Is the following procedure safe/ok?

  1. MCU turns on the optocoupler (MOC3063, datasheet)
  2. Optocoupler output turns on a triac (BT139-600, rated 16A, 600V AC)
  3. Triac supplies 220V AC voltage to the bobbin of a contactor (contactor ratings: 25A, single phase, 240V AC max for contact, 220V AC bobbin)
  4. The contactor switches a submersible well water pump. (Rated 1.5HP (=1500W), 230V AC, 5.2A)

I've read here and there that,

  1. Do I need a diac for triac? Don't know where and how to use one.
  2. Do I need a snubber circuit to protect the triac from the inductive kickback of contactor bobbin?
  3. Are these ratings (the motor, the triac, and the optocoupler) matched and ok?

P.S. I'd be very happy to get rid of the triac, as I just read about them this week, and have no experience working with them. Any suggestions to serve this purpose (switching such motor through contactor using 5V mcu signal) is welcomed.

Also I can't use Solid State Relays (SSR).


  • 1
    \$\begingroup\$ You’ll need dc for 1)2) so use a relay \$\endgroup\$ Jun 25, 2021 at 19:18
  • 1
    \$\begingroup\$ 1. no, those are most helpful for dimming. 2. yes. 3. yes. an AC SSR is a triac. You could use a 5v DC relay instead of the triac. \$\endgroup\$
    – dandavis
    Jun 25, 2021 at 20:02

1 Answer 1


Relay boards with opto inputs are online (cheap) 4ch will replace everything between uC and contractor coils. An RC snubber will extend the life if you want >>50k cycles.

  • \$\begingroup\$ "RC snubber extends the life". of relays? \$\endgroup\$ Jun 25, 2021 at 20:42
  • \$\begingroup\$ Yes it bypasses some of the arc current on flyback, sparing some wear \$\endgroup\$ Jun 25, 2021 at 21:30

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