I'm interfacing a Digispark Pro to drive a 12V automotive relay. I'm planning on using the high side driver circuit as outlined by Nick Gammon in another thread:
I plan on implementing this in as small a package as possible so I've started by choosing an appropriate SOT-223 P-channel MOSFET that can handle the relay's coil current (Vishay IRFL9014 60V@1.8A). The Digispark will obviously output an active high +5V signal to activate the relay. What I'm hoping someone will suggest is an appropriate part for Q3, the NPN BJT, in an SMD form-factor. Any suggestions?
That MOSFET has a large Rds(on) of 0.5 ohms, so can we clarify what the
relay current is? Automotive relays can be quite power thirsty so you need
to check the power dissipation.
I'd normally expect to see Rds(on) around 0.05 ohms or less in a modern MOSFET.
BTW don't expect to drive a MOSFET to anywhere near its max current
continuously, its usually the thermal limit with infinite heatsinking, an impractical
value. Its not a parameter you need to look at really...
You calculate the thermal dissipation from the Rds(on) and ensure enough heatsink
if necessary or choose a better device.
The relay current is 546 mA. I said automotive application but actually it's a motorcycle start relay. Ids on the MOSFET at 100°C is 1.1A. That's about twice the expected draw of the relay worse case. Do you believe the high Rds will compromise the thermal capabilities of the MOSFET I've chosen? I'm planning on using copper planes on both sides of the PCB with multiple via holes to transfer heat. The nature of the application is to start the motorcycle engine, so it is intermittent current draw lasting 5-10 seconds maximum.
