Measuring negative voltage with voltmeter - Page 1
Forum section: electronics / beginners

author: tony359
author_rank: Frequent Contributor
timestamp: March 03, 2025, 06:16:39 pm
content: Hi there,I am building a PSU box which has three outputs: +5, +15 and -15V. I've purchased some voltmeter from Ali to display Voltage and Current so I can monitor all three rails.I forgot that those voltmeters are not rated for negative voltages... However, I just need to wire them properly (-15V to the black wire and Ground to the red one) and it works, I can live without a (-) sign!The issue is that on the device under test, all ground points are obviously connected. So when I plug this contraption to the device, I am effectively shorting -15V to ground.Is there anything I am missing here, before I look for a different type of voltmeter? The diagram is below. I have verified that the two black leads - the one supplying the voltage to the controller and the one for the current measurement - are unfortunately connected.Thanks!

author: Rick Law
author_rank: Super Contributor
timestamp: March 03, 2025, 09:47:37 pm
content: You have a mixed problem here but your diagram doesn't help clear it up. Your PSU has 3 output (as you said in your post) +5V, +15V, -15V, but you didn't say how the connection is arranged (the PCB image doesn't show enough, the edge jumper header is possibly output connector as well):Arrangement as in one of the following:1: Single set of output post (one +- pair of connection to power, with a switch of some sort selects +5V, +15V or -15V) So the +5, +15. -15 are not done simultaneously.2.Two sets of outputs. One post for +V(a selection switch (the VR?) on your PSU PCB to set +5V or +15V) and one for -V (-15V), with shared ground post or separate ground post.3: Three sets of output. One for +5, one for +15, and one for -15V.4: (Your diagram seem to suggest this is not it) Your device has 3 (or more) connections (+15Vin, -15Vin, and ground) directly corresponding to the outputs of the PSU.Also, is your voltmeter the 3 wires kind or 2 wires? The 3 wires kind have a separate power-in for the voltmeter. The 2 wires kind just take the power off the +Vin.Each case will have a different preferred solution.Based on the way you presented the question, Iassumeyou are case 2 described above, two sets of outputs one for +V and one for -V. I alsoassumeyou have 2 wire voltmeter since you only talked about red and black wires but not a third wire.If my assumptions are correct, you would want to use two voltmeters, one showing the plus side and one showing the minus side. No fuss and you don't need to worry about the ground of meter1 connecting to v+ of meter2. They are separate differentials..Alternatively, a less preferable way is to add a 2p3t (2 post 3 throw) slide switch. The single voltmeter with 3 choices of where it connects (+5, +15, -15) on where you want the 2 posts of the voltmeter connected. Unless you have a part store near by, getting the switch may be nearly-as or even-more expensive than getting an extra voltmeter. You can even go 2p4t, with the 4th one being left open for voltmeter off. With 3 wire voltmeter, you may need 3p3t and that would be hard to find if you can find it at all. So you have to connect the 3rd wire to the red and skim the power off the voltmeter's Vin.EDIT: Modified wording to make it easier to understand

author: tony359
author_rank: Frequent Contributor
timestamp: March 03, 2025, 10:35:17 pm
content: Hi,Apologies for not being clear. The diagram is for the voltmeter, it's not my final setup.My PSU is case 3: all three outputs work simultaneously. I made a box with three of those voltmeters and I'd like to see the voltage and currents of those three voltages. See attachments below.Those three voltages share a ground on the PSU.The voltmeter has 3 wires, you can see in the picture on the previous post: thin BLACK/RED is power, YELLOW is the voltage you intend to sense. The thicker RED/BLACK wires are current sensing, to be connected in series with the DUT, but it reads continuity with the thin black ground wire, so I cannot have -15V there.The voltmeter does not read negative voltages - I can wire -15V the other way round so that the voltmeter has -15V on the black wire and ground on the red wire and it works, just the minus sign is missing which is fine. By doing that, I am putting -15V on the current sensing plug and when the circuit goes back to the PSU, that gets shorted to GND.In short, I can read the voltage but not the current.I thought about powering the voltmeter with 5V from the other circuit and sense -15V but the voltmeter does not read negative voltages at all.I made a little drawing to try to clarify. Thanks for your help!

author: ledtester
author_rank: Super Contributor
timestamp: March 03, 2025, 11:27:20 pm
content: This stackexchange discussion might help:https://electronics.stackexchange.com/questions/516682/the-issues-of-common-ground-with-voltmeter-and-ammetersI think you will have issues with just the positive rails if you want to measure each rail current individually.Consider just the +5 and +12 rails. Normally the return current for both of those rails is the GND connection. However, since those meters only do "low-side" current measurement, you will need separate return entry points in order to distinguish the return currents from the two rails.So, you would have "+5 OUT", "+12 OUT" and "+5 RETURN" and "+12 RETURN" but those return nodes would not be the same and also would not be the same as the power supply GND.Attached is the schematic. The +12, +5 and GND nodes on the left of the meters are the power supply outputs. The nodes on the right are what you connect your application to.Note that in the meters there is a shunt resistor between RET and V-.A better way to do this is to measure the current on the high-side for the positive rails - then all three rails could share a common ground.

author: radiolistener
author_rank: Super Contributor
timestamp: March 03, 2025, 11:31:02 pm
content: Your issue is that the voltmeter needs its own PSU, and for a negative voltage voltmeter, you need a PSU with reversed polarity.The solution is to add a third PSU (PSU3), whose ground is galvanically isolated from PSU1 and PSU2. This allows you to connect the "+" terminal of PSU3 to the ground ("-") terminal of PSU1 and PSU2, solving your issue.Usually, such issues are solved by adding separate low-current transformer windings with dedicated voltage regulators for voltmeters and other auxiliary modules, ensuring they are galvanically isolated from the main PSU.Additionally, if your device is not sensitive to strong interference and noise, you can use ready-made switching-mode power isolation modules. In these modules, both the ground and the power rail at the output are completely galvanically isolated from the input lines, allowing you to power the isolated part of the circuit. Usually their power is about 1-2 W, which should be enough for voltmeters.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2025, 08:20:59 am
content: Build three, galvanically isolated power supplies, connect the meters to their outputs, then common the load side of the meters.

author: tony359
author_rank: Frequent Contributor
timestamp: March 04, 2025, 02:16:42 pm
content: Thanks all.Yes, I thought about adding an insulated "buck converter" module. Noise might be an issue - it's powering an audio device - but if I don't see the noise when I am testing the outputs, it's fine. Alternatively, I'd imagine I could install a very small linear transformer with a bridge rectifier and a capacitor to derive a working voltage from 15V and power the third meter?So basically you're saying to get say the 15V line, run it through one of those converters to output - say - 12V and that would power the voltmeter in charge of reading the -15V?@ledtesterI'm not sure I follow your point. Yes, grounds are all connected but the shunt is before the grounds get all connected? I think I align with the second answer given in the link you provided.

author: Rick Law
author_rank: Super Contributor
timestamp: March 04, 2025, 11:16:47 pm
content: Quote from: ledtester on March 03, 2025, 11:27:20 pmQuote from: tony359 on March 04, 2025, 02:16:42 pmYou have more problem than just noise.ledtestorpointed out there is a "low-side" measurement issue as well. In your case, as you said in your OP that you are measuring -15V by swapping polarity - using your meter's Vin (Red) to the PSU's GND and meter's Gnd (Black) to the PSU's -15V. This created two different "Grounds" introducing a measurement error. And, you have a separate Ground with the 5V to as well. Noise can be filtered out, but the "low-side" issue can't be filtered out.Just addressing the power source of the voltmeters will not help this issue. You need to understand the "low-side" measurement better in order to address that, particularly since also have a -15V to measure (which reversed the low-side to a high-side). All three voltmeters are of the same design. If the voltmeter is designed as a high-side measurer, your -15V is reserved polarity so now is it now a low-side (ground side) measurer.Those voltmeters in your photos doesn't look like high-precision. You would be lucky if the native (voltmeter's own) error is at least +- 5%, likely > 5%. Soin my opinion, it is not worth a huge effort. If they work together by just reversing polarity on the -V side, just let it be. If you really want a more accurate solution, it will take a redesign which higher precision differential voltage measurement.Let me go into the different grounds and measurement issues deeper below.Usually, measuring Current is done by inserting a sense resistor into the circuit and measure the voltage drop across the sense resistor. Many DMM would use a 0.1ohm sense resistor to do the job. So let that be our example. When you measured a 0.1V drop across the 0.1 ohm sense resistor, using Ohm's Law (I=V/R), you have 1 Amp current flowing through that resistor, easy math. 2Amp flow would be 0.2V drop, so on.That leaves the question, where do you insert the sense resistor? You can have it at the "high side" or the "low side".High side: PSU +Vout --> current sense resistor --> Device VinLow side: PSU Gound --> current sense resistor --> Device GoundMost lower cost voltmeters using low-side measurement since it is easier to measure between 0.000V to 0.001V vs measuring between 15.000V to 15.001V as the percent change is much lower percent.Looking at just the +15V out of the PSU and say 1amp output. When using "high-side" current measurement, after the sense resistor voltage drop, the Device Vin got only 14.9V, and ground is 0V. That is typically no problem.Now if you use "low-side" measurement, the Device Vin has 15V, but the Device Ground is sitting at 0.1V, not 0V which is PSU ground. Typically no problem if you only have +V, but...Now add in your -15V measurement which is polarity reversed. Your voltmeter's Vin is connected to PSU's ground (0V) which is sitting 0.1V below the Device's ground. So whatever you measured here will be off by another 0.1V as compared to what the device is actually getting. You can see the problem there. This voltage delta can't be filtered out and this voltage delta is variable depending on +V side current.But you also haveat least two ground outputson the PSU- one with the +5V and Ground, the other with +-15V and Ground. If they are all connected to the same device, you have more issues. All wires and connections have some resistance. Most of the current will find the path closer to the 0V ground rather than the ground sitting on +0.1V at the sense resistor (or some voltage varying depending on current flow) thusavoided measurement.A proper design should put all the sense resistors on the high side, so you have a single 0V ground.

author: Zero999
author_rank: Super Contributor
timestamp: March 05, 2025, 08:29:25 am
content: Here's a simplified schematic of your meter.These are typically based on digital meters with a range of a couple of hundred mV or so, with a potential divider to scale the voltage down and a shunt resistor for the current. The problem you have is all your supplies are connected to together.You either need three separate power supplies, rather than the single PSU you have, or separate meters.

author: tony359
author_rank: Frequent Contributor
timestamp: March 05, 2025, 10:00:27 am
content: Thanks Rick for taking the time to write a comprehensive reply.QuoteAbsolutely but I don't need that.I use that box to test equipment which reaches my lab in poor shape. They're usually shorted or almost shorted. What I've done so far, I plug power in and check with the DMM that all rails are working. If I see one rail "jumping", that rail is shorted somewhere.I just thought I'd made a fancy box with the indicators. And since I was there, I added the current measurements. It doesn't have to be accurate as long as it's repeatable.That is, if a good device draws 2.45A at 5V, I can use to quickly compare others. I don't care if the actual current is 2.55A insteadThe box is only going to use for testing, a few minutes every time."low side measurement", interesting thanks! That makes sense, but one thing.Let's ignore the -15V line for a moment, you say that even with positive voltages I'd end up with slightly different ground points and that would affect the reading.But why is so?I made a drawing. In my mind, AFTER the ammeter, there is 0V. Where is my mistake?Do you have some recommendations for alternative ammeters? I'm looking for cheap stuff from Ali, well aware that they're not going to be reliable/accurate

author: Rick Law
author_rank: Super Contributor
timestamp: March 05, 2025, 09:02:45 pm
content: Quote from: tony359 on March 05, 2025, 10:00:27 amYou are welcome. Time consuming as it may be, it is a pleasure to share knowledge with one who wishes to learn! So you are welcome indeed.If your +15V and +5V go to different devices and there is no connection between the two devices except PSU ground, it makes no difference, one is not going to affect the other. If there is a connection between the two devices, some Current may leak into the other device.To understand that, it is easier with an analogy. Now that you understand low-side measurement affects ground voltage (as compared to PSU), you can imagine the two devices sitting at different ground level.Think of two lakessitting at different elevation (device ground) above sea level (PSU ground).One at 200 feetandthe other at 201 feet. Both lakes has a small creek to the sea and each creek carries a certain amount of water to the sea. You can now picture if a small tunnel is dug between the two lakes. Some water from the lake at 201 feet will flow to the lake at 200 feet. Both creeks will now carry different amount of water to the sea because of this tunnel.Also in the same analogy, water flow has resistance - there is contact friction (between water and the inner surface of the water pipe), there are other external influences (such as ambient temperature or truck passing over a pipe compressing the pipe by however small an amount)... Even actual sea level is not the same every point on earth. So we define sea level to be a specific value using averages, and we define PSU ground as 0V the same way.Current in your device will do the same. Current will flows from higher voltage to lower voltage if there is a connection. The magnitude of the flow will depend on the voltage differential and the resistance to the flow. That is physics that you can't change.Instead of your devices being as lakes at different elevations, your device grounds are sitting on top of their respective sensor resistor each with their own unique voltage drop. We choose to define PSU's ground as 0V. 1V within device 1 would be 1V+X when measured from PSU ground where X is the voltage delta caused by sensor resistor of device 1. Same for device 2, but it would be 1V+Y as voltage sensor delta would be different.Normally no issue if PSU ground is the only mutual connection.But with any other connection, you have issues. The connections are the "tunnels connecting the lakes" leaking water (current) between them. That will change the flow in the creeks (sensor resistors) to the sea (PSU ground).Even within your PCB, Ground level at one point (point 1) may be differently elevated than ground connection at another point (point 2) on your PCB -- because the copper electrical connections (traces) on your PCB will have electrical resistance. Each trace to the ground could have different a resistance value thus a different elevation to 0V ground. Point 1 and Point 2 are both connected to ground but are both not exactly 0V. The difference could be tenths of a volt, milli-volts, nano-volts, pico-volts, whatever. However small, itwont bepractically impossible to be exactly 0.Search for "ground loop" and you can explore this subject in much more details.EDIT:Replaced "wont be" to "practically impossible to be"It could be theoretically 0V but in practice it is highly improbable.

author: tony359
author_rank: Frequent Contributor
timestamp: March 05, 2025, 09:12:43 pm
content: QuoteVery kind of you to say, thanks!I understand the analogy (thanks!), I am aware of ground loops - which turn into humming noise in sound equipmentWhat I struggle to understand is why this applies to my use case. What is incorrect in the drawing I posted above? The difference voltages you mention are before the current sensor resistors, not after? In my mind, I have 0V at ground on both rails. I'll re-read your message and ruminate on itAnd to clarify, those voltages - all of them - end up on the same device. So all grounds are all tied together.Thanks again!

author: Zero999
author_rank: Super Contributor
timestamp: March 05, 2025, 10:26:54 pm
content: The problem is, your meter measures current on the low side i.e. negative rail only. Note R3 and that +in is connected directly to +out. This doesn't matter if you're just using it to measure one load, but if you connect another meter R3 becomes short circuited.What you need is this.You might be able to modify one of these meters so it measures current on the high side, by moving the shunt and adding a current amplifier IC, such as the ZXCT1009, but I don't have time at the moment to post any more schematics.

author: Rick Law
author_rank: Super Contributor
timestamp: March 05, 2025, 10:42:40 pm
content: Just sawZero999's reply as I begin to post my reply, He is exactly right. So I stop my posting process and reedit to remove what would be merely repetitive.I kept this as it may be directly related:If PSU ground is the ONLY connection between your equipment, there won't be an impact. It takes > 1 connections for issue to occur....The PCB picture you post has what appears to be asingle 3-pin plug for +15V, Grd -15V,. Say if you do use an equipment with 3 wires to the PSU with for +15v, -15V and ground, you have a >2 connections between the +15V side and the -15v side. One at the shared PSU ground, and the other at the equipment's ground, plus whatever else within that equipment. The equipment ground is going to short the current sensor at low-side measurer rendering that measurement to be largely meaningless,

author: Rick Law
author_rank: Super Contributor
timestamp: March 06, 2025, 02:28:29 am
content: One final thought...Since you experimented that by reversing the polarity (yellow to the -15V) it works but without showing the minus sign, I suggest you may want to stick with that if they all 3 meters work together... That would at least give you voltages of the three outputs.Assuming all three can work together showing good voltage only...Here is aslightly better waythat would give youbetter results: don't connect the current measurements at all. No voltage drop in the readings, and no worry about other shorting created, just 3 independent meters measuring volts.You should try this:Test it with just the the thin wire connection onlyfor all three meters. The thick wires looks like they are for inserting the current sense resistors into the flow. Do it with just the thin wires (3 pin socket yellow, red, black) and leave the 2-pin socket with thick wires open. I can't think of a reason why it would measure -15V aok, but fails when two totally separated (+15V and +5V) are added. High probability of success here.Without the thcik wires, the Current reading cannot be done -- but without the current sensor inserted, you have no voltage drop. Better accuracy, and less complications.EDIT: adding this:If the three meters works together without current reading, it suggest to me that possibly the current sense measurement and voltage measurement is entirely separate. This leaves the possibility of moving from high-side to low-side measurement! But let see if this voltage only connection works... One step at a time.

author: tony359
author_rank: Frequent Contributor
timestamp: March 06, 2025, 10:00:09 am
content: Thanks Zero999,I had to make a drawing myself and now I see your point. Understood.Rick,At least the -15V meter does work without current measurement attached. I'll try the other two as well but the current measurement pins show continuity with the black thin wire so I'd say it's not possible.Meanwhile, is there a cheap option for measuring Current as well on the high side this time?Thanks!Update: I tested the contraption today - ignoring the current readings - and I see a LOT of noise being introduced by those gizmos which end up on the sound outputs of the device I am testing! I shall try by removing the current readings on the positive rails as well - since they are useless - but I think I might need to continue doing the manual way with my trusted Fluke DMM

author: Rick Law
author_rank: Super Contributor
timestamp: March 07, 2025, 01:04:21 am
content: Low cost high-side current reader is going to be rare if exist at all. High-side requires two measurements for differential so it take a bit more complexity and cost. There are chips design specifically for high-side measure, but I have not seen them in cheap voltmeters with LED displays.The easiest way I can dream up is toaddtwo similar voltmeters asAmpMeters, Each will be need a separate power source. You don't need to add a third -- the -15V should be able to do both volt and current measurement -- more on that later.Since you already have multiple such meter, you can test that by using an existing one as AmpMeter running off a 9V battery. The AmpMeter's ground (and it's independent power source) will be tied to at the sensor resistors' low-end. Say you are testing the +15V, you use the existing one as VoltMeter and "borrow" the +5V's voltmeter as AmpMeter to do the test. Your VoltMeter for +15V shouldnothave any current-sense connection (the two-pin socket should have nothing). Just the volt-sense (thin yellow) is connected. Power source for VoltMeter during test can be the PSU's 5V or a 9V battery. The AmpMeter should use a 9V battery as power source and only current sense connections. The high side current sense wire (i.e. the PSU's+15v wire) is connected to this meter's current sense socket +pin, and the low side (power to DUT) is connected to the -pin (ground to the AmpMeter). When that works, you can have the two switch roles and test the PSU+5V. If both works, you can decide if it is worth investing in another 2x meters and 2x isolated power source.By the way, from your posted pictures, I can discern that the 2-pins socket is for current sense, and the U-shape bare-wire next to it is the current sense resistor.I cannot confirm which pin is really the low (ground) sideby looking at pictures only. Looks like the -pin is the low/ground side but I am not certain. Continuity test will likely ignore the sense resistor and report it as continuous since the sensor resistance is <= 0.1ohm. Even DMM ohms display would be difficult with only an additional 0.1ohm or not. You need to be sure of that. So, before you disconnect the setup (as pictured), is it registering the proper amperage with -pin being the PSU ground?(Later after things works) For the VoltMeters, since you already removed the current measurement due to noise, you can short the two-pins (from pictures, it looks like that is where you insert the current sensor). It could remove some jitters -- the ADC is going to measure a solid zero quickly instead of counting a free floating pin acting like a antenna for more noise.For the -15V side, reverse polarity should work even with current-measurement connectedifyou areNOTusing the PSU's +5V or +15V for voltmeter's power,. The -15V side's meter must be fully dissociate the voltmeter from the PSU +V side and do a full reversal: PSU's ground is the meter's +Vin, and PSU's -15V is the meter's ground as well as the current-sense wire connections.Once fully dissociated from the PSU's +V side, one could use an independent power source for the meter or skim it directly the from the -15V side,. According to seller's posting for the same board (labeled M4430), power source can be up to 30V but recommend 4V to 12V. So it could run at 15V for a while during testing. Once tested to work, insert a resister to drop the meter's Vin (thin red wire) down to within the 4-12V. recommended range. On this side, the reversal must be complete. Connection to the sensor insertion (2 pin socket) and meter's ground (thin black) must now use the PSU's -15V line. The voltage sense (thin yellow) and meter's Vin (thin red) go to PSU's ground line.Good luck. I'm sure this is getting a lot more complicated than you had imagine. But EE is fun....

author: ledtester
author_rank: Super Contributor
timestamp: March 07, 2025, 02:49:44 am
content: For high-side measuring you might look at the INA226:https://www.eevblog.com/forum/projects/ode-to-the-ina226-voltagecurrent-sensor/The INA219 can also do it and Arduino-style module boards are available. You'll have to interface a microcontroller to them, write some code, hook up a display, etc.

author: Zero999
author_rank: Super Contributor
timestamp: March 09, 2025, 12:37:41 pm
content: You might be able to convert it from low side current measuring to high side.The meter will have a similar internal block diagram to this. It might not have amplifiers for the current and voltage, but that doesn't matter for the purposes of this discussion.What you need to do is move the current shunt (R3) to the positive side and add an amplifier to shift the voltage to the negative side, which is connected to the current input of your module. I suggest the ZXCT1009 because its easy to use and only has three pins.https://www.mouser.com/datasheet/2/115/ZXCT1009-93594.pdf

author: Rick Law
author_rank: Super Contributor
timestamp: March 09, 2025, 10:31:45 pm
content: INA219 is a great little chip. Arduino Nano + INA219 + display is my go-to solution for my discharge or V/I monitoring. I have a dedicate Nano+INA219+Nokia5110 permanent set up just for whenever I need V/I/Watt monitoring.But I don't think the OP is ready deal with the learning and complications of added the MCU to the mix. The INA219 can only deal with the +V side, but not the -15V. If the OP is ready to deal with the Arduino Nano, plenty of choices.Mean time, OP, if you are ready to give it a try/test using two your voltmeters for Volt and Amp reading on the +15V (or+5V), I attached a drawing on how to do it -- much easier to follow that words.. Two of your existing voltmeters, one to read voltage and one to read current on the high side. If you like that, this is directly applicable to the +5V. We can figure out how to wire the -15V side (which actually can do both Volt and Amp on the same meter.)

author: tony359
author_rank: Frequent Contributor
timestamp: March 11, 2025, 09:37:56 am
content: I've tinkered with Arduino and RPi a bit. I could consider this solution if it works.At present, the meters I showed you have been disconnected as they introduce a very subtle 500Hz noise on the lines. It would not be audible in real world but I am testing a sound device and I can see the spikes on a Spectrum Analyser which I use to confirm that all is well and any noise there is going to spoil the measurement. Just to clarify, the noise is 10/15dB over NOISE FLOOR so you wouldn't hear it from a speaker.What would be your recommendation for my use case? I'd prefer something ready made to be honest. But also happy to put together some code and wire an Arduino. One other requirement is that the meters should really display a value as soon as power is applied. Sometimes those boards come to me with a short: with the Fluke I see the voltage jumping and I immediately remove power (yes, I do check for shorts beforehand, sometimes I miss something!). So ideally I don't want to wait x seconds before the display comes up to life.I know, lots of requirementsThanks for now, I appreciate your help!

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2025, 09:48:01 am
content: I recommend a INA219 with a suitable micro controller and display, rather than hacking your existing panel meter modules.

author: tony359
author_rank: Frequent Contributor
timestamp: March 11, 2025, 12:31:13 pm
content: Thanks.Is there an already made project available I could use? Such as a pre-made PCB I can have manufactured and build myself.

author: tony359
author_rank: Frequent Contributor
timestamp: March 11, 2025, 10:03:20 pm
content: I see there are these boards available. Would I be able to measure all three voltages with them and with no noise injected in the line?

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2025, 10:43:05 pm
content: Anything with a clock i.e. AC waveform, has the potential do induce some noise, including the INA219, which is why good layout and filtering are necessary.The question is why were those panel meter modules so noisy? Have you tried reverse engineering one?

author: tony359
author_rank: Frequent Contributor
timestamp: March 11, 2025, 10:47:38 pm
content: I wouldn't say they are noisy - as I said the noise is only above noise floor, in normal use I doubt you'd hear it.I'm not skilled enough to reverse engineer something like that I'm afraid.I might get one of those modules and test it, unless there is a recommended "good" one?

author: Rick Law
author_rank: Super Contributor
timestamp: March 12, 2025, 04:43:29 am
content: Quote from: tony359 on March 11, 2025, 10:47:38 pmAlmost, 2.5 out of 3.First,some background needed for this discussion: You will need an MCU, say an Arduino NANO V3, and a display. The INA215 uses I2C (bus) for communication, so an I2C 2004 LCD seem natural. 2004 means 20 characters/line, 4 lines. 2004 I2C LCD is common and code examples are easily available, The module you shown looks like a clone of my earlier version AdaFruit INA219 module so the code examples and the instruction pages from AdaFruit will help a lot.The reason why 2.5 but not 3:The INA219 is a high-side measurer, it works well on the +5V and +15V rails and they can even be on the same MCU. High side is away from ground. Low side is ground side and measuring there causes uneven grounds, On the -15V rail, you have to run the INA219 (and it's own MCU+display) in "reverse", PSU's ground is high, and PSU's -15V is low, So now the INA is taking measurement at ground again and nowuneven ground is introduced. Your DUT at +15V or +5V has different ground than the -15V.If you are going to venture into Arduino land, there is a way to do all 3 (+15V, +5V, -5V) on a single NANO with minimal additional hardware/complications. Less learning curve (no INA219), less hardware. Plenty of example code for the LCD and how to do ADC (amperage or volt) measurement. Just a few resistors for 6 voltage dividers, may be 3 current sense resistors (or use the ones nicely mounted in your existing voltmeters) and perhaps some voltage drop resistors to drop voltage so it can use the -15V rail to power the NANO. That will give you PSU-Vout and/or DUT-Vin and Amperage reading for each rail.Once the basics are done, there is an upgrade path to give you more (such as showing more info like system run time, better accuracy, show PSU temperature etc., etc.). The NANO (10 bit = 1/1024 resolution)is probably as good as or even better than the accuracy your 3 existing voltmeters at hand. The INA (12bit 1/4096) is better than the NANO. Perhaps even a pair of ADS1115 (4 ADCs each = 8 ADC, you need 6). That will give you 15 bits (1/32768)... Upgrade is endless, but that is after the basics.If you are interested, we can open that discussion.Edit1:Swapped two sentences to make it easier to read.Edit2:Strike out one sentence "...is probably as good as or even better than the accuracy your 3 existing voltmeters at hand..."I am thinking voltage measurement only in that statement. For amperage, they may use differential to amplify the measurement. NANO's 10 bit ADC measuring 30V via voltage divider will give.0293V (293mV) resolution. Changes below that cannot be discerned. Using a 0.1ohm sense resistor, that givesamperage resolution at 293 mA. It will change at steps of 293mA. Changes smaller than that will not be discerned. Make sure you found this acceptable before giving this a go.

author: ledtester
author_rank: Super Contributor
timestamp: March 12, 2025, 05:03:17 am
content: Quote from: tony359 on March 11, 2025, 10:03:20 pmSimply multiplexing LEDs could very well introduce noise - especially in the audio frequency range.

author: tony359
author_rank: Frequent Contributor
timestamp: March 14, 2025, 05:13:05 pm
content: I've tinkered with Arduino before. I wouldn't be able to start a sketch from scratch but with some copy and paste I can get somewhere. I also have an Arduino somewhere!

author: Zero999
author_rank: Super Contributor
timestamp: March 14, 2025, 10:40:46 pm
content: The voltage is easy. Virtually any panel voltmeter will do.It's not measuring negative voltage or negative current which is the problem. It's measuring positive current. Your existing meter measures negative current. Use one for the negative side.For the positive supplies, I would use a voltmeter across a suitable current shunt. 200mV is a common range for a voltmeter.Here's an example of a suitable product.https://www.rapidonline.com/voltcraft-dvm-230w-dc-digital-panel-meter-51-5392It will need an isolated 9V power supply. A small isolated DC:DC converter can get 9V from your existing PSU. This one works off 5VDC.https://www.rapidonline.com/recom-10000590-ro-0509s-1w-dc-dc-converter-5v-in-9v-out-49-7116

author: Rick Law
author_rank: Super Contributor
timestamp: March 14, 2025, 11:21:15 pm
content: Quote from: tony359 on March 14, 2025, 05:13:05 pmStarting Arduino from ground up is not difficult. Arduino was designed with low entry barrier as a design goal.The codeyou need would be mostly already be in the examples. Start with an example and, copy and paste from other examples into appropriate location into your code. Once the basic is done, than the optional refinements.To get the basics working, the most difficult part is really getting the LCD to work and the complication of Arduino grounding at PSU's -15V. Your Arduino's USB ground is not the same as your plug-in PC's ground (which is, if connected, same as PSU's ground which would be V+ of your NANO). Your NANO needs a laptop running on battery power should you want to log data or communicate out via the NANO's USB serial.This can cause real bad problemsif you forgot and then just connect up the NANO-USB to a PC to log the data.The main drawback of this approach is the infinite temptation of enhancement, I can see that the first upgrade you may want to do is to add an OpAmp as differential amp to increase the Amperage measurement resolution for each of the rail.. Simple job, so rather attractive. Then more enhancements such as better ADC to make it more accurate, reducing noise, etc, etc... The job just never seem to finish.These tasks, while almost all "copy & paste", are complex. You will learn a lot, but it may take weeks even before the first enhancement!Your best bet is to buy something suitable ready-made off the shelf. If not, yourreally have to wantit to venture into making it.

author: tony359
author_rank: Frequent Contributor
timestamp: March 19, 2025, 11:01:58 pm
content: I've used an LCD display with Arduino before - and I have one or two around. So I'm not scared by that. What I cannot do is the HW implementation, for that I need some guidance.QuoteI cannot unfortunately. They introduce noise on the lines they measure...

author: Rick Law
author_rank: Super Contributor
timestamp: March 20, 2025, 05:01:42 am
content: Quote from: tony359 on March 19, 2025, 11:01:58 pmSince you've already done LCD+Arduino, next step is, you can do some voltage measurement. I attached a picture of how it could work (phase 1, raw). I know this set up works I did this exact thing on +12V +5V and -12V . But I was not concern about noise.With noise being a prime concern, before we go further, noise should be tested by doing some simple measurements. You could connect your Arduino (with LCD). See if noise is acceptable first:1. Nano power by battery, and measure just the PSU +5V without any voltage-divider. Nano ground is PSU ground for now. See how much noise it injects into your system. This is minimal noise injection. Acceptable?2. Next test is letting it powered by battery ground at PSU -15V and measure all 3 rails one at a time with a voltage divider and test the noise level when the PSU -15V is ground. Acceptable? (PSU +15V is 30 volt to the NANO which is grounded at PSU's -15V)May be the noise level is not at auditable range. I am an amateur with some experience but not a lot. I don't know how to remove noise other than throwing some capacitors at it and watch the scope. May be that will work for you too (or some other will jump in with better filter than throwing capacitors at the problem). But if we have acceptable noise so far measuring 1 of many voltages, we know we can possibly have a working system adding the other measurements.The picture attached show bare basic hence I call it phase 1. As I pointed it out before, Voltage resolution is not too bad (30/1024 = 0.0293V resolution for the +15V rail since Nano is grounded at -15V) but Amperage resolution is very poor (.1ohm sensor at this resolution give you 293mA resolution each measure. But when back luck strikes, upper measure can be 293 high while lower is 293 low, so differential error could be as high as 2x293mA). Thus, I would think you may want to use a differential amp (phase 2) to increase the Amperage resolution to (5/1024, perhaps more if necessary). Noise not withstanding, I think a good final setup is to add a "buck converter" power-in by PSU's +15V, while Buck and NANO ground at -15V.

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2025, 07:33:06 pm
content: Just use an op-amp and transistor for high side current sense. In this case, you just need an op-amp which will work up to 15V plus a bit of margin and the common mode input voltage range needs to include the positive supply.A common op-amp which will work is the TL072, but not very well because it has a high input offset voltage.VOUT= I*R3*R2/R1

author: tony359
author_rank: Frequent Contributor
timestamp: March 30, 2025, 03:47:54 pm
content: I'm going to be lazy and suggest the laziest method maybe: why not buying 3 extra-cheap multimeter, disassemble them and use the current meter function to measure everything I want?Those work on the positive side so shouldn't be affected, right? Yes, they'd need a battery.Would that be a feasible idea?

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2025, 07:33:59 am
content: Quote from: tony359 on March 30, 2025, 03:47:54 pmThat would work, but obviously each meter needs its own battery.

author: Rick Law
author_rank: Super Contributor
timestamp: April 02, 2025, 06:50:13 pm
content: Quote from: Zero999 on March 31, 2025, 07:33:59 amYou don't need 3 new meters- just two more for the +5V and the +15V. Your -15V rail's meter runs in reverse polarity (PSU's ground is +V for meter, and PSU's -15V is ground for meter, which was the way you originally tested it). Thus the low/high side is also reversed. Now (measure near meter ground) is now on the the PSU -15V side (before it goes to DUI's -15V in) so it can measure voltage as well without DUT & PSU having different grounds.You do need independent battery power for all 5 meters since each meter has their own ground that differs than the other meters' ground.Disassembly is also unnecessary -- since (from pictures of the posted meter) connections to the Sensor resistor are separate (the 2 pin plug). So for the amperage (current-meter), you merely need to connect to the 2 pin plug to the appropriate PSU +/- V out, and battery power to the thin wires. As for voltage wire, DUT voltage is going to be just a tag different after the sensor reg, you can decide whether to show PSU voltage or the DUT voltage (see picture attached) for the + rails. For the -15V rail, you got no choice, your +V is going to be PSU ground and your meter-ground is going to be the PSU -15V. You will be reading the PSU's -15V actual voltage but in reverse (no minus sign).Good luck. This is a much simpler solution, just 5 more batteries to worry about.