I want to control a freezer using a microcontroller and relay, and be able to set a higher higher temperature for dry curing meats. I realize there are off-the-shelf solutions but I kind of want to build it from scratch. I'm familiar enough with microcontrollers and small signal relays, but I've never tried to switch mains voltages or inductive loads. If the freezer draws 2A (AC steady state, at 110v, not sure what starting current is) what relay would you recommend, and how would you recommend I drive it? I'm thinking of using a 15-25A DPST relay for the load w/ 12V coil voltage. DPST because I'd switch both hot and neutral. And a 12V relay (with V_coil matching my uC supply voltage) in front of the power relay to drive the power relay.

Here's a selection digikey relays matching the power relay criteria I've mentioned... http://www.digikey.com/scripts/dksearch/dksus.dll?FV=fff40010%2Cfff80367%2C16040015%2C16040017%2C16040018%2C16040019%2C1604001a%2C16080007%2C16080008%2C1608000a%2C16080039&k=relay&vendor=0&mnonly=0&newproducts=0&ptm=0&fid=0&quantity=0&PV72=13&PV72=35&PV72=2

BTW, I'm comfortable wiring mains voltage. I don't mess around with it unless the circuit is OFF! I've just never interfaced my electronics stuff to my house wiring.

One point about DC power source: I'll probably use a silver box from an old PC to provide the uC voltage and 12V relay supplies. This will be on the same circuit as the power relay for the freezer.

I can provide more clarification if needed. Thanks.

Edit: One person I spoke to had suggested switching both line and neutral, and I'd read it on a forum somewhere as well (can't remember where). I plan to keep the GND connection hard tied to a common ground throughout the system / house.


1 Answer 1


This will drive a relay from a micro-controller pin: -

enter image description here

The relays you have described are fine and putting a contact in live and neutral is fine too. Just make sure you select your final one to switch 10A and check to see it is rated for the voltage and has a good gap coil to contacts (or is specified as being suitably safe).

The transistor in the circuit has to be able to supply the relay coil current so it may need to be rated at 200mA depending which relay you choose. Voltage rating 20V or above. If you are using a "hungry" coil and it needs more than 100mA you might want to reduce the 10k to a 2k2 to make sure the transistor turns on properly.

If the relay doesn't have an in-built diode protection circuit then you'll need to add a diode to the circuit across the relay. It's normally not conducting with the anode at the collector of the transistor. 1N400x type will do.

  • \$\begingroup\$ That makes sense to me, thanks. Turns out I have a monster contactor in my spare parts bin. It is a 3PST that can handle up to 600V at 25A F.L.A. / 100A L.R.A max, and from what I can tell from the marking on the side, a 2HP single phase motor at 120V. The coil is driven with 120V AC, and I think I have a spare relay that can be connected from the uC as you've shown to drive the contactor. Would I need a snubber circuit on the driver relay (in addition to diode), or any sort of protection on the AC coil of the contactor? Thanks again for your help. \$\endgroup\$
    – MJD
    Apr 13, 2013 at 13:32

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