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How Do I Test A Negative Power Supply Using Dave's "Current Sink Dummy Load"?

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DaveFan:
I'm developing a small automated tester with 4 e-Loads to characterize some Low Voltage Power Supply Boards which have both positive and negative power supply outputs.
I want to be able to apply loads to all 4 outputs simultaneously and load each output accordingly via software control, and measure the output voltages for each load.

I want to be able to use a current sink dummy load to test both negative, as well as positive power supply outputs, but am not sure how to go about doing this.
I've combed through tons of posts regarding the Current Sink Dummy Load and watched many Youtube videos demonstrating how it works, but not one post on how to test a negative power supply. I assume that due to seeming lack of interest or activity on this subject there must be a way to use the same device to test both positive as well as negative power supplies, but I just haven't discovered it yet.

My first thought would be to connect the negative power supply backwards to keep the current flow in the same direction as for the positive power supply, but this would connect the negative power supply directly to ground would it not?

Additionally, I would like to use the same circuit (current sink) as a constant current selector, preceding the adjustable voltage regulators for both the positive and negative sources.
This will be DAC controlled via OpAmp comparision of the differential voltage across the sense resistor. This will require two OpAmps, one as a differential amp for measuring the voltage across the sense resistor which is fed back to the second OpAmp which compares the DAC voltage and adjusts the output current. It's the same exact current source circuit as Dave's except instead of measuring the voltage across the sense resistor referenced to ground, I will take a differential measurement across the sense resistor.

My second question is, I already know how to do this for a positive supply, but how is it done for the negative power source input to the regulator?

I assume the answer to the first question will lead to a solution for the second.

Please see the two rough sketched schematics to get an idea of what I am trying to do. I can add gain in the feedback OpAmps if necessary, and I know these will work for a positive supply, both for the constant current configuration as well as the dummy load. Problem is, I don't think it will work for a negative constant current & negative dummy load. Please give me some guidence on this...

Can someone please help me with this? THANK YOU!!!!!!!

fcb:
You could build a bipolar current sink, or you could float a standard layout current sink and connect to it with a bridge-rectifier.

DaveFan:
@ fcb: Regarding the bridge-rectifier, not sure I understanding what you mean. Can you please explain? Thanks!

fcb:
Take a bridge rectifier (BR).
Connect your bipolar power-supply under test to the AC terminals on the BR.
Connect your unipolar (floating!) dummy-load between the + and - terminals on the BR.

The potential disadvantage of doing this is that you will add approx. 1.2V to the minimum voltage that you can be sunk by your dummy-load. e.g. if your duumy-load is designed to operate down to 2V, then it will now operate down to 3.2V

You'll also have to design your dummy load to float - but this might not be a bad idea anyhow.

DaveFan:
Now that's just plain cool! I get it, pretty smart idea indeed. And, since I'm using a differential amplifier to read the voltage across the sense resistor I don't actually need the ground reference for the sense OpAmp. What if I use a DPDT relay to connect the two power supply outputs to the load? Would I be able to go down to the minimum without the diode drops?

If I use the relay and leave the load floating will I need to connect the reference ground of the Gate drive OpAmp to the power supply somehow, and how would I go about doing this. I am assuming (I need to draw a schematic to get a bigger picture) with the diodes the ground will take care of itself?

Thanks!

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