Current sink

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Gregfox

Joined Jul 23, 2011
16
Hello all, I’m working on a project, a Battery powered Current sink with low voltage cutoff. I want to be able to connect a dummy load to a project, or run tests on batteries etc. and be able to set a low voltage cut off point. The Sink will go up to 1.5A.
In the attached schematic, the current sink works and the low voltage cutoff also works, but not well.
The problem is that when the low voltage cutoff is set with R5 it does not shut off clean. As it’s shutting off power by grounding T1 there is a lot of rail to rail chatter on IC1C, and the current drops off slowly, and also is not clean.
I hoped R9 would provide about ½ volt of hysteresis but it does not. The hysteresis is for battery testing, so that when the set point is reached, the circuit will not “chatter”, when the voltage drops to the set point, Q1 turns off, and the battery voltage rises slightly.
My problem is:
1. I can’t figure out how to stop the chatter on IC1C.
2. Add hysteresis when Q1 turns off, so that it doesn’t chatter.
Any help will be much appreciated.
 

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