Dear all,
I have an issue.
I built a power source from ATX; +5V, and variable (plus others but it does not matter). Nothing fancy, simply buck/boost convertor, no protection, no other circuitry.
Now, I want to power 12V bulb using variable and measure current. I have standard (Chinese) 5 wire Volt/Amp meter that needs to be powered with 5V.
When I power the voltmeter externally, the current measured is 0.223A and everything works properly.
When I try to power it using the ATX's +5V (which was my intention), the current immediately drops to 0.140A (or so) - and it happens as soon as I connect negative wire (question mark in the image) - the positive (power) wire even does not need to be connected...
It looks like the current is "splitting" into two negative wires (+5V and variable have separate negative wires that "join" only on the ATX board). But why does the current go through "voltmeter power wire"?
Could you please suggest why this happens? And what is more important, how to avoid this?
Thank you for your advice!
R.

 

What Volt/Amp meter are you using? The ones I am familiar with requires a totally independent power source; the can’t be powered from the same source that it’s measuring. Your ATX power supply’s 5V isn’t isolated.

 

sounds like the Volt/Amp meter can't be powered from the same circuit to be measured. Take an old mobile phone adapter with output 5V and power up the meter

 

The Volt/ammeter combo I have used before measures the current in the +ve wire not the -ve. Can you provide a link to your meter...

 

The Volt/ammeter combo I have used before measures the current in the +ve wire not the -ve. Can you provide a link to your meter...

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Hi and thank you for your reply. I have always seen Ammeter in negative line... This is from the seller: (ignore 5-24V info, I need more than 24 that's why I need separate powering of the meter).



Anyway, I will try to put it in the positive line.

 

Do you have a link to the volt/ammeter that you have? The diagram you supplied implies a separate, isolated power source for the board (the black rectangle labeled DC5-24V).

Can you try with a wall adapter (wall wart)? Or 4 AA batteries.

 

Do you have a link to the volt/ammeter that you have? The diagram you supplied implies a separate, isolated power source for the board (the black rectangle labeled DC5-24V).

Can you try with a wall adapter (wall wart)? Or 4 AA batteries.

Hi and thank you. Yes, I do need separate voltage (power supply). I only didn't know that there would be problem with "common negative". The thing is that I have a big box with many outputs, multiple converters but only one power source that powers all the convertors which means that all share common negative pole (the convertors are not isolated). I am thinking about adding "wall adapter" which should be isolated by integrated transformer (I guess and hope).