Advice on using 3.3V I/O to drive 12V solenoid

[admin]

Can anyone advise me on a valid choice of transistor / FET to allow me to use a 3.3V digital IO line in order to operate a 12V, +-800mA solenoid?

At present I'm just thinking of using a NPN transistor with the IO line driving the collector, with the solenoid between the emitter and +12V supply rail (and back-EMF suppresion diode across the solenoid coil), but I'm not sure about which particular transistor to use..

Does anyone have some advice for me? I've already tried to use the IRF540 MOSFET, but that design required me to use a 5V IO line (at least that's what I experienced).

Thanks.

[Wings]

Go to digikey.com, search for RFP12N10L-ND.

It's a 12A, 100v N-channel MOSFET in a TO220 package so it should be easier to use than a surface mount device (assuming that's one of your goals). At 3v drive it will pass 800ma with ease and without a heat sink. You will still need to use a suppression diode (1N4001 - 1N4004) across the coil of the solenoid to protect the MOSFET.

If you are looking for something to use in large quantities, there are other devices probably better suited since this part is close to a buck in single quantities.

Hope this helped.

[admin]

Thank-you - the datasheet on this devices looks exactly right.

I notice that the "equivalent circuit" shows an internal diode across the FET. Is it still recommended to fit an external one anyway?

[bigmac]

Hello,

For a N-channel MOSFET the intrinsic diode will only "catch" negative voltage excursions (below ground potential).  However, when the solenoid turns off, the energy within its inductance will drive the drain of the MOSFET positive (above the supply potential).  The intrinsic diode will do nothing in this instance.

The connection of a diode in parallel with the coil of the solenoid, as was previously mentioned, would require that the anode of the diode be connected to the drain, and the cathode to the positive supply.

Regards,

Mac

[admin]

Mac,

Thanks, yes - it occured to me while I was in the shower (good place to think) - the diode across the solenoid shorts out the back-EMF produced by the collapsing magnetic field in the solenoid, which the internal diode probably doesn't help with.

Thanks for clearing this, though.

Hmm-- I wonder if battery-powered applications could gain any use from trying to recover some of this current and recharge the battery - like regenerative braking, but for solenoids...