Simple electronic load for negative supply

[samihawasli]

Hello all,

I had a question regarding the simple electronic load Dave featured a few times on his channel. I have been using it to test postive voltage supplies with no real problem. But for the first time, I need to test a negative supply. Could my same circuit and N-Channel MOSFET be used to to test a negative supply? Would it be as simple as the setup below?

If I am testing a -6 volt supply, Applying -6 volts on the input would do nothing, adjusting the Op-amp's input to -5V would give me 1A? Would this N-channel device work like this?
posted the datasheet for the MOSFET too for reference.

If this isn't right, what is the best way to modify what I have to work on negative supplies?
Thanks,
Sami

[wraper]

If electronic load is floating and not electrically connected to anything else as it normally should be, just connect positive to positive PSU terminal, negative no negative terminal. It's not like you need some special electronic load. And if PSU is floating, then it does not even matter if electronic load is floating or not. I think you have some weird idea that you need to connect electronic load terminals in opposite way when you use negative supply. Not to say supply is negative or positive only in a way how you look at it's reference point.

[Doctorandus_P]

For the output of the load it does not matter whether you write "+6V" and "GND" near the terminals, or "GND" and "-6V" They're just pencil scratches.

What does matter is where the power supply from the opamp comes from, and how you generate the reference voltage.

Stop working with those numbers for the voltages, and take a closer look at what the circuit is actually doing.
The opamp is just comparing the voltage differential on it's input terminals, and from that it either increases or decreases it's output voltage.

Without showing how the reference voltage on the other opamp terminal is made, there is not much useful that can be said about this partial circuit.

The circuit below is from a simple load I coughed up some years ago, and I designed it to work without a power supply and a reasonably low output voltage.

How it works:
U101A generates a reference voltage of around 1.2V, which is is twice the voltage over the diode. (Because of R103 and R102)
Then the potentiometer makes the reference voltage asjustable, and opamp U101B compares the adjusted reference voltage with the voltage over power resistor R107.
By using Shziklay pair for the amplification between the opamp and the big current sink terminal the power supply voltage can be kept low.

The whole circuit only has 2 external connections, and can be used as either a current sink or current source.
Because the power supply for the opamps are taken from the same 2 terminals, there is a minimum voltage (Just below 3V) and minimum current (around 20mA) for this circuit.

The use of the transistors and opamp is not critical.
LM358 is a quite nice choice because it works from a low power supply voltage and it's input terminals work upto the V- rails.
The transistors on the output are just what I had in the parts bucket.
I did have to tweak it a bit to get it stable. So you will have to add some capacitors too.

[samihawasli]

Thanks all. I realize my previous drawing left off a lot of context. I was worried about "negative" voltages because I'm testing a system with multiple rails, and your exact question about the reference is what I am after.
The picture below is normally how I would connect things in order for everything to share the same reference. But as it, I would have a negative Vds across my MOSFET? Is that bad, or does that matter? Not sure why having this negative rail is throwing me for such a loop.

Thanks again for all the help

[rascalsailor]

Hi samihawasli, (I know this is an old post)
I came across your CCT and just wondered if I could ask you :
Any chance that the inputs of U101B should be swopped?
Following the circuit - If the pot slider is turned towards the top, U10B output will go lower switching Q101 more off (right?)
Then the voltage on R107 will drop etc.
Also as Q101 turns off, the base of Q102 will float - should it have a pull up resistor from base to emitter for safety?
Thanks and regards
Russell