Current shunt location question

[Chris Wilson]

I think I may be doing something daft!

I have  Chinese made 0 to 300V /  0 to 50 Amp dual digital meter.The DC volts reads fine, red wire to + 100V DC at the bridge rectifier / electrolytic caps , but no current is displayed. If I disconnect the shunt and feed the yellow and black wires with a few mV the current section of the display starts working. I attach the schematic of the circuit I am using it with, and I have the shunt in a break in the 100V DC ground wire. In other words the bridge rectifier, smoothing caps negative wire is not taken directly to chassis ground, it now goes to chassis ground through the larger of the shunt terminals. I know the device is drawing a good few Amps when the current meter section remains at zero. Thanks for any help.

[tautech]

Chris, ensure the Com and meter supply negative are not commoned.
IME with similar meters, they require their supply is isolated from the rails to be measured.

[Chris Wilson]

Hi again, and thanks for the reply. I am not sure I can actually use this dual volts / amps digi meter in this circuit. I managed to blow one up trying I think the issue is having the 100V DC supply negative tied to ground *AND* the 12V DC regulated supply negative also tied to ground. I can measure amps by putting the shunt in the 100V DC positive supply wire, and running the meter off a battery, but it appears even with the black negative power supply wire to the meter floating trying to *ALSO* read the 100V DC + wire won't work.

For whatever reason the shunt in the negative 100V DC wire, before it is grounded, didn't seem to give an amperage reading.

I have a couple of amp only digi meters that read 0 to 50 amps, using the same 50 amps / 75mV shunt, but they need a 5V DC supply. I am now wondering how to test one to see if the 5V supply negative needs isolating from ground. I can use a 5V regulator on the 12V supply in the amp, but do I need to float it somehow above ground?

These things are neat, cheap as chips, but have complexities I hadn't considered over and above good old analogue meters! And hole sawing a round hole is a lot quicker than setting up to mill a rectangular hole with tight corner radii

[PSR B1257]

I think the issue is having the 100V DC supply negative tied to ground *AND* the 12V DC regulated supply negative also tied to ground.

Why???
As your schematic shows, there is no connection between the auxiliary supply and the supply to be measured