hi, after a schematic that will turn on a relay at about 13.2 vdc and stay on , until voltage drops to about 12.8 then turn of .,

anyone help ?
mark k

 

For the average magnetic relay you would need to set up a comparitor circuit as the hysteresis of the average relay is way more than that.
Max.

 

For the average magnetic relay you would need to set up a comparitor circuit as the hysteresis of the average relay is way more than that.
Max.

True. The last time I looked, an automotive relay was guaranteed to pull in at 80% of the label voltage and persist below 50%.

 

You could use two LM339 or LM393 comparators in a window comparator configuration to control a transistor driver that energizes the relay.

 

Here is something I did for somebody about a year ago:

 

Here is something I did for somebody about a year ago:

That's why I save most of my drawings. The same questions come up, year after year.

 

I like your circuit Mike.
It's a simple voltage comparator and has a stable-trip point basically independent of the supply voltage.
I think it meets Albert E's criteria.