I've been trying to build my own dummy load based on Dave's design (seeing as how everyone else it seems has built one), and I followed his schematic almost exactly. I'm using an FQP30N06L logic-level MOSFET instead of the MTP3055 that Dave used. Also, instead of an LM324 I'm using two LM741's. The other difference is I used a 100k pot instead of a 50k but that's not the source of my problems.
What seems to be the problem is that the second LM741 (the one connected to the MOSFET) is constantly outputting the highest voltage it can. This is because the inverting input (V-) is much lower than the non-inverting input. The non-inverting input (V+) is following the voltage from the pot perfectly, so I know the first op-amp is doing its job perfectly. I really can't figure out why V- isn't higher -- the MOSFET should be more than fully turned on (the op-amp is putting out something like 4.4V to the gate, which according to the datasheet is fully turned on -- the Vgs threshold is almost identical to the MTP3055 specs). So essentially, when I turn the pot -- nothing changes. The other problem is that even when the pot is turned all the way down the first op-amp will only go down to 0.9V (but I know this is because of the nature of an LM741 -- I suppose I should try and find a better op-amp for the job -- I've got a whole pile of different op-amps but I'm still learning about how to choose the right one for each job. In this case, just to get it working, I figured the LM741 would work)
Now, technically speaking, it does work -- the power supply has about 770mA being drawn from it constantly (there is, accordingly, about 0.77V on the source side of the MOSFET, which is, of course, connected to V- on the second LM741). So as a 'fixed' dummy load, it's working. But of course I want to be able to adjust it.
Can anyone see a glaring error I've made? I'm sure there is one... is it just that the LM741 can't handle the job? Or is there some spec on my MOSFET that I'm missing? Any insight at all would be much appreciated! I can draw a schematic if needed but honestly it's identical to Dave's minus the part changes. I also added an ON/OFF switch.
Thanks guys!!!
EDIT: Well part of the problem was because the power supply I was testing it on couldn't even supply enough current to get the inverting input up high enough for the op-amp to balance out. So I switched things around and now I have the power supply input connected to my 60W bench supply. Now with the current knob turned all the way down it draws 1.9A. Once I turn the current knob up high enough I am able to dial in current between 1.9 and about 2.2 (the limit of the voltage divider + the other losses). So now I'm wondering why it can't go below 1.9A? Should I run the op-amps from a split supply? I'm fairly sure that would help... I'm finally really wrapping my head around what is going on in the circuit. So if I were to run the op-amps from +/-5V, the op-amp would be able to 'pull' the output down more? Or am I missing something else... I know that this would solve the problem of op-amp number 1 above not being able to go lower than 0.9V.
However even with the second LM741 having 0.9V on the inverting input and 1.2V on the non-inverting input the output is still at the positive rail. This doesn't make any sense to me as my understanding of op-amps dictates that it should be trying to make its inputs equal.