Bench equipment: adjustable DC load - can we isolate circuit under test?

Thread Starter

cspwcspw

Joined Nov 8, 2016
67
When I've wanted to look at stability, ripple, noise, etc. of various DC power sources, I don't have an obvious load. There are a number of "adjustable DC load" circuits knocking around, based essentially on the principle of using a power transistor / MOSFET as a current limiter, but shorting the output to ground to simulate a load at a given current. From what I've seen, the output stage of the load is usually a high-power MOSFET with a sense resistor and some feedback to control the MOSFET.

However, at least one resource tries/ claims to maintain electrical isolation between the circuit under test, and the load control circuit.
upload_2018-7-19_17-40-1.png
The sense resistor R1 is a 4-terminal Vishay resistor that has an isolation voltage of 100V between its 100 milliOhm sensor, and the voltage it reportes from such sensing. But I cannot see (or don't understand) that the MOSFET in this situation provides similar isolation between the component providing the power, and the component simulating the load.

I see that we do get less-common 4-terminal MOSFETs. Is the motivation to provide for isolation between the MOSFET "control" and the MOSFET "output"? If not, and I did want some electrical isolation between the control circuit and the tested supply (say using a more conventional 3-terminal MOSFET), what might be the suggestions?

Thanks in anticipation...
Peter
 

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danadak

Joined Mar 10, 2018
4,057
Not sure this is useful but by definition an electronic load is itself, its
controlling element, not isolated from DUT. The isolation onc could seek
is from power source of the electronic load and the DUT power. Is that
what you are concerned with ?

Regards, Dana.
 

Thread Starter

cspwcspw

Joined Nov 8, 2016
67
Thanks Dana. Your comment gives me new things to think about. Yes, I'd like my artificial load to be isolated from the Device Under Test. I was questioning whether I could get isolation at the point of test (which is what I understand the 4-terminal sense resistor provides). Clearly if I just need an ON/OFF load, a relay or an optical coupler could switch in an isolated load. So I wondered if some kind of MOSFET configuration was somehow capable of isolating control from load flow.
Of course if I just make sure my adjustable DC load circuitry is isolated from mains or ground, I should also be okay. It's more in my head for inquisitiveness than a particular need I have. Before watching the video I had no idea 4-terminal sense resistors even existed.

Peter
 
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