Using a MOSFET and comparator together as a switch

[Shinnog]

Hello all, I was advised to use a MOSFET and a comparator together in a solar charger that I am building to stop the solar panel from charging the batteries when full. Seems logical, right? The only problem is I've never used MOSFETs or comparators, so I'm not really sure how to. I need a comparator with an internal reference voltage of 5.5V and a MOSFET rated for 100 ma. I looked around and found a MOSFET but I couldn't find a comparator with an internal reference voltage... does anyone know where I could find one? And once I do find one how to hook it all up?

 

[Bob S]

You should use a standard voltage comparator along with a separate 3-terminal voltage regulator like the Motorola MC7806 (see
http://www.datasheetarchive.com/pdf-datasheets/Datasheets-21/DSA-405486.pdf
and use two resistors (voltage divider) to get 5.5 volts.

 

[berkeman]

Be sure to put a couple hundred millivolts of hysteresis into your positive feedback on the comparator, to avoid noise switching and buzzing.

 

[fleem]

You want to stop charging when the battery reaches a certain voltage? Just charge the battery with the output of a voltage regulator set at that cutoff voltage. It'll cut off the current when the battery voltage reaches the output voltage.