potentiometer tweaking for the Daves CC dummy load

[lk]

Greetings,
i was wondering if it would make sense to offset the potentiometer, so that turns are not wasted, while turning the potentiometer in the range that will not set the voltage high enough to drive the fet gate. In my head at least with a potentiometer on a 5 volt v+ and the fet does not turn on until 2 volts, gives 4 wasted turns, on a 10 turn potentiometer.

I have included a schematic drawn in yenka, that shows what i had in mind, but i don't know if its the optimal way to do it. I plan on using the same mtp3055 as used in eevblog #102

Is this the right approach or is there some other way, or should i just not worry about it?

-lk

[icon]

Hi

I think you're misunderstanding what the pot is doing. It doesn't directly provide the gate voltage; the op amp does that, because the MOSFET is inside the feedback loop. On my example, the op amp output sits at about 3.5V when the current is zero, which I assume is the gate voltage at which my MOSFET starts conducting. As soon as the pot starts to turn, the op amp output voltage starts to rise and current starts to flow.

Build it on a bread board and measure voltages around the circuit - I found that very illuminating when I built mine! I also discovered interesting things about op amp stability and oscillation, but that's another thread.

John

[lk]

Thank you,

i build it again with a different opamp, and now i see reaction for the entire range of the potentiometer, that was not the case before, it would only start doing anything when i had it turned most of the way up.

-lk

[electroguy]

Ik
yes you can, but as icon said, you aren't directly driving the fet gate, the op amp does that. The voltage range you are modifying with the pot, is the voltage drop across your sense resistor, and the op amp does the rest to make it match.

So, in the diagram you have drawn, you can use the top resistor as a "maximum current limit setting" if you like, so that even if you wind the pot all the way up, you don't burn your load out or burn the sense resistor. And likewise for the bottom resistor, make sure you make it small enough so you can turn the load down to your minimum required load.