Protecting FET From Relay Coil

[Noggin]

A guy wants me to make a device that will switch a relay on and off (will provide a ground), but the relay will be external to my system. How can I protect the FET or BJT (havn't decided which I'm going to use yet, though it won't really matter to the design)? The relay will switch on for 10 seconds, and off for 5 seconds so long as the car is turned on and the AC is on. Are BJT's as sensitive to those high voltage spikes as FETs?

The relay is the one for driving an AC system in a car, I don't know if it has a built in flyback diode or not. I suppose I could just put the diode at the drain/collector to 12v of the car, no reason it has to be right next to the coil...

 

[Papabravo]

How you connect the diode depends on how the load is referenced. Is the relay coil referenced to body steel(ground) and activated with a high side switch, or is it referenced to (V+) and activated with a low side switch?

You may also want to consider what happens on an alternator load dump which is independent of what the relay is doing.

 

[Noggin]

The device I build will be a low side switch

 

[mramos1]

anyway you can draw your two outputs and the relay coil? And upload it?

 

[Noggin]

Schematic is attached. I'm going to add some diagnostic LEDs, and the TPS input is a voltage level proportional to the throttle position (as it was described to me) from 0 to 5v. It will be fed into an ADC so that the AC will be switched off at full or near full throttle.

 

[Noggin]

You may also want to consider what happens on an alternator load dump which is independent of what the relay is doing.

Are you referring to the 80+ volt spike as if the battery is suddenly disconnected? I was planning on using a zener or an MOV in front of the voltage regulator. I know the 7805 isn't meant for a car application, but this board has to remain cheap and small.

 

[Papabravo]

Yes, I think you're in good shape.