- Jul 23, 2015
- Jul 23, 2015
I'm hoping that someone can see what I've done wrong in this circuit, (attached). It looks fine to me, but I haven't done any electronic design for some years and must be getting rusty. It monitors a 9V Ni-Cd battery - the top comparator should go high when the battery voltage is 7.2V or higher, and the bottom comparator should go high when the battery is at 6.0V or lower.
The comparators will be connected to the 'Set' and 'Reset' inputs of a 4013, with the 'Q' output switching a P-channel MOSFET via NPN transistor to switch the battery voltage on or off to the following circuitry.
I could have gone with a single comparator with hysteresis, but this should give more accurate results. (Should)
I've built the circuit on a breadboard, exactly following my schematic. The transistors/LEds are temporary, for diagnosis, and will not be in the final circuit.
Instead of clean switching, I'm getting a slow response, with the LEDs gradually getting brighter/dimmer over about 0.2V of input voltage variation. An LTSpice simulation gives me fast switching, but the actual circuit doesn't.
Is this likely to be caused by the breadboard, and a problem that will disappear when I make a final PCB, or should I make some changes? (Maybe adding a little hysteresis to both comparators)
Any insights much appreciated. I haven't done any circuit design for many years, but thought this would be simple.....
(This is my first post here, by the way. A great forum, from what I've seen so far.)