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
17,496
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
30,712
Make sure that the meter is 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
824
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.
 

DonFring

Joined Jul 23, 2020
10
This is not a time constrained post in that it does NOT expire because the situation is the exactly the same and will be for as long as these same meters are released into the public domain but missing a shunt. I just got one of these exact meters and I wonder if the OP ever resolved his issue and thus was able to solder the proper wire shunt that was missing on the board.
 

DonFring

Joined Jul 23, 2020
10
Just resolved the issue about 5 minutes ago. If you have received one of these with no shunt and you wish to use it for the 10A application it is advertised for:

1- set the current POT to the half way point (marked on the board as I-adjust)
2- strip about 2" of 18AWG solid copper wire
3- start with about 1" (inch) of wire through the loop marked for the shunt to make a horse shoe shape and let about 1/2" extra wire poke out on the opposite PC board component side
4- connect a precision meter and the device in series
5- check amps; if the amp reading is too high on the device display, then wet the solder with an iron and pull through some more wire using long nose pliers to shorten the loop. For example if the device display was reading 1.6A and the precision meter was reading 0.8A, the the wire length would need to be halved.
6-get the reading as accurate as possible, matching the precision meter and then adjust the PC Board side if "I Adjust" (depends on models, some POT positions are switched, so read the PC board
7-if you are able to fine adjust using the "I- adjust" POT, then trim any excess wire from the end of the shut wire sticking out on the PC board side
 

DonFring

Joined Jul 23, 2020
10
That is good news. Apparently these are distributed with "many issues". I could not trim anything because there was no shunt included with the meter, as with the OP and with my own experience. (It reads 999 if the shunt is missing). The pot is a very minimal trim and cannot trim out the wrong resistance or the wrong shunt and assuming one is found and tested. Hopefully your advice to check the pot if there is a shunt and my technique of creating a missing shunt can help others who come along and have also received meters with no shunt in the box and none soldered on the PC board.
 
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