# Read current from two ground-connected supplies

I have two separate variable DC PSUs (15V@2A, 20V@4A) from which I read the voltage and current through a microcontroller. For the current reading I use a low-side shunt.

The ground output of the supplies are connected toghether with the uC ground, then each one fed through the shunt resistor to the loads.

I would be interested in a way to somehow connect the load ground leads together and still maintain the separate current reading. I'm assuming that current will flow only through the used power supply so theoretically it should be possible.

Maybe I misunderstood your configuration but if it's like the following I don't see the problem of measuring the individual currents.

Or is this what you want to do?

In this case there is no way to measure individual ground currents since there is only a combined current so it only leaves the high side shunt option.

Note that there is a solution for high side measuring with no need for supply like needed for INA138.
I refer to a solution like ZDS1009 which uses a current mirror.

• Indeed, I went with the first configuration a few months ago, previously (before asking this question) I was using the second configuration. I believe this is the most accurate response and useful for people searching to achieve something similar. Commented Jan 8, 2014 at 9:46

If both 0V shunts are connected to power supply ground and then you connect the load end of the shunts together, electrically you get one shunt resistor and both readings will be the same.

If your "real" need is to have no shunts in the ground connections (so that both loads can be commoned to ground together) then maybe a high-side current measurement circuit is going to be better.

Here is a chip from TI that can do the job. It's a INA138-Q1

• Thank you for the answer, I should've mentioned I know about INA138/139 or that I can simulate it. I was thinking that maybe I'm missing out on an obvious way to do this with low-side shunts. Commented Jul 9, 2013 at 11:44
• @brainwash I don't think there is a way of doing what you want on the low side Commented Jul 9, 2013 at 14:12