3 wire voltmeter module for low voltage - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: June 28, 2017, 12:01:35 pm
content: I need to measure 0-1V are there any sensibly priced volt meter modules out there that I can use or am I asking for the impossible.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 28, 2017, 01:30:23 pm
content: Give us some specs. Any multimeter can measure 0-1V. Since you are asking, I'm going to guess you either A) require higher accuracy than an average multimeter can provide. If this is the case, go buy yourself a better multimeter B) You need a module to be integrated in a systemEither way, give us more information!What is your budget, required accuracy, etc.What is a three-wire voltage measurement anyways? I've never heard of this.

author: Simon
author_rank: Unknown
timestamp: June 28, 2017, 01:32:13 pm
content: I'm looking for a little panel meter. Most measure their own supply voltage, I need to measure 0-1V so it needs independent power.Sent from my Moto G (4) using Tapatalk

author: tszaboo
author_rank: Super Contributor
timestamp: June 28, 2017, 01:38:47 pm
content: There you go:https://www.aliexpress.com/item/DC-Analog-Meter-Panel-1V-AMP-Current-Ammeters-85C1-0-1V-Gauge/32685489360.html?ws_ab_test=searchweb0_0,searchweb201602_3_10152_10065_10151_10068_10304_10136_10137_10060_10302_10155_10062_437_10154_10056_10055_10054_10059_303_100031_10099_10103_10102_10096_10052_10053_10142_10107_10050_10051_10084_10083_10080_10082_10081_10177_10110_519_10111_10112_10113_10114_10182_10078_10079_10073_10123_10189_142,searchweb201603_16,ppcSwitch_4&btsid=e7b8597c-807c-4862-99ca-dd6dfd0c86ad&algo_expid=b66f39e4-1ce1-4413-83d0-017952075797-3&algo_pvid=b66f39e4-1ce1-4413-83d0-017952075797

author: Simon
author_rank: Unknown
timestamp: June 28, 2017, 01:49:36 pm
content: Think digital will be preferred.Sent from my Moto G (4) using Tapatalk

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2017, 03:16:47 pm
content: There are a lot of modules with 3.5 digits for something like a 200 mV or 2 V range. The older ones based on ICL7106 or similar. There are relative cheap Chinese ones with 5 digits. So it really depends on the source you want. Plenty of modules to chose from. Just beware that some need a supply that is not connected to the voltage to measure - so more like 4 wires.

author: HighVoltage
author_rank: Super Contributor
timestamp: June 28, 2017, 03:23:24 pm
content: I have found one brand to be the best and sticking out like no other.And I have used hundreds of these over the yearsECKMANN+EAGLEwww.bue.deThe most used one is 0 to 19.99VBut they also have a 0 to 1.999Vor 199.9 mVor even Amp meterWith and without lightHere are a few pictures of the one I usually have in stock.Prices are around 20 Euro each

author: jeroen79
author_rank: Frequent Contributor
timestamp: June 28, 2017, 04:04:54 pm
content: Quote from: TheUnnamedNewbie on June 28, 2017, 01:30:23 pmYou can get panelmeters with different wiring configurations.2 wire: Combined supply and measurement, will have a minimum input voltage.3 wire: Shared ground, separate high for supply and measurement.4 wire: Completely separate supply and measurement.

author: Simon
author_rank: Unknown
timestamp: June 28, 2017, 04:36:09 pm
content: Quote from: HighVoltage on June 28, 2017, 03:23:24 pmUnfortunately none of the main distributors have them, but you give me hope, I will have a proper look round.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 28, 2017, 04:46:19 pm
content: The most common and cheapest units are based in the old ICL7106 chip.Just be aware that, without some additional circuit, the ICL7106-based devices cannot measure a voltage which is referenced to its own supply common.

author: Simon
author_rank: Unknown
timestamp: June 28, 2017, 05:09:37 pm
content: I found a +/-2V one but boy £38!!!! I can buy several multi-meters for that!http://www.mouser.co.uk/ProductDetail/Murata-Power-Solutions/DMS-30LCD-1-5B-C/?qs=b13bmSh8y6x6JXWo0o5iqQ==&utm_source=findchips&utm_medium=aggregator&utm_campaign=DMS-30LCD-1-5B-C&utm_term=DMS-30LCD-1-5B-C&utm_content=Murata

author: HighVoltage
author_rank: Super Contributor
timestamp: June 28, 2017, 05:10:29 pm
content: Quote from: Simon on June 28, 2017, 04:36:09 pmYou can order them directly from the manufacturer.Or if you just need one, let me know and I can send you one.

author: Simon
author_rank: Unknown
timestamp: June 28, 2017, 05:25:40 pm
content: Thank you, alternatively these are a steal:http://www.ebay.com/itm/V27D-Digital-Voltmeter-LED-Panel-3-Digital-Display-DC-0-40V-Voltage-Meter-3-Wire-/192225701698?var=&hash=item2cc18b6f42:m:mtKBQrgMIQPjHXuQTTSUlgQ

author: DBecker
author_rank: Frequent Contributor
timestamp: June 28, 2017, 05:29:37 pm
content: Quote from: schmitt trigger on June 28, 2017, 04:46:19 pmThe ICL7106 is obsolescent. Not for performance, but cost, convenience, cost, price and cost.The most common and cheapest panel meters are based on 8 bit microcontrollers. They use their internal 10 or 12 bit ADC with lots of averaging to display 3 or 4 digits of resolution at a 1-5 Hz refresh rate. More accurate ones have an internal bandgap reference that they correct against, but plenty of them just use the supply 7805 as a voltage reference (which admittedly does provide generous dithering noise for input averaging).These are readily available in 2 and 3 wire models. If you do find a cheap 4 wire meter it's probably using pseudo-differential inputs. That's good because it lets you eliminate the current draw of the LED display from the reading and doesn't require an isolated supply. It's bad if the input range limit hits you.You can still get the typical old-style four wire meters that require an isolated supply, but what you think you are gaining in accuracy might be defeated by the noise out of a typical DC-DC converter. It's not they have a lot of noise, it's that the remaining noise can be really difficult to cheaply eliminate. And with the variable frequency and pulse width, it's difficult to be certain that you won't hit a bad regime.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 29, 2017, 03:32:57 pm
content: Quote from: DBecker on June 28, 2017, 05:29:37 pmIndeed it is. It is a 1980s era chip. But there are plenty of panel meters based on them. So, I only wanted to make the OP aware of the limitations.

author: Simon
author_rank: Unknown
timestamp: June 29, 2017, 05:11:07 pm
content: I have gone with these:https://www.rapidonline.com/Catalogue/Product/51-5393

author: Zero999
author_rank: Super Contributor
timestamp: June 29, 2017, 09:18:13 pm
content: Not backlit or branded but cheaper. I haven't personally ordered from JPR Electronics before but the school I went to did and they seemed reasonable.https://www.jprelec.co.uk/store.asp/c=1076/LCD-Multirange-Panel-Meter

author: Simon
author_rank: Unknown
timestamp: June 30, 2017, 07:03:49 am
content: Well I'm quite happy to spent a tenner a pop but 40 quid is taking the piss when I can buy an Amprobe multi-meter for £30

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 05:20:51 pm
content: So, panel meters arrived..... yes you guessed it, hidden away in the documentation is a little picture that seems to indicate that the supply voltage cannot be in common with the measured voltage. WANKERS!So, what does this actually mean ? How much resistance between the 2 grounds does not in common constitute ? I am measuring 0-100mV off the back of a 0-5V pot, and with the input impedence of the panel meters being very high I can easily use 100K-1M resistors in my potential divider.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2017, 06:31:22 pm
content: Quote from: Simon on July 06, 2017, 05:20:51 pmIt could mean that the input of the meter consists of a differential amplifier, internally biased somewhere around half the power supply voltage and doesn't include 0V in its common mode range. There could be an easy work around using an op-amp to shift the voltage to the meter's internal bias point.Try shorting both inputs together, powering the meter and measuring the voltage on the inputs, with respect to 0V. Hopefully the 10M impedance of your meter won't pull the panel meter's inputs outside their common mode range and it will continue to read 0V.A more foolproof solution is to just power the meter off an isolated DC-DC converter but you may need extra RFI filtering.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 06, 2017, 06:34:31 pm
content: Quote from: Simon on July 06, 2017, 05:20:51 pmThat is one reason why I suggested to you the German made one.I went through a lot of different digital panel meters that did not work correctly, before I found that one from above.

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 06:36:33 pm
content: Well I still have to get 24V down to 9V so I could use an isolated switcher brick to power them.

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 06:38:39 pm
content: So if I can't connect grounds together what stops me connecting the +in to power - ? I don't care if the measurement is negative.

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2017, 06:41:48 pm
content: Quote from: HighVoltage on July 06, 2017, 06:34:31 pmI thought Voltcraft are German?Anyway, it says on the data sheet. This isn't surprising. The meter supports measuring negative voltages and would require a negative power supply if it worked with the inputs below the negative rail.Quotehttps://www.rapidonline.com/pdf/126568_an_ml_01.pdfQuote from: Simon on July 06, 2017, 06:36:33 pmSeems like a good idea to me.Quote from: Simon on July 06, 2017, 06:38:39 pmWhat do you mean?The common mode range of the input terminals, when in 0 to 200mV mode (2V will have a potential divider), is likely to be from around 1 or 2V above the negative rail to 1 or 2V below the positive rail or an internal voltage regulator, if it has one.

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 06:44:40 pm
content: I mean if I use the positive input as the negative one and ground it. It's probably safer to use an isolated brick, I've got 30 quid to spend on it

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 06:45:50 pm
content: I think voltcraft is a trading name of conrad, say no more.

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 06:57:48 pm
content: oh and it's 0.5% +10 counts!, not a problem in this case as it's just a 0-100 indicator for a pot position and I'd already seen that before I bought the, but give us a break, why bother with the last digit at all.....

author: HighVoltage
author_rank: Super Contributor
timestamp: July 06, 2017, 07:29:11 pm
content: Quote from: Hero999 on July 06, 2017, 06:41:48 pmVoltcraft used to be good and many items made in Germany.Now, I think everything with the name Voltcraft is made in China and partially with very bad quality.These days, I stay as far away from Voltcraft as I can.

author: stj
author_rank: Super Contributor
timestamp: July 06, 2017, 07:50:54 pm
content: there are lots of 3-wire modules at banggood for beer-money.

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 07:52:38 pm
content: There were plenty on ebay for less than 2 quid each! The rapidelectronics (conrad) website is now worse than ever to navigate, you can't open links in new tabs, I don't think I'll be ordering much there again, I'd be lucky to find what I want in the first place.

author: stj
author_rank: Super Contributor
timestamp: July 06, 2017, 08:09:06 pm
content: you can open stuff in a new tab at Rapid, just need to avoid rollover or pulldown menu's - that i f-ing hate!

author: Simon
author_rank: Unknown
timestamp: July 06, 2017, 08:20:51 pm
content: not any more

author: Zero999
author_rank: Super Contributor
timestamp: July 06, 2017, 09:35:41 pm
content: Quote from: Simon on July 06, 2017, 06:44:40 pmNo because, as I said above, the common mode range most likely doesn't include the negative supply. That means both inputs need to be above the negative supply, in order for it to work properly. If the inputs are just outside the common mode range, the reading will be unreliable but if the inputs are way outside the common mode range, i.e. outside the power supply rails, then the meter will be damaged.Try this: connect the meter to a 9V supply and vary the voltage on the inputs between 0V and 5V, which shouldn't damage it, as it will be within the operating range of any internal voltage regulator. You'll find the meter only gives sensible readings, when the inputs are both within a certain voltage range.

author: technix
author_rank: Super Contributor
timestamp: July 07, 2017, 03:09:14 am
content: If you don't mind building something on a perfboard, you can try the ATmega328P + HD44780 LCD module stack. The ADC on the ATmega328P can be referenced to its internal 1.1V reference to both save component count and improve dynamic range. And the bulk of the code can be written using Arduino. Upgrade to a ESP8266 module if you need connectivity (e.g. pull the reading over Wi-Fi from your phone.)

author: Simon
author_rank: Unknown
timestamp: July 07, 2017, 06:32:04 am
content: Quote from: technix on July 07, 2017, 03:09:14 amWell that is what I was considering until I found these "nice" cheap panel meters. I've ordered a DC/DC brick for a tenner and I needed a regulator anyway so it's fixed.

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2017, 08:21:42 am
content: Quote from: Simon on July 07, 2017, 06:32:04 amGood. Presumably you were going to use a linear regulator? The switched mode will also reduce the power consumption too.

author: stj
author_rank: Super Contributor
timestamp: July 07, 2017, 10:13:26 am
content: Quote from: Simon on July 06, 2017, 08:20:51 pmso i see.i just messaged them on the subject, telling them the site was no longer viable and another supplier would have to be used.no right-click = no useability = no sales.i'll see if they respond - assholes.i had a list of stuff to get from them too!!!

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2017, 10:49:22 am
content: A differential amplifier might be an alternative solution to an isolated supply.http://www.eetimes.com/document.asp?doc_id=1272328The op-amp must be suitable for single supply use, i.e. its common mode range must include the negative supply.REF goes to the meter's negative input.VOUT to the positive input.IN- to 0V.IN+ to the voltage you want to measure.You could also swap the REF with VOUT and IN+ with IN- and it should still work.Make the resistors as higher value as possible. If the input impedances are too low, add a unity gain buffer between Ref and the meter and IN+ and your source.This will probably be cheaper than a DC-DC converter module but it's more components and you still need a 9V power supply so probably not worth worth it.

author: Simon
author_rank: Unknown
timestamp: July 07, 2017, 11:47:05 am
content: Quote from: stj on July 07, 2017, 10:13:26 amI'm planning to do the same, I did fed up when it crashed and went to farnell

author: stj
author_rank: Super Contributor
timestamp: July 07, 2017, 12:14:02 pm
content: i got mailQuote

author: technix
author_rank: Super Contributor
timestamp: July 07, 2017, 01:59:00 pm
content: Quote from: Simon on July 07, 2017, 06:32:04 amFor direct reading meters those ready made panel meters are good ideas.If you need connectivity you may be stuck with ESP8266 module + your own code though.Personally I would prefer the modules as making the readings available on the home network means I can use my home server to poll the readings and keep a log on it (and text me if there is anything abnormal.)

author: Simon
author_rank: Unknown
timestamp: July 07, 2017, 04:26:13 pm
content: Quote from: Hero999 on July 07, 2017, 10:49:22 amFor all of the messing about on veroboard I prefered to spend a tenner and know that it's sorted for what is a one off bit of R&D kit I have been asked to make and have charged enough for if I don't spend hours hand wiring and troubleshooting bits.

author: rdl
author_rank: Super Contributor
timestamp: July 08, 2017, 09:27:19 pm
content: I don't see how a meter could measure anything with just 3 wires (power +, power -, and signal voltage) without being common ground. Requiring separate power connections for the meter implies a 4 way connection. One advantage to this is that the voltage to be measured can be below what is needed to power the meter.

author: Zero999
author_rank: Super Contributor
timestamp: July 08, 2017, 09:41:44 pm
content: Quote from: rdl on July 08, 2017, 09:27:19 pmUnfortunately the meter Simon purchased is no a 3 wire meter. It has a differential input tied to an unknown reference between its power supply rails and the common mode voltage is unspecified. There are two ways to give it a proper differential input: add a differential amplifier or power it from an isolated supply.

author: rdl
author_rank: Super Contributor
timestamp: July 08, 2017, 09:58:55 pm
content: Ah, I see. He did not actually buy a "3 wire" meter. I would just buy some of the cheap micro based meters from ebay or similar. The 2 and 3 wire versions are cheap.I bought some for $1.59 each shipped. They use less than 30 mA and agree with my Fluke 87-III to within +/- 2 digits.I wish I could find some similar in a 4 wire version because they have the smaller 0.36"/0.40" digits, but all I can find are 0.56" digit size or ones that cost way more than I want to pay.

author: Simon
author_rank: Unknown
timestamp: July 09, 2017, 08:01:02 am
content: Unfortunately unless you are buying expensive units and or out of stock units the documentation is poor.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 11, 2017, 01:55:01 pm
content: Simon; OT questionIs that Freddie Mercury in your Avatar?

author: jimmc
author_rank: Frequent Contributor
timestamp: July 11, 2017, 03:44:21 pm
content: How about these meters?http://www.ebay.co.uk/itm/LED-5-Digit-DC-0-4-3000-33-000V-Digital-Voltmeter-Voltage-Meter-Car-Panel-UK-/192066249579?var=&hash=item2cb80a636b:m:mw5SK0EFa7T0OF0sxR05ZFg(There are a lot of other suppliers.)I bought 2 a couple of years ago, they are surprisingly accurate for such cheap meters.Points to watch are that the input resistance is about 350k and they will only measure positive voltages.Jim

author: Simon
author_rank: Unknown
timestamp: July 11, 2017, 07:39:44 pm
content: Quote from: schmitt trigger on July 11, 2017, 01:55:01 pmYes, it's just a standard avatar available from the forum software. I decided to change my avatar because people can do reverse image searches etc and being a moderator you do get the odd idiot, I once found place of honour on some weirdo's hall of hated things or something tom that effect so decided something generic would do. It's amazing how easy it is to join the dots up with a few minute googling.

author: Simon
author_rank: Unknown
timestamp: July 11, 2017, 07:40:40 pm
content: Quote from: jimmc on July 11, 2017, 03:44:21 pmThank you but a bit late now.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2017, 09:58:46 pm
content: Quote from: Simon on July 11, 2017, 07:40:40 pmAnd, more importantly no data sheet, so not much use for anything other than hobby use.

author: Simon
author_rank: Unknown
timestamp: July 12, 2017, 06:27:31 am
content: Well the voltcraft datasheets didn't turn out to be much. We have a rapid power supply at work which I guess is a rebadged voltcraft and it's current readings are 10% out!

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2017, 07:40:26 am
content: Quote from: Simon on July 12, 2017, 06:27:31 amAnd what makes you think it's the meter? Whilst that's a possibility, you're jumping to conclusions. It could just as easily be the current shunt or the gain of an external (to the meter) differential amplifier being 10% out.

author: Simon
author_rank: Unknown
timestamp: July 12, 2017, 11:42:30 am
content: Oh I know it might not be the meter itself but the whole supply was probably made by voltcraft or whoever they buy stuff from to rebadge so not that impressive.