Using an adjustable load with negative voltage supplies

[mikejp56]

Hi All,
I have just finished building a constant current load as in EEVBlog #102. Thanks Dave; that was just what I have been looking for.
The MOSFET specs call out V (br) DSS (drain to source breakdown voltage) as 55 V. But I want to test some negative voltage power supplies. So does the negative lead of the power supply go to ground and the ground lead goes to the input of the load? Won't this short the power supply to ground?
The alternative is hooking the negative lead of the power supply to the input of the load, and the ground lead of the power supply to ground. But then the drain to source voltage will be negative. Will this destroy the MOSFET? Is the alternative to build a second constant current load specifically for negative power supply testing?
Thanks again to Dave for a great little project, and thanks all for reading and adding your feedback.
Regards,
mikejp56

[magic]

The alternative is hooking the negative lead of the power supply to the input of the load, and the ground lead of the power supply to ground. But then the drain to source voltage will be negative.

Power MOSFET contains a bulit-in "body diode" which conducts current freely in reverse direction, even if the MOSFET is otherwise turned off (zero gate voltage). So this can't work.

So does the negative lead of the power supply go to ground and the ground lead goes to the input of the load? Won't this short the power supply to ground?

This can work if there is no external connection between the ground of the PSU and the ground of the load.

For example, if the PSU under test is earthed through mains and you connect an oscilloscope probe alligator clip to the ground of the load circuit, the PSU will be shorted through the oscilloscope.

If the PSU is not earthed then this can work. In particular, batteries are no problem at all.
If the load is not earthed, and no earthed equipment like oscilloscopes is connected to it, then it will work too.

[mikejp56]

Hi magic,
Thanks for the prompt response!
I completely forgot about the body diode.
I am doing a bipolar multi-voltage power supply build and I wanted to test it under various loads.
I guess I need to invest in an isolation transformer, huh?
Thanks again and regards,
mikejp56

[magic]

I am doing a bipolar multi-voltage power supply build and I wanted to test it under various loads.
I guess I need to invest in an isolation transformer, huh?

If the power supply under test contains a transformer and the secondary is not earthed and you are testing the secondary then no, you don't need yet another transformer.

Also, if the load is powered by a floating (unearthed secondary) PSU and isn't earthed by any other means (like through test gear connected to it) then you don't need isolation either, unless the negative voltage which will appear on the load's ground is high enough to be dangerous.

edit
Testing a negative high voltage PSU would be difficult, because either the load or the PSU will have its "ground" at high voltage, which would likely pose danger to the operator. This is one case where it may make sense to build a special negative load.

[alm]

If you want to test both positive and negative voltages, then another option is to put a bridge rectifier in front of the load.

[magic]

And what does it change?

It's still fireworks if the PSU is grounded at its positive terminal and the dummy load is grounded at its negative terminal.

[alm]

And what does it change?

That you won't forward-bias the body diode when applying a negative voltage like when:

I am doing a bipolar multi-voltage power supply build and I wanted to test it under various loads.

It allows testing both positive and negative voltages with a unipolar load without switching connections. It even (sort of) works for AC.

It's still fireworks if the PSU is grounded at its positive terminal and the dummy load is grounded at its negative terminal.

Yes. So don't ground both the power supply and the load. That was discussed already in this topic.

[mikejp56]

Hi All,
Thanks for all of the responses. So the electronic load is currently in a beta state. It is built on perf board but not yet enclosed in a chassis. I have been powering it from an earthed bench top supply. If I power it from batteries, then will this solve the problem? It will have it's own internal panel meter powered by the batteries, so it will be completely isolated from the mains ground.
Sorry I didn't mention this earlier.
Regards,
mikejp56

[alm]

If the load is fully battery powered and not connected to ground (like sitting on a grounded metal surface or connected via a USB cable to a grounded computer), then you should be able to safely connect it to the power supply as long as you observe polarity and maximum voltage of the load. In case of negative polarity of the power supply, make sure to switch connections to the load so the + input of the load is more positive than the - input of the load, or add a bridge rectifier in between with the load on the "DC" side as I described in my previous post.

[mikejp56]

Hi alm,
The active load will be completely ground isolated; it will be battery powered and have its own internal panel meter.
So this should allow me to just hook up the negative power supply with the ground lead of the power supply under test to the positive lead of the active load, and the negative output lead of the power supply under test to the ground of the active load?
This makes sense as long as there is no earth ground connection.
Regards,
mikejp56

[alm]

So this should allow me to just hook up the negative power supply with the ground lead of the power supply under test to the positive lead of the active load, and the negative output lead of the power supply under test to the ground of the active load?

Exactly. To make it less confusing, while the load is battery powered it may make more sense to call the inputs the negative (-) and positive input (+), which is how commercial loads that are isolated from ground are generally labeled.

[mikejp56]

Hi alm,
Thanks for your responses and your help.
Regards,
mikejp56