Dummy Load Experiments

[PsychoMaster]

Hi
I have ordered lots of different op amps and I am trying them out on Daves dummy load circuit as they arrive. I am using a 10k pot, a irfz44n mosfet and a single 100 watt 1 ohm resister as current source for a meter and checking this against a inline multimeter  . A lm358 works either fully on or fully off no matter what voltages .A single lm741 will not take negative feedback from the mosfet source pin but works beautifully when sourced at the gate and is very accurate. Putting a car headlight bulb before the mosfet takes heat  away from it  and still I get very acurate meter readings.Two mosfets in direct parrallel do not share the load equally and require a ballast resister on the source.Some say this works without ballast but I have found it not so.One was cold and the other burnt my fingers but warmed up when I removed the hot one.

[PSR B1257]

a single 100 watt 1 ohm resister

Fairly high value unless you stay under 1A load current.

A lm358 works either fully on or fully off no matter what voltages

It shouldn't. Have you connectet it correct?

Some say this works without ballast but I have found it not so.

Right. You can connect MOSFETs in parallel only in switching applications. But if they operate in linear mode they do not spread the load current even (do to different gate-source theshold voltage and thermal effects).

[PsychoMaster]

Hi
With the lm358 I have tried using just one internal opamp as well as using one as a buffer but still I get the same affect.My lm324s have just arrived which I will now try.I now have my first oscilloscope (475A Tektronix) so I can see what is happening better. I did not expect the LM741 to give such good results on 5 volts and a 50% attenuation ratio.Remember Daves psu circuit when he first used a op amp to drive the current LED and the output would swing straight from low to high?, I think this is behaving in a simular manner.