Ok I have an Op-Amp as a comparator,
Its the TCA0372… whole amp it can source, not needed just nice.
and I have a relay,
http://www.mouser.com/ds/2/315/jw-catalog-1075968.pdf
Its driven by 12v.
So with flyback diode and all I have it set up as a regular Op-Amp comparator to the coil of the relay. This worked great everytime without fail when I did this on a bread board. I put this onto a PCB board I made and had fabricated and populated and now it works for a few seconds and never works again untill power cycling and giving it time. (almost sounds like overheating)
The Op-amp V+ is 12v and V- in Ground. Also I know an Op-Amp is not always best to use as a comparator but I had it laying around and verified on bread board it would work.
I check the + input the problem is not there as it gets the full 3.3v im using to drive it. Also the Vref is set with a voltage divider and iv verified its at 1.6v. I power on my board and this whole thing works a time or two and then it never works again. The output is around 0.43v. Its almost as if its over heating and needs time powered off to recover. however this Op-Amp can source a while amp and with the internal resistance of the relay coil it should draw no more then 44mA from the output of the Op-Amp.
Anyone know what could be going on. Iv tried the reference voltage extremely low and also tried pull up and pull down resisters on the output.
Best Answer
You should use a Schottky diode in this application. Try the SMT version of the 1N5819 rather than the 1N4001 type you are using.
That will prevent the coil current from being shared with the chip output isolation junctions (to an appreciable degree) at turn-off.
Injecting currents from voltages below ground can cause latch-up which can damage the chip if there is enough supply current available.