Chinese voltmeter/ammeter has no shunt

Thread Starter

carpenter940

Joined Apr 24, 2020
1
I received a cheap voltmeter/ammeter from eBay but it did not have a shunt on it (the thick wire mounted on the meter next to the thick BLACK and RED wires) and because of that, all I get in AMPS on the meter is 104 when I hook any load to it. I've put a short copper wire (approx 10AWG 2" long) across the two thick wires and a DC motor increased in speed but the meter read 00.0 AMPS. I put a 10 ohm resistor across the two wires and a DC motor increased in speed but the meter read 40.0 AMPS. I then tried a 5 ohm across the two wires and a DC motor increased in speed but the meter again read 00.0 AMPS.
My questions are...
1. What resistance wire can I put in there to make it complete?
2. If I'm putting the shunt in the wrong place, how should I add the shunt? (I have tried soldering directly to the board)
3. Should I just trash the meter? (I got a refund from the seller)

p.s. The meter works fine for reading VOLTS.
p.p.s My power supply only puts out 12 amps at best!
 

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wayneh

Joined Sep 9, 2010
16,390
I don't know about your device but typically using the shunt requires a different hookup than a normal voltage or ohms measurement. It's essential that the shunt is in series with the load, not in parallel. The resistance of the copper shunt wire is in milliohms at most. I guesstimate they'd want 20mV at a 10A load, meaning the resistance is just 2mΩ.
 

MrChips

Joined Oct 2, 2009
21,047
Make sure that the meter connected properly.

The small RED and YELLOW wires go to the positive side of the power source and the LOAD.
The small BLACK wire is not used.

The fat RED wire goes to the negative (return) side of the LOAD.
The fat BLACK wire returns to the negative of the POWER source.

VOLT-AMP-METER.jpg
 

bassbindevil

Joined Jan 23, 2014
94
It sounds like they sent you a 50A (or 100A) meter which requires an external shunt. Searching ebay for 50a shunt brings up some which are specified as 75mv at 50A, or 0.0015 ohms. So, since you don't need 100A full scale, try 0.015 ohms, although the decimal point will be wrong. There are low ohm resistors hiding in lithium-ion BMS boards, if you have some old battery packs to scavenge from. I'd be inclined to confirm the scaling using a known resistance (the displayed amps vs the shunt voltage), then use a wire resistance table to determine the correct length of solid copper wire for the desired resistance. There's trimpots on the DPM which presumably adjust the voltage and current readings.
 
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