As part of a project I am controlling 24VAC solenoid sprinkler valves like this https://rayshobby.net/wordpress/understanding-24vac-sprinkler-valves/ using a triac (BT1308W) http://www.mouser.com/ds/2/302/BT1308W_SER_D-111990.pdf and optocoupler (MOC3012) https://datasheet.octopart.com/MOC3011-M...-Fairchild-Semiconductor-datasheet-627034.pdf

My design is based on the reference circuit on the optocoupler datasheet for inductive loads:

enter image description here

My current design:

enter image description here

A few questions :

1)I added a TVS (D1) to protect from reverse voltage spike when the solenoid is turned off. Should I also add a snubber or with the TVS is enough? Do the protect from different things?

2)Is it a good idea to add a 10K resistor between triac's gate and VAC2?

3)I'm using the R19 and R20 values from the suggested datasheet circuit but, how are they calculated?

4)C21 is rated at 50v, is that correct?

  • \$\begingroup\$ Remove the TVS and place a RC snubber accross the triac. 2)Is it a good idea to add a 10K resistor between triac's gate and VAC2? NO, why? \$\endgroup\$ Oct 12, 2018 at 11:03
  • \$\begingroup\$ What would the RC snubber circuit look like? \$\endgroup\$
    – ilium007
    Oct 23, 2020 at 12:36

1 Answer 1


you don't have to worry about a spike because the triact turns off when the current is zero. the voltage step when this happena (because current lags voltage) can cause re-triggering however. so use the snubber

  • \$\begingroup\$ The triac can not be damaged by the spike caused by the inductive load? \$\endgroup\$
    – Marc
    Oct 13, 2018 at 8:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.