The circut is here:
The idea is to discharge a NiMH AA cell at 500ma down to the point where the cell voltage is 1V. I picked the specific parts because I already have them in my junque bin.
The idea is that R1 is the sense resistor which should have a voltage across it of 0.25V if 500ma is passing through it. Op amp B controls the pass transistor such that the current stays constant at that level.
Opamp A is supposed to keep the cell voltage from dropping below 1V.
I'm not very experienced at this, and would appreciate comments on whether this will work. I am most curious about:
- What will happen as the discharge voltage approaches 1V. As the output voltage of Opamp A drops, the current will be reduced. But when the battery load is reduced, the voltage will go back up again. So I wonder if it will start to oscillate, which I don't want.
- I have a blind spot with PNP transistors. I have R2 in the curcuit to limit the current that Opamp B tries to sink out of the transistor base. But I think maybe I don't need that resistor. And if so, it would be better without it since the opamp may have some trouble driving the transistor in the first place. So I guess the question is - if the voltage on the base is lower than the voltage on the collector, will current flow through the base?
And if it matters, there will be another duplicate circuit for the other cell, which will share the same quad opamp and 1V reference.
Thanks for any suggestions.