I have a 3D printer that has a "dumb" 12V DC fan on the extruder. It is always on and starts with printer; but unless the (12V DC) heater is turned on, it is useless and just makes noises.
I can't attach manual switch to it - if I'd forget to turn it on, heater will burn if turned on. I can't simply wire fan relay to heater - fan needs to cool heater down for few minutes after heater goes off.
I need to add a switch that will turn the fan on as soon as heater turns on, BUT this switch must be "sticky" - it must stay on onward, until 3D printer is powered off. I know how to add MOSFET switch, but can't figure out how to make control input "sticky".