EEVblog #102 – DIY Constant Current Dummy Load- CR mode

[reagle]

Hi Dave,
was just watching your episode 102, and you mentioned that in your  original battery capacity tester design, you could do various modes (CC, CR, CP etc). I can see the Constant Current and Constant Power being pretty easy to do by knowing load current/voltage and setting PWM duty cyle.
But what did you do for constant resistance? Wouldn't it require breaking the feedback loop from the current sense and making the gate control opamp into a simple follower so its voltage going to the gate only depends on the uP command and not the current?  Or do you just calculate what the current should be for the given voltage /requested resistance and keep adjusting that on the fly as the voltage changes?I suppose that would work, just not as fast as the hardware only  control of the CC mode

[EEVblog]

I measured the voltage and current hence just calculated the desired value for constant resistance.
Yes, it's not as fast because it's calculated value, but that's usually just fine for say battery capacity testing.

Dave.

[scrat]

My simple guess is that fixed duty PWM is already a resistance control: given a certain switch and duty-cycle, voltage and current behave almost linearly.
Feedback should be needed for precision only (so it doesn't need to be so fast), unless the control is linear and not PWM.
Am I wrong?