I have an automotive cooling fan which is nominally 12v and draws about 15-25A with a BIG inductive spike at startup (will happily blow a 30A fuse).
These are traditionally wired via standard automotive relays, but I'd like to drive it from a FET or similar solid-state circuit, possibly PWM but that's a glory job for later, for now let's just switch it on & off with minimal magic smoke escape ;)
This device presents a few challenges:
- Big inductive spike at startup / switchoff
- When the vehicle is moving but the fan is NOT powered, we need to allow it to freewheel.
- When the vehicle is moving AND the fan is powered, it may be driven "overspeed" by the motion of the vehicle, again we want to either disengage (freewheel) it, or at least not try to brake it.
- In both cases the driver circuit will have to tolerate the conditions (freewheeling open-circuit the motor may generate far more than 12v, for example) as well as switch between them without letting the smoke out.
I haven't started doodling circuits yet as I suspect the answers to this question will dictate things like device choice & circuit topology. I'm assuming the freewheeling requirement will mean going down the H-bridge route (or at least half an H-bridge, there's no real call to reverse the rotation of your cooling fan!)
I mentioned that PWM control would be nice, other niceties would be soft start (to avoid the massive startup kick) and over-current protection (if the fan tries to kick in when something's jammed in it, the vehicle is 4x4 so the fan could easily be full of mud/sticks/squirrels at any given moment).
So, I'm looking for guidance on what sort of topology I could use to drive it, which families of devices would be most tolerant of this sort of situation, parameters/configurations to look out for or avoid, and what's achievable or a right pain from the "wish-list".