Meter and Shunt Wiring Question

Hi BFL13,

Now you know why I hate shunts.

Thanks for the good explanation. I was able to get it to read volts but it showed "0" for amps trying it every which way. But also my test set-up may have been wrong.

I don't know how many amps it should have said with the set-up I had to be able to use RJ's advice on that either. I will be using it to measure charging amps between say 156 amps down to about 20 amps. Other than that it would be for as a voltmeter.

I need to set up a better test. It is getting dark, so I will get back to it tomorrow.

At least now I know what to try.

BFL13 wrote:
In diagram #2, the IN+ is on the outer end of the shunt and COM at the inner end, so it would show the amps from the battery to run the motor?

If the thing is a battery charger instead, you put IN+ on the battery end of the shunt and COM on the outer end, and it will read amps into the battery from the charger?

Yes and yes.

It may help to understand that the ammeter part is really just measuring voltage and applying some scaling factor, and the shunt is really just a low-value resistor. The current through the resistor causes a voltage drop, and the meter part measures that voltage drop and shows it in numbers that (hopefully) correspond to the current flowing. The COM lead is the negative lead for the ammeter's voltage measurement, and the IN+ lead is the positive lead. (The PW+ lead is the positive lead for the voltmeter part of the meter, and the COM also its negative lead. Apparently COM and the power supply - are internally connected together. The + power supply lead is, of course, the positive supply for the meter circuitry.)

When current is flowing from the battery to a load, the battery negative terminal is the most negative side of the resistor, so the COM lead goes there. When a charge current is flowing, the sense of the current and the voltage across the resistor is the opposite—the battery really is the load in this case, and the charger the power supply—so the end of the resistor opposite the battery is the most negative and gets connected to COM.

I don't know how you can miss hooking up a meter w/ ext shunt. But I would be curious how low amps you want to read, because I have a brand new 200 amp one here that won't read anything till near 10 amps. These 200 amp meters are for reading starter current, or very big converters. They are not accurate at lower amps at all

In diagram #2, the IN+ is on the outer end of the shunt and COM at the inner end, so it would show the amps from the battery to run the motor?

If the thing is a battery charger instead, you put IN+ on the battery end of the shunt and COM on the outer end, and it will read amps into the battery from the charger?

The two diagrams differ mainly in that the first one has the meter's power supply isolated from the supply for the thing being metered, while the second the meter and the thing share the power supply. Since you don't need an isolated power supply, you'd presumably use the second diagram with the controller and the motor.

The + and PW+ wires go to positive from the battery (presumably via an appropriate fuse).

The COM wire should go to the more negative side of the shunt, which for your use would be the chassis ground side, assuming you're putting it as usual between the chassis ground and the negative battery lead.

The IN+ wire should go to the other side of the shunt, i.e. the battery side.

If for some strange reason the amps reading shows the wrong direction, swap the two leads from the shunt.

There's a very slight chance that using an isolated DC-DC converter for the power supply (which would be connected to the power supply + and - connection) that you might actually be able to get a current reading in both directions. You could test that by hooking it up to a 9V battery or similar, I suppose, and seeing what it does. Most likely it won't show negative current.