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Voltmeter for negative voltage

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AngraMelo:
If the meter reads current going both ways without having to switch the thick leads, this is the only way I figure it can be connect. But I still believe I need to toggle the thin black lead. If everything goes right the meter will treat the negative rail as ground and will see the 30V coming from ground to power it

I did the wiring again

perieanuo:
I think I made a mistake, I'll draw a schematic tomorrow to clear up.
Current wires needs also to change wiring from sensing common gnd to negative branch
Sorry, I'll revise asap
Pierre


Envoyé de mon iPad en utilisant Tapatalk

fsr:
mmm... i don't know, you need to find some documentation, because who know how is this thing on the inside. It uses a positive supply to power the meter. The yellow wire seems to be to measure the voltage, and then the thick red/black wires to measure current (well, black probably for voltage too).

First, measure both black wires to make sure that they aren't shorted inside (probably won't, but just to be sure...). Then, on the points labeled "power supply for the load", you could connect GND to + and VEE to - to measure your PSU's negative output, or connect VCC to + and GND to - to measure your PSU's positive output.
(VCC means your PSU's positive output wire and VEE your PSU's negative output wire).

Well, at least that's what makes sense to me, but no warranties without documentation.

I think that you need to switch the signals on "power supply for the load", because even if the ammeter works in any polarity, there's only the yellow cable to measure voltage, and that means that the other cable for the voltmeter is probably the black one for the ammeter, so you cannot just leave the ammeter wired directly.

And that's not all, you also need to connect your PSU's output directly when the meter is connected to the other side, so you have to switch VCC, GND and VEE on two different possitions, further complicated by the fact that the meter needs to be in series with the load to measure current. Worse: you have only one GND terminal, and it's shown that the ammeter goes in series with the load, over the black wire. If that's the only way to use it, well, that's a problem, because it's a split supply, so that the current from both sides will return on that lead (but going in different directions).

That seems like several things to test, then draw how the meter would need to be connected to measure the positive and negative output, then see how a switch can provide that kind of connections. And even then, moving the switch will briefly interrupt the output.

perieanuo:
Hi, you can do it like in schematic attached (eagle schematic also attached):
1. you need to use switches that "break before make"
2. all switches needs to commute synchronised (or find a correspondent switch, I was lazy to find one, like a old rotactor)
3. avmeter works in this case for voltages between 3.5-28Vdc
For point 3 solution is regulate power supply for avmeter with 7805 for example before LM317 positive branch and DC-Dc converter + 5V regulator on negative branch
Point 1 is very important.
I don't like none of the approaches, I suggest you did it the right way: 2 avmeters with 2 dc-dc's and 2x5V regulators vor avmeter power supply
I have one avmetter that resemble yours and thich black and thin black are in fact one common node, thin black isn't connected in case of using with same PS for powering the meter and measure for correct measurement.
If you want to open the schematic cut the .txt from sch file, open with eagle.
regards,Pierre

AngraMelo:
Thank you so much for taking the time to draw everything.
The wiring looks like it would work. My issue is that A 8 pole switch is very hard to find. It would be something like a rotary switch of a multimeter. The absolute best I could find is a 4 pole triple throw rotary.

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