Voltage meter hook up

@jeffreyjene

I copy pasted your post to chatGPT.

It gave answers but I upset someone once before posting chatGPT answers but if you have no problems with using AI you might give it a try?

I don't mind chatGPT. I just know not to always trust it. 🙂 I'll take a look.

@jeffreyjene 

What I like about AI is it takes an "interest" in your project whatever it is.

Also it doesn't have emotional issues that result in it clicking the dislike button.

As you say you shouldn't trust that is is always accurate.

Google: "funny results of trusting AI".

@robotbuilder

Posted by: @robotbuilder

↑

...but I upset someone once before posting chatGPT answers...

No, this isn't the way to be. 🙂

AI tools like chatGPT are a good source for discussion here. Their benefits far outweigh the defects. I'm sure there are many people here who would benefit from your demonstration of how to use the tool and we'd all benefit from the resulting discussion of the proposed solution.

We need more, not less, examples of how to use the AI tools to analyze the problems.

(From a selfish POV, I enjoy it watching you toss your ideas at the AI to see what comes out.)

Don't let one grumpy response deter you. 😉

YMMV

Posted by: @robotbuilder

↑

As you say you shouldn't trust that is is always accurate.

(I couldn't help myself.)

Query:
Why should DroneBot Workshop forum member irobotbuilder
continue to post his chatGPT responses to the forum?

CoPilot Response:
I’m an AI programming assistant focused on software
development. I can’t advise on forum moderation or
personal posting decisions. If you want, I can help
convert ChatGPT responses into concise, code-focused
summaries, verify and improve code snippets, or draft
posts that follow forum style and licensing guidelines.

Query: Are chatGPT responses appropriate for discussion
in programming forums?

CoPilot Response:
Yes — but only as a starting point and with safeguards.

• Label the content as AI-generated (ChatGPT) and follow the forum’s posting rules.
• Verify every claim and run any code; AI output can be incorrect or insecure.
• Prefer concise, runnable examples and explain trade-offs so others can review quickly.
• Cite sources or docs when asserting facts; don’t paste large copyrighted excerpts.
• When you adapt an AI answer, show what you changed and why to encourage constructive discussion.

Practical workflow: post the suggested snippet + a brief
test or repro, note it came from ChatGPT, and ask for
review or improvements.

Hi @jeffreyjene,

   I do use a little free AI for odd things, but not in respect to this answer ... of course, it might have been a better answer if I had ... 😀

These combined voltmeters and ammeters can be good value, but also can lead you in the wrong direction. I'll try to explain the 'innards' and perhaps you will see the pitfalls.

Starting with the two thin wires,  yellow and red. One provides the positive power for the device (typically red), the other (typically yellow) provides the positive voltage being measured. The 'better' vendor websites will provide a diagram, like the one below from AliExpress: 

Inside the meter, there will probably be a small 3.3V preprogrammed microcontroller, with two analogue voltage inputs. One of those inputs will be connected to the yellow wire, via a voltage divider.

The two black wires will joined at a point inside the meter, and this point will also be the 0V line for powering the meter and 0V reference for the analogue voltage measurements. 

For the example above, I would expect the yellow wire and black wires to go to a voltage divider with a ratio of (100/3.3) : 1, so that if the 'power' voltage was (say) 50V, then 50 / (100/3.3) = 1.65V would be applied to the first analogue voltage input of the microcontroller, which would be measured, and shown as 50.0 V on the meter.

The supply for the meter on the thin red and black wires, would be sent to 3.3V Voltage regulator, whose output is the supply voltage to the microcontroller and the display.

The main (conventional) current flow, in the circuit goes from the positive terminal of the power source, to the positive input of the load, through the load, from the negative load terminal, through the thick red wire, into the meter, through an internal low value shunt resistance of say 20 milliOhms, to the thick black wire emerging from the meter and back to the power negative terminal.

The voltage drop across the shunt will be quite small, say 200 mV when the current is 10A. So this voltage is amplified by a factor of (say) 1.65/0.2 = 8.25, making 200mV shunt drop into 1.65V. This voltage is sent to the second analogue voltage input of the microcontroller, where it is measured, and results in a current reading of 10.0A on the display. 

As the two black wires connect to the same point inside, some suppliers will omit the thin black wire, and require the user to use the thick black wire for both the power feed to meter, and as the return connection for the main current flow.

Note that the thick red wire is the load current return point, and whenever a current is flowing through the load, will be at a slightly different voltage to that of the negative '0V' of the main power supply.

When the main power source voltage is relatively low, then the yellow and red wires can be connected together, but this implies it is within the range that the internal regulator can handle.

This voltmeter and ammeter arrangement is fine for many applications, but it implies that the load is not directly connected to '0V'. This can cause difficulties when the 0V must be connected to a common 'Earth' line. It is also inappropriate when the equipment has more than 1 voltage, say 5V and 3.3V, when the user may wish to monitor the current for each voltage.

I hope this sermon explains the situation ... but I'll understand if sends you searching for a friendly CoPilot, Gemini, Claude, etc...

Best wishes, Dave

Thanks, I appreciate it. AI is fine, but I prefer the answers of people myself. Otherwise, I'll just ask AI "what it thinks of my project" and so on. So what's the point of having the forum? I Like the opinion and company of human beings best. 🙂 In any case, AI was incorrect with my question anyway. My 12V power source comes in and connects to a switch. Past the switch it hooks to a terminal. One line from the terminal goes to a buck converter which has a USB for the Pi and the other to the motor power pins for the TB6612FNG. My question is, in this setup what is the "negative of the load"? Where does the thick red wire hook so I can see the amps? Connecting the small red and yellow and the large black only show me the volts.

JJ

Hi @jeffreyjene,

   As I said, the answer I gave did not come from AI ... I wrote it in the early hours of this morning (UK time), based on my memory of some reverse engineering I did on a pair (one voltage, the other current) of 4 digit meters a few years ago, that also originated from AliExpress sources, and I am pretty sure the combined one you have will use the same approach.

So I have tried to describe exactly what the circuit in your voltmeter/ammeter consists of, so that you can understand how it fits into your scenario.

The first decision you need to make concerns which current (and voltage measurement) you are interested in, as you effectively have at least two, and probably three current loops (sorry my confusion is my fault, but your wiring description leaves me with an ambiguity concern) :

1. Input to the buck converter and resulting return current 
2. From buck converter to whatever it is powering and return (probably a Pi)
3. Any power passing to a load that is 'directly' from the source, and not via the buck converter (probably motor and TB6612FNG).

Each of these three loops has its 'own' current flow, but you have only one ammeter, and it is 'tied' to negative current side of your circuit. 

As I tried to explain, the voltmeter/ammeter can only 'see' and hence measure the current flowing back to the power source, via the thick red wire, through the meter, and onwards through the thick black wire to the power source. So to even begin to give a definitive answer, I need a precise wiring diagram, involving both the 'positive' and the 'return' wiring.

In a 'fully instrumented' case, you could have three ammeters connected on the 'positive' side of each power source, one of them being the output from the buck regulator. This is 'easy' if each ammeter is a 'standalone' device, like an old moving coil meter or a self-contained/powered multimeter, but more tricky when the ammeter is a chip measuring a voltage drop, that must be powered by the source it is measuring. Of course, there are methods of measuring 'high side' current flow, but they add complexity and cost which is far beyond the simple voltmeter/ammeter you are using.

You could decide to measure the total (conventional) current flowing back to your 12 V power source, by connecting all the returns from buck regulator, motor, Pi, etc to a single point, and connect that point to the thick red wire. 

However, if you want to measure just one current component, (e.g. the return from the motor alone), the situation becomes more tricky, since any return via the thick red wire will at a slightly different voltage to that returning directly to the power source negative terminal. In some cases, this may be acceptable, in others it will cause problems with noise, etc.

I am sorry if this sounds rather negative, complicating and non-authoratitive, but real circuits can behave in manners that are much more complex than a simple approach might suggest.

Best wishes, Dave