Ampmeter Feedback issue on power supply

I’ve attached a video of my problem as well as a schematic

Basically, I have a mini amp meter that I want to measure the output of my power supply. It’s built into the power supply and powered internally. After frying one circuit, I learned that these amp meters need to be powered from a source separate from what they are measuring. I now have an isolated power module to power the meter and then I’m running my output through the meter as well. Whenever I turn that output on, the amp meter reads around 6 amps. I haven’t left it on for more than a second or two cuz I don’t want to fry more things.

I gave more details and images in the video. Let me know if more info is needed or what ideas you have.

Thanks tons

Draw a schematic of how you connected it. I do not think many persons are interested in watching a video for several minutes without getting the information they need when spotting the problem in a schematic takes 30 seconds.

Alright, chances are you’ll find more wrong with my amateur schematic than the actual circuit, but here it is. Sorry it’s hand drawn. Hope it is clear enough.

P.S. Haven’t ever heard of viruses on dropbox, but don’t watch the video if you don’t want to.


I was able to view the video but I'm unable to find an example of the "all in one meters" that you speak of. I did a search on ebay, and all of the search results came back with devices that have a 3 wire input (power, gnd and test point).

None of them seemed to have independent power inputs.

KenF, here is one like I have -

Sorry, not sure if posting links to stuff like this is ok.


Since you have 3 digits on the model you're using, I did a bit more searching to try to find something even closer to what you have.

I found this one, but the diagram at the bottom, although agreeing with your concept of a separate power supply, seems to be wired somewhat different.

0.36" mini Red LED 0-100A 99.9A DC Digital Panel Ammeter

You're probably not going to like what I'm about to say but it is customary to perform a standalone bench
test with a test load and test supply to test a piece of equipment like the ammeter before actually installing
it in a system. You need to remove it, or somehow disconnect it from everything else and power it with a
battery or some other power supply . You might be able to use one of your buck converters but it can't be
the one that it was connected to when having the problem. Think outside the box, meaning where can you
find a power source "outside the box" you have everything installed in ?

12V/1A = 12 ohms.
12V x 1 A = 12W.

In order to connect a 1A test load to your converter, you need to put a 12 W 12 ohm resistor across the output of the power supply. The ammeter will also need a separate power supply (preferably completely
disconnected from the rest of your box ).

If the ammeter reads 6A when separated from the box and in series with a DIFFERENT buck converter
that is SEPARATED from the box (like running off a car battery under the hood), then I think it would be
safe to say there is something wrong with either the buck converter or the ammeter. If the ammeter
reads 1A, then I would say the problem lies in the box wiring. You have too many variables. Somehow,
some way, you have to ISOLATE the UUT (Unit Under Test) from everything else.
Each and every one of those buck converters should have had a standalone bench test and each ammeter
and volt meter should have a standalone isolated test. If I installed all the parts in a system without
testing them independently at work my engineer would horse whip me. (verbally of course)

Your ammeter is wired incorrectly. Please look at the schematic for the eBay part you provided. The large red wire should be connect such that the positive supply passes through the load and then to the large red wire. The large black wire must go to the negative supply. You do not have it wired that way...

You do not need isolated supplies for the meter you show on eBay, you only need to wire it correctly. On the eBay ammeter, you'll see the small black (supply negative) wire connected together with the large black (shunt low side) wire which shows that since commons are already connected together, common supplies will not be a problem. This in fact the most common way to connect supplies, with the negatives all tied to the same reference point.

As general rule, ammeters are wired in series with the load, voltmeters wired in parallel. You might also have another error in your wiring as I see the ammeter reading does not change with the variable output which leads me to believe the large red wire is connected to a fixed voltage supply.

I wanted to check that too but couldn’t find any information on the wiring (since the OP didn’t include wire colors) . How did you figure it out ?

Thanks for the responses -

@raschemmel - I did test the amp meter before installation and it worked correctly. I also had it wired as it currently stands, only I powered it externally from a 9v battery and it worked fine. So I think the problem is definitely something with my current wiring (no pun intended).

@avr_fred - Now I’m starting to see what you’re saying. Let me clarify to make sure. I drew a picture of how I have it vs. my understanding of what you are suggesting based on your reply. (ie. put the meter after the load rather than before?)

wire it like this?

Either way, am I fine to leave the meter powered through the isolator?

Raschemmel, the wires were in the eBay photos, no magic insight on my part.

Bradix14: The isolated power, no problem and since they are already wired in, I cannot think of a reason to change it.

Both drawings you posted are incorrect. :slightly_frowning_face:

The eBay drawing is easiest way to show the correct connection path. Just change "battery" to your variable output regulator module and connect the smaller red/black wires to your isolated supply which appears to be okay as-is since the ammeter powers up okay.


Alright, I hooked it all up according to the image. Still read around 6 amps with no load attached to my jacks. Then I disconnected both fat ampmeter wires. It still read the 6 amps!!! So my problem is something in between the isolated power module and the ampmeter...

any ideas or next steps.

thanks for the troubleshooting!

You've already stated there is nothing wrong with the ammeter because you tested it before you installed
it. I said you need to isolate it from the box, specifically get it out of the box. It would appear you are
reluctant to do that. You are going nowhere fast. Sooner or later you have to face the fact that you need
to get that thing out of the box and back onto the bench. (ie: back to square one). If it is still reading 6 amps it must still be in the box because you stated it worked fine "outside the box". I realize you spent a
lot of time building that thing but it is worthless to you if it doesn't work. Take it apart, piece by piece until
you find the wiring error, which undoubtedly you will find. You're asking for advice. We are giving it to you.
It's not worth much if you don't follow it.

Hi, if you have the amp meter only connected to power and the thicker current measuring wires out of circuit, I would say you have cooked the internal shunt if you only get a reading of 6A.
With a DMM, and the meter not powered up, measure the resistance between the REDand BLACK heavy wires coming out of the meter unit.
It should be very low, less than an ohm.

If it is higher then you have probably cooked the internal shunt.

Tom.... :slight_smile:

@raschemmel - you’re right. I was being reluctant to pull it clear out. I had it all so nicely in there, I just wanted to make sure it wasn’t a simple wiring error before I started surgery.

I will yank it out and do some more troubleshooting and error elimination.

I appreciate the help.

Thanks for the tip TomGeorge

Hi, again, on your diagram you show LED’s as output indicators, are these LEDs with built in current limit resistors, or have you fitted them but forgot to put them in your circuit.
Can I make a suggestion, reverse engineer what you have constructed, that is, start with a blank piece of paper and draw your circuit from what you physically have constructed, painstakingly trace every wire.
I know it can be a pain, I have to do it quite often, but I get payed to do it.
It will tell you what you have got compared to what you think you have.

When attempting projects like this it is usual to keep a circuit diagram as you design and assemble it.

It looks like it will be a nice bench electronics supply, but check your wiring.

Tom… :slight_smile:

@raschemmel -- the amp meter is toast!! Thanks for making me take it out. Took it all the way out and it still reads 6 amps. The resistance is still almost 0, but obviously something is wrong. I'll have to order a replacement.

@TomGeorge - the hand drawn schematic is drawn as it is currently built. I just drew it yesterday for the purpose of receiving troubleshooting help. The LEDs do have resistors, I just didn't draw them in. Other than that, I traced every wire.

Try starting with the schematic first next time you attempt to do a project like this. With all components in and all issues clarified. That will take a lot less time.

My first electronics project was a bench power supply built into a nice wooden cigar box. It had machine
screws for terminals. It had some fixed voltage regulators and a bipolar +/-12V supply using an LM723,
similar to some of the circuits here. I tested it by running a metal can opener back and forth across
the terminals , shorting them and generating sparks. Passed. All the voltages were still there after the test.