Making panel meters for my psu. Any suggestions ? - Page 1
Forum section: electronics / projects

author: gabri.peic
author_rank: Regular Contributor
timestamp: May 11, 2022, 09:46:04 am
content: Hi. I just finished making the main pcbs for my bench power supply based on elektor 1982 precision power supply. It seems to work as it should, But now i need to think of the panel meters. As much as i like the vintage look of analog meters i still prefer a digital display for everyday use. So is there anything i can buy or build for this purpose ?I would like to have 4 digit 7 segment led displays although 3 digit would be ok too.I am open to using microcontroller, I am not good at programming but i am willing to learn.Also it is important to note that i will have two completely isolated supplies in the same enclosure so i need to keep two sets of panel meters completely isolated.

author: LateLesley
author_rank: Frequent Contributor
timestamp: May 11, 2022, 10:09:59 am
content: Well, a quick search on eBay show some candidates. It's hard to choose, because we don't know the Max voltage or current, but I thought I'd link the search to give you a starting point to peruse.https://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=4digit+voltmeter&_sacat=0&LH_TitleDesc=0&_odkw=4+digit+voltmeter&_osacat=0

author: RoGeorge
author_rank: Super Contributor
timestamp: May 11, 2022, 10:12:47 am
content: There are many models of 7 segments panel meters on Aliexpress, cheaper than one can build. Various colors and segment sizes, most of them with 4 digits. Some are single indicator, some are grouped in 2-3 lines for V, A, W. They usually have a trimmer to calibrate them.I bought a batch of ten voltmeters some years ago, and they were OK. 2 wires for power and a 3rd to measure the voltage. Pretty accurate after calibrating the trimmer. What they couldn't do was to be accurate in both sub 1V and over 1 volt at the same time.

author: themadhippy
author_rank: Super Contributor
timestamp: May 11, 2022, 10:26:17 am
content: Quotecombine the 2,the beauty of analogue with the convenience of digital

author: xavier60
author_rank: Super Contributor
timestamp: May 11, 2022, 11:35:31 am
content: Those cheap LED Volt/Amp meters can be a lucky dip. The 4 digit ones tend to work better because they have auto-zeroing for the Amp section.The biggest annoyance is the trend for slowing the refresh period which can be as long as 2000ms despite it being advertised as 300ms usually.

author: gabri.peic
author_rank: Regular Contributor
timestamp: May 11, 2022, 05:51:35 pm
content: Quote from: xavier60 on May 11, 2022, 11:35:31 amThe slow refresh is exactly what i was afraid of with the cheap ones from ebay...Maybe i will look around and give some a try.Are there any dedicated meter ic chips out there 🤔?

author: RoGeorge
author_rank: Super Contributor
timestamp: May 11, 2022, 06:05:55 pm
content: Quote from: gabri.peic on May 11, 2022, 05:51:35 pmICL7106 or ICL7107, 3 1/2 digits.I don't remember mine being 2 seconds slow, that would be unusable slow. Found the description page I've saved back then, at the checkout payment ($10.36 total for a pack of 10 voltmeters). The description says something about 0.3s, and that's about how I remember them, not blazing fast but OK.QuoteFound a picture, they were 3 digits, not 4! Sorry.

author: strawberry
author_rank: Super Contributor
timestamp: May 11, 2022, 06:24:56 pm
content: ICL7135 4.5 digitsslow display refresh rate (weak CPU)slow ADC sample time (may hide REF or input noise problem/immunity)

author: RoGeorge
author_rank: Super Contributor
timestamp: May 11, 2022, 06:41:35 pm
content: Or buy 4 pcs of cheap DMMs (some can count up to 6000 for about $10) and fix them behind the front panel of the power supply.If not, some ARM devbords have many channels of 16bits ADC for $15, like this FRDM-KL25Z:https://www.nxp.com/design/development-boards/freedom-development-boards/mcu-boards/freedom-development-platform-for-kinetis-kl14-kl15-kl24-kl25-mcus:FRDM-KL25Z

author: ledtester
author_rank: Super Contributor
timestamp: May 11, 2022, 07:22:10 pm
content: The INA226 and INA219 are popular with the Arduino crowd. They can report current, voltage and power back to the Arduino which then can send the values to whatever display you want.Here's a good video on how these kind of sensors work in your circuit:#321 7 Sensors tested: Measuring Current with Microcontrollers (Arduino, ESP32, ESP8266)https://youtu.becG8moaufmQs

author: TimFox
author_rank: Super Contributor
timestamp: May 11, 2022, 07:31:56 pm
content: There are lots of decent digital panel meters out there.Be careful, though (assuming you don't use battery power as in a DMM):Many (if not most) of them have a common connection between Input Low and Power Supply Common (-), so small extra power supplies may be required.

author: Dr.Krieger
author_rank: Contributor
timestamp: May 12, 2022, 12:06:10 am
content: I am impressed by some cheap eBay meters, although some are worst than others.One of them had smaller LEDs and was losing a digit when measuring less than 10V, but then again i wouldn't trust the last digits of those meters. The good thing is that you get a fairly accurate X.XX reading (ignore the rest) and you don;t get that with the cheaper $1 3-digit voltmeters.I am not sure about the actual listings and sellers because its been 4-5 years since i bought mine, but here are the ones i find today:0.36" that loses one digit <10V:https://www.ebay.com/itm/1324745890550.56" that is fairly accurate:https://www.ebay.com/itm/274758652402Some of those eBay meters have (or had at least) 18bit ADC;s in them according to this post:https://www.eevblog.com/forum/beginners/43-000-count-voltmeter-from-10-bit-adc-how/

author: gabri.peic
author_rank: Regular Contributor
timestamp: May 12, 2022, 01:26:11 pm
content: Thanks. I will probably go with one of the ebay ones.Only one problem.The ones suggested above are all just volt meters, So are the same meters available but with current insted of voltage ?In that case i would put 4 meters on the front panel, Two per channel for current and voltage.Obviously i would supply meters for each channel with isolated power supply to keep the isolation between the channels.

author: Mortymore
author_rank: Frequent Contributor
timestamp: May 12, 2022, 01:39:40 pm
content: There are cheap panel meters with both Volts and Amps

author: TimFox
author_rank: Super Contributor
timestamp: May 12, 2022, 01:40:43 pm
content: A normal digital panel meter is merely a voltmeter across a "shunt" resistor.Be careful again: you don't want to lose too much voltage across the shunt, 100 mV should be available in a panel meter.Of course, you are limited by the accuracy of the shunt resistance, and probably need a four-terminal connection, and you need to wire the voltage meter after the current meter.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 12, 2022, 07:58:23 pm
content: With a lab supply one may be able to use the same shunt as used for the current regulation also for the current measurement.In the best case, especially with a lab supply that uses a separate supply for the regulator part (e.g. the Harris / HP design), one may use the auxiliary supply also for the meters.

author: Benta
author_rank: Super Contributor
timestamp: May 12, 2022, 08:03:39 pm
content: I've never bothered making panel meters myself. Even if you find the right bezels etc, in the end it still looks amateurish.My personal preference is Red Lion panel meters. Not cheap, but very, very good. The biggest difficulty using them is making the rectangular front panet cut-out.https://www.redlion.net/family/panel-meters-0EDIT: scratch that. I just saw the latest prices. Ignore this post, please.

author: Dr.Krieger
author_rank: Contributor
timestamp: May 13, 2022, 11:12:10 am
content: I have seen 4-digit "red-blue" V/I LED Panel Meters but i can't seem to find any on eBay atm... All i can find are the cheap 3-digit ones like in Mortymore's post, but those are NOT accurate at all, for example you can have a 30-70mV error and that is after you calibrate it using the tiny pots on the PCB. The 5-digit Voltmeters i showed are "pretty accurate" (don't have a good voltmeter to compare them i'm afraid), you can still get a small error like 10-20mV on the lower part of the range (like under 1 or 0.5V) but i think you can live with that.There are also 5-digit Ammeters but as you said you will have to use 4 for a dual PSU...Also note that there are 5/50mA meters that you can configure using the correct resistors, like in the circuit i show below. So a 5mA (reads 0.0000-5.0000) can be made to show 3.0000 at 3A with fairly good accuracy.https://www.ebay.com/itm/325088488516https://www.ebay.com/itm/284289515988Also pay attention to what TimFox and Kleinstein said above, the Voltmeter has a very high input resistance and causes no problems (besides the supply it needs) but an Ammeter measures a voltage drop on a "shunt" resistor and if you don't connect it correctly the shunt's voltage drop will be an error on your output, so if for example you have 50mV drop at 1A on your shunt and Vout is 10V (your accurate meters measure 1.0000A and 10.000000V) , then you will only have 9.95V on the load, so there goes your "load regulated" voltage... And those Elektor guys even gave you a 4-wire output (remote sense) so you can have exactly 10.00V on your DUT.If i am right about the circuit you use, then you kinda have to use a different Shunt to measure the Amps, because the one the OpAmp uses to measure current is on the "high-side" (on the Positive side of the output voltage) and most if not all eBay meters measure from Ground to a positive voltage, so your options are either use a ground-referenced meter and use the output of the big 33V/4A transformer to make a aux supply for the meter (i have used a 5-part Zener Regulator before to power such meters from a +48V supply and it works fine with a TO-220 NPN like a BDX53C or similar an a tiny heatsink), or use an isolated aux psu with either a small 50Hz transformer with Mains input or a smallisolatedDC/DC supply using one of your existing rails.You could try and use the already existing high-side 0.22Ω resistor (R22), put a divider on it and use an "accurate" meter like the ones we talked about before (or a ICL7107 meter or something as long as the input range goes low enough).You can even try to mess with the circuits 0.22Ω/3W shunt (that should really be 5W+) and change it to a smaller value true shunt (cheap 75mV@5A Chinese examplehttps://www.ebay.com/itm/282285363253) if you know what you 're doing, maybe for better accuracy vs temperature because 3A thought a 0.22Ω is 1.98W and that is a ton of heat that will definitely affect the value of the resistance. I think the cheap white 5W resistors have ~500ppm/C (these ROYAL OHM ones are +-400ppm/Chttps://www.tme.eu/Document/46f95e5fd8d12d8d5e15683d74cddc76/ax5w_axial_royal.pdf) while a "proper" shunt can do at least 20 times better (20ppm for this onehttps://www.tme.eu/Document/9a11044aeb9919912b385f857afa61b7/oar.pdf).Just don't go too low with the shunt value because those 741 are not exactly "cutting edge" stuff and you may lose accuracy because voltage drop is too small for the Op Amp...So i don't know if you can find accurate V/I "combo" meters, but you can for sure use 5-digit meters for Voltage and Current without much fuss, you only have to make a small Zener Regulator (maybe with a 78L05 on the output?) to lower (and regulate because it will have lots of ripple under load) the main transformer's ("33V/4A" )~50Vdc output (on zero load) to a "ground-referenced" (well actually "C10's negative pin referenced") small aux voltage like 5V and you can use that for both meters, AND if you place the Ammeter like i show below, BEFORE R22/R24, then you will have zero error from its tiny onboard shunt!*example drawn is with a combo V/I meter but the same works for two separate 5-digit meters.

author: Dr.Krieger
author_rank: Contributor
timestamp: May 13, 2022, 11:49:34 am
content: Better schematic, with the 5-part Zener Regulator.Transistor and Zener dissipation depends on the current the meter/s draw, a 500mW Zener is enough but the BJT can have a Vce of over 40V so even a 30mA current means it will dissipate ~1.5W so a heatsink is needed.If you use two meters it is possible need more than 50mA so maybe use a bigger Zener value (like 12-15V instead of 5.6V) and let the meter's onboard 3.3V reg do some of the work.Like i said i have used this circuit to put those cheap and inaccurate but "close enough" 3-digit red-blue V/I meters on the output of 48V PSUs and it works fine as long as you put a heatsink on that TO-220 (i use BDX53C).*"close enough" for general monitoring of a PSU that powers a 3D printer or something like that, not for a Lab PSU ofc!OFC a switch-mode DC/DC step down would be way more efficient but those cheap eBay LM2596 modules can't handle a 50V input...

author: tunk
author_rank: Super Contributor
timestamp: May 13, 2022, 01:27:31 pm
content: Quote from: Dr.Krieger on May 13, 2022, 11:12:10 amYou could try to search eBay with this: ammeter 4bit

author: Dr.Krieger
author_rank: Contributor
timestamp: May 13, 2022, 03:42:53 pm
content: Quote from: tunk on May 13, 2022, 01:27:31 pmWell damn, it was that easy!I don't know why i couldn't find them, i guess i was using both "voltage" and "current" in the search bar of something like that...In any case thank you sir!This variety seems to be in the 100/200V range, i don't know if <30V accuracy suffers from that higher overall range, and i haven't used one of these to know how accurate they are, just remember vaguely seeing them.Anyone used them before?

author: gabri.peic
author_rank: Regular Contributor
timestamp: May 13, 2022, 09:32:15 pm
content: Wow thanks for all that feedback.I will read all of the suggestions later or tomorrow (its getting late over here).But i just wanted to quickly ask if anyone had any experience with these meters...https://www.ebay.com/itm/323892139594?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=t_-H60NrTzC&sssrc=2349624&ssuid=AvwScfSORWO&var=&widget_ver=artemis&media=COPY

author: fkfaraz
author_rank: Regular Contributor
timestamp: May 15, 2022, 11:07:55 am
content: some Panels available on Ali ExpressClaiming to be OLED Displayshttps://www.aliexpress.com/item/1005002950979761.html?spm=a2g0o.detail.1000014.24.1ef27dc2bJF4Ej&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.40050.281175.0&scm_id=1007.40050.281175.0&scm-url=1007.40050.281175.0&pvid=46f64d2b-8ad2-4fb9-b72b-409d46a031d2&_t=gps-id:pcDetailBottomMoreOtherSeller,scm-url:1007.40050.281175.0,pvid:46f64d2b-8ad2-4fb9-b72b-409d46a031d2,tpp_buckets:668%232846%238113%231998&pdp_ext_f=%7B%22sku_id%22%3A%2212000022928175654%22%2C%22sceneId%22%3A%2230050%22%7D&pdp_npi=2%40dis%21PKR%21%211133.5%21%21%21%21%21%40210312ef16526125808314290ecf5c%2112000022928175654%21rechttps://www.aliexpress.com/item/33019908970.html?spm=a2g0o.detail.1000014.16.7eb657a9zTrdgX&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.40050.281175.0&scm_id=1007.40050.281175.0&scm-url=1007.40050.281175.0&pvid=e226f384-4815-40ba-903b-c49ec2ebdc76&_t=gps-id:pcDetailBottomMoreOtherSeller,scm-url:1007.40050.281175.0,pvid:e226f384-4815-40ba-903b-c49ec2ebdc76,tpp_buckets:668%232846%238113%231998&pdp_ext_f=%7B%22sku_id%22%3A%2267203017554%22%2C%22sceneId%22%3A%2230050%22%7D&pdp_npi=2%40dis%21PKR%21%211139.29%21%21%21%21%21%40210312ef16526126331334775ecf5c%2167203017554%21rec4 1/2 Voltage Meter displayhttps://www.aliexpress.com/item/32856546181.html?spm=a2g0o.productlist.0.0.c1a33c95A7Yheh&algo_pvid=b86ad202-d697-4cec-a189-f66fea443ba0&algo_exp_id=b86ad202-d697-4cec-a189-f66fea443ba0-12&pdp_ext_f=%7B%22sku_id%22%3A%2265256055499%22%7D&pdp_npi=2%40dis%21PKR%21%211542.18%21%21%21%21%21%402101e9d016526123633072192ee7a4%2165256055499%21sea

author: gabri.peic
author_rank: Regular Contributor
timestamp: May 15, 2022, 01:15:50 pm
content: Ok, i read all the suggestions and decided to go with the pre made panel meters.As far as powering the meters i will have the auxiliary probably 12v transformer anyways for the temperature measurement, overheat protection and fan speed control circuit so i willl probably just take 12v from that supply and send it through isolated dc to dc converter to keep two channels isolated.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2022, 02:49:21 pm
content: Quote from: RoGeorge on May 11, 2022, 10:12:47 amIf you can trust the specifications; it is not trivial to achieve 4 digit performance.Quote from: strawberry on May 11, 2022, 06:24:56 pmThe ICL7135 is a real instead of toy ADC intended to have 1 count accuracy over 20,000 counts. Typical applications would require a low sample rate to suppress noise and achieve that sort of resolution. The datasheet recommends 2.5 or 3 samples per second maximum to still deliver 1 count accuracy.

author: Dr.Krieger
author_rank: Contributor
timestamp: May 16, 2022, 02:54:04 am
content: I am not sure about the accuracy of Chinese ICL7107 Meter kits (https://www.ebay.com/itm/402533876034), for that price the chip must be a Chinese knock-off and not the real thing...Don't have one to test though, could be accurate enough and gabri.peic said the 3-digits are enough if accurate (don't need 1mV accuracy), has someone bought a cheap kit like that?Quote from: David Hess on May 15, 2022, 02:49:21 pmYea, a $10 7-digit Ali/eBay voltmeter is not the same with a $$$ bench Keithley 7-digit voltmeter, obviously...That is why i said that those X-digit cheap meters are nice but trust only a couple of decimals points and ignore the rest.Quote from: fkfaraz on May 15, 2022, 11:07:55 amThe first two have incredibly small screens, its ok for a handheld toy or something but not for a Bench PSU, too small. I have one of those and can take side-by-side pics with other LED panel meters.

author: Capernicus
author_rank: Frequent Contributor
timestamp: May 16, 2022, 03:01:08 am
content: Why bother learning programming at all? I dont think if u can just tap the position with the multimetre directly on the power supply circuit you even need any microcontroller at all for this, just chuck the metre where the multimetre was.

author: David Hess
author_rank: Super Contributor
timestamp: May 17, 2022, 04:12:00 am
content: Quote from: Dr.Krieger on May 16, 2022, 02:54:04 amIt is not even that. I really mean that 4 digit performance is not trivial, especially if cost must be kept low.Even assuming a perfect ADC, any signal conditioning, like an input buffer to provide a high impedance input, is not trivial. 1 part in 10,000 requires 80dB of common mode rejection to keep the error less than 1 count, but that is a difficult specification to meet for the JFET or especially CMOS input amplifier needed for a high impedance input. (1) The CMOS ICL7135 and its predecessors were designed very cleverly to null the common mode error of their horrid CMOS input buffer as part of their automatic zero cycle. Other contemporary ADCs which did not do this had to use a very expensive external precision JFET amplifier instead.The amazing part is that there are relatively simple discrete ADC designs from that era that manage it, but they are so difficult to understand that even then an integrated part was preferred. Today if I was designing something from scratch, I would use one of the many integrated delta-sigma ADCs (2) (3) with a microcontroller to directly drive a multiplexed display, although for a high impedance input this would still require great care of the input circuitry. Duplicating the performance of the ICL7135 is still not easy.(1) There are some amazing (non-chopper) parts now that handily meet this requirement, but the best option for a long time was a discrete design, a LM308 or LM11, or later the LT1008/LT1012 bipolar precision low input bias current operational amplifier. Analog Devices, PMI, and Burr-Brown had suitable precision JFET parts if you could afford them.(2) The LTC1043 switched capacitor building block could also be used as part of a discrete ADC to achieve this level of performance.(3) There are sampling ADCs with the needed precision now, however they would require external processing to remove 50/60 Hz line noise. Instrumentation delta-sigma converters can be configured to do this internally.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 17, 2022, 05:00:48 pm
content: For around 20-25$ usd for the icl7135 at 4 1/2 digits, you can't go wrong with them, at 199.99 volts you still have 2 digits after the decimal ...Was never satisfied with the ones based on a mcuAnd used in a power supply (0-80vdc 0-8 amps), i added (lots of empty space) two floating 5vdc supplies just to be sure to have a non common ground, 1 for the voltage and 1 for the current, you have lots of resolution to play with, i replaced 2x needle meters lollAnd after a few years, they are rock solid and still very preciseDont go the hard way designing a pcb, buying parts etc ... not worth the price / time lost / etc... my 2 cents

author: Zoli
author_rank: Frequent Contributor
timestamp: May 17, 2022, 05:36:36 pm
content: Quote from: coromonadalix on May 17, 2022, 05:00:48 pm2ndsource 7135 is around 2CAD:https://lcsc.com/product-detail/Analog-To-Digital-Converters-ADCs_XINLUDA-XL7135CD_C513493.html. Calsheet is too big to attach(22MB), but the store page has direct link; drop me a PM if you want a test board(I have 4 left).

author: cdev
author_rank: Super Contributor
timestamp: May 19, 2022, 01:44:44 pm
content: Are these cheap ADCs fast and cheap enough to make low cost SDRs for receiving HF shortwave?Quote from: Dr.Krieger on May 12, 2022, 12:06:10 am