here at least (plus some further explanation)https://eddy-em.livejournal.com/73375.html

author: floobydust
author_rank: Super Contributor
timestamp: April 10, 2022, 02:34:39 am
content: Careful, there's two different styles of these 5-digit panel meters. I look at the pictures to figure out which is being sold but even then, pic is 4-wire and I get 3-wire. If a vendor does not show a pic of the board, I would not buy.SOT23-6 A/D (MCP3421) and some modules have only three wires for power and ground and V input, others have two connectors etc.SOIC-16 A/D it's a chinese dual-slope part and I forgot where I posted the part number.

author: magic
author_rank: Super Contributor
timestamp: April 10, 2022, 06:33:40 am
content: Here.Are those with SOIC16 ADC any good?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2022, 07:09:43 am
content: The ADCs in SO16 case are likely OK too. The MCP3421 is not that special: just small, cheap and easy to use.The other likely Chinese ADC version may be a bit cheaper but probably similar quality. It may even have an advantage with better 50 Hz supression. The MCP3421 has good 60 Hz suppression, but no good 50 Hz supression (one can improve on this by averaging over 3 consecutive readings).Knowing the ADC and HW configuration would in theory give the option to use the Chinese HW and just make a custom SW version.I somewhat doubt that the 16 pin ADC is a dual slope - they went largely out of favor because they need a relatitvely bulky capacitor (usually some 100 nF...500 nF with low loss). The ICL7106 even needs 2 more large capacitors. SD ADC are made with all parts on chip and are mean-while quite cheap. Also modern handheld DMMs use SD ADCs and usually no more dual slope.

author: b_force
author_rank: Super Contributor
timestamp: April 10, 2022, 12:39:56 pm
content: The amount of bits doesn't say that much to begin with.I have seen ADC's with an higher amount of bits, but with an higher amount of error a well.Resulting basically in a ADC with much lower bits. (since the rest of the bits vanish into those errors)Also a proper voltage reference is always needed of course.This can be somewhat stretched when being manually calibrated.I just calculated the total relative error estimation (with those wonderful partial derivatives).These devices will just give me enough accuracy.I was just hoping there was some of the shelf solution.Maybe I can write those Ebay/Ali shops a message if they can reprogram something for me.I am also happy to make my own PCB, it's pretty straightforward, but those MCP3421 are out of stock (like most other things atm)

author: floobydust
author_rank: Super Contributor
timestamp: April 10, 2022, 09:10:54 pm
content: I found it, SOIC-16 "22 bit" A/D marked "BX5815" and nobody knows where it comes from in the gutters of Shenzhen. It looks like a TL431 is the reference.At first I thought a knockoff ofTC500dual-slope.Or the IC is possibly a remarked Titan Micro delta-sigma 24-bitTM7707or 16-bit TM7705. They appear to be "enhanced" knockoffs of Analog Devices 16-bit delta-sigmaAD7705.I wouldn't doubt it's 16 bits, as the display at 99,999 count almost 17 bits worth.more in this threadhttps://www.eevblog.com/forum/testgear/are-sub-$10-5-digit-ammeters-worth-buying/

author: magic
author_rank: Super Contributor
timestamp: April 10, 2022, 09:26:47 pm
content: If TL431 is the reference then they likely aren't as good as MCP3421, although the latter's TC specification is only "typical"That being said, mine shows 5.000 when measuring a MAX6250 5V ±1mV reference chip.Now it makes me wonder if those meters also cheat and round to full volts. Anyone tried?

author: floobydust
author_rank: Super Contributor
timestamp: April 10, 2022, 10:53:48 pm
content: I tried a couple I have (MCP3421A/D model) and it's kind of hilarious. There is no evidence of rounding to favorite values, and they are within about 2 counts verses a 34401a across the span of 0-25V, and VCC changes do not affect readings. But not so great firmware.Both only show 4 digits i.e. 9.999V then the 5th comes on past that i.e. to show 10.000VOne module shows 5 digits up to ~4.3V but it freaks out:If it is already in 5-digit display mode, it will move the decimal point at around 4.3900V i.e. increasing input it displays 4.3899V then goes to 4.390V (it does flicker 31.895V as it moves the DP to the right).If it is already in 4-digit display mode, it will move the decimal point at around 4.3100V i.e. decreasing input it displays 4.310V then goes to 4.3089VOf course a cold-boot does aggravate all this as well. In slow motion video, cold boot with input steady 4.3087V it displays: 0.000/0.430/0.5037/3.4920/4.3083V and settles there.What I dislike about these modules is the averaging, if you input 5.0000V and add 50mVpp sine the numbers are constantly changing, at 10-30Hz they're wandering too much to make sense of. The A/D slowest is 3.75sps for 18-bits.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2022, 06:50:03 am
content: I have not tried one of those modules, but the MCP 3421 in a DIY circuit. I did not see any problems and with averaging of 3 readings 50 Hz supression is OK.The claimed 18 bit resolution looks about right - not like many 24 bit SD ADCs where this is only the number of bits provided for the data format. So 5 digits is a bit of a stretch - not a stable last digit, but also not wandering all over the place (e.g. RMS noise similar to last digit).That an AC background makes the signal wandering is normal, if not at the notch frequency. With some additional averaging the SW could get a more stable, but also slower result. With 60 Hz hum the MCP3421 should have reasonable hum suppression, similar to what you get from a handheld DMM.The factory trim of the scale factor looks quite good, though there is some doubt they actually get 0.05% accuracy after soldering.With the additional dividider at the voltmeter input, there is anyway likely a cal factor in the software. It is not that much effort to get the adjustment right.

author: b_force
author_rank: Super Contributor
timestamp: April 11, 2022, 12:30:07 pm
content: Quote from: Kleinstein on April 11, 2022, 06:50:03 amYes, so with a 0.05% accurate voltage reference, the max theoretical error can only be as good as 0.05%.On a 10 volt signal, that means 10.000±0.005V.With a 30V signal that means 30.000±0.015V -> 30.00±0.02V so much so for those 5 digitsFully assuming that any additional voltage divider won't attribute (which is mathematically and physically impossible)This is however without any other additional issues, like gain error, match error and DC offset (as well as drift).With external components you will makes this error a lot worse very quickly.According the datasheet, the gain error is 0.05% typical, but can be 0.35% maximum.Point is, we don't know.I am a little confused what they mean with the PGA error, since there is only one PGA in there?Usually this is just (quite literally) just an amplifier stage, which it could mean the gain error in de positive vs negative input.If that's true, this can cause a lot more issues with a typical 0.1% error.in 18 bit mode it only does 3.75 samples per second, so you won't be even able to measure 50/60Hz hum.Any low-pass kind of behavior will automatically mean an averaging filter to begin with.Only in 12 bits mode you will be able to really measure this 50/60Hz hum.In the end it's always a little odd, having so many bits with a reference that's only 0.05% accurate.Meaning that it's fine to measure relative differences very well, but absolute differences you still need an external voltage reference and some calibration inside the microcontroller.Or in simple words, you can detect a difference in signal very well, but the value of this difference a little less.(the main reason why most professional multimeters/bench meters are not more accurate as they are)The contribution of an external voltage divider can be calculated as well.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2022, 01:16:33 pm
content: The simple SD ADC linke the MCP3421 essentially does an integration over certain time and this time determines how much of 50/60 Hz hum comes through with the result. You don't have to be able to really measure the 50/60 Hz with this, one actually prefers not to see them and have a blid spot at exactly that frequency. Nominally a blind spot is at 60 Hz (for 14 bit and more), but there are huge tolerances: 14 bit mode is between 44 and 82 SPS, with 60 SPS nominally. So it depends on luck where the hum suppression works well. Nominally 60 Hz, but some units may have the sweet spot even below 50 Hz.The accuracy of the reference and scale factor limit the accuracy one can get without an extra calibration / external calibration factor. It can be corrected with an external factor. When using the extra divider one is essentially forced to use such an extra factor and it is well possible the Chinese could as well use the right factor instead of a fake. With that extra factor the initial accuracy of the reference does not matter.The HP34401 DMM and most other 6 digit DMMs use a LM399 ref. with 2% accuracy.It is also quite common to have more resolution than accuracy. This especially comes up with higher resolution. A typical 6 digit DMM has 40 ppm uncertainty on there best range (except frequency), so that is 40 LSB steps and other ranges can get quite a bit worse. It would be strange the other way around. With low resolution meters one is used to get the last digit essentially stable (1 LSB peak to peak). With higher resolution the more common case is an RMS noise of about 1 LSB steps and thus the last digit with about 6 steps peak to peak noise. That is the common way to define the resolution, as the higher grade ADCs tend to give out more bits and the limit is from the noise and often not the quantization.The PGA part in many SD ADCs is not actuall a true amplifier, but more like a change in the sampling frequency. Essentially all the single chip SD ADC wort with switched capacitors and sampling the input at a rather high rate ( in the MHz range). This transfers charge to an integrator for the conversion. What they call PGA gain is actually only using a higher frequency (and in some cases maybe 2 parallel channels) for the input and this way operating the ADC with gain. The neat feature of this is that the "gain" is very stable and accurate. I am not sure the display units would use that gain settings other than for the current mode.One can see an effect of the this more frequent sampling: the input differential impedance is specifies as 2250 Kohms /gain.

author: b_force
author_rank: Super Contributor
timestamp: April 11, 2022, 02:24:23 pm
content: Quote from: Kleinstein on April 11, 2022, 01:16:33 pmYes, that's called external calibration, which gives an absolute result that is only as good as its reference.In this case they don't use the LM399 for the absolute voltage reference, because that would mean your error/accuracy in the end will be only as good as 2%.After they calibrated the device, they use the LM399 for a reference for drift over time and/or temperature.Drift over time (long term stability) is often not specified for most ADC's and seen the teeny tiny page of this specific ADC probably not so good.Let alone having some kind of active heating element (like the LM399 has)So a smart person already learned something from this.As long as you have a good and stable reference yourself and you can calibrate your devices, one can make cheap gear pretty accurate, as long as you calibrate them on a (very) regular basis!This is also standard procedure in some research, to calibrate your instruments before you're doing the actual measurements.btw, the effort to get the calibration right isn't difficult on paper.In practice this is a pretty labor intensive job, since you have to check every device by hand.Since wages in Asia are so low, 99.8% of those Chinese meters are all just calibrated by hand with some bench meter or so.In the end it's all just statistics, there is a chance that your 10 euro dinky meter will be better than a 100k bench meter.Like I said, you just don't know and the chances willlikelybe very low!Anyway, I think we are going off-topic at this point?

author: Kleinstein
author_rank: Super Contributor
timestamp: April 11, 2022, 04:16:49 pm
content: With the extra divider from relatively cheap resistors they essentiall have to calibrate / adjust the meters. Even with a very accurate reference they should check the accuracy anyway and doing the adjustment does not have to be much more complicated and time consuming. The procedure would be to power the circuit up in a test fixture and let it measure 0 voltage and an externally provided reference voltage (e.g. 10 V or 20 V, depending on the meter) and than send a special command to the meter to tell it to do the adjustment. So much of the adjustment would be internal to the small meter.An accurate adjustment may take a little more time for averaging or waiting for the circuit to warm up and a more frequent check of the reference.Ideally there would than be a check if the display reading is actually OK as it should be.Adjustment would be time consuming the old style with a trimmer. So even in China they tend to prefer software calibration over mechanical trimmers.

author: sperb
author_rank: Contributor
timestamp: April 13, 2022, 02:06:05 am
content: As requested, I included some additional information on my post with the alternative firmware for the meter.here:https://www.eevblog.com/forum/projects/$5-voltmeter-with-5-digit-(0-1mv)-resolution/msg3730789/#msg3730789Please let me know if that helps.Regards.

author: floobydust
author_rank: Super Contributor
timestamp: April 13, 2022, 04:54:02 am
content: I didn't see the firmware file(s), where are they located?I think the STM8 is used for these panel meters because its the lowest cost, small MCU with EEPROM for the calibration. I have not tried soldering in the pushbutton switch to see what it does.

author: sperb
author_rank: Contributor
timestamp: April 13, 2022, 04:56:00 am
content: floobydustCheck my previous post (link above).It is on the attachments.Regards.

author: floobydust
author_rank: Super Contributor
timestamp: April 13, 2022, 05:03:45 am
content: Check again, there's no software in the attachments of post #141, just the two pictures

author: sperb
author_rank: Contributor
timestamp: April 13, 2022, 05:05:55 am
content: Are you sure?

author: floobydust
author_rank: Super Contributor
timestamp: April 13, 2022, 05:12:07 am
content: AH now I see it, pic filenames I thought are on top. Sorry.Do you know how much codespace is used, I think the parts have 8KB of flash.