I have a question about controlling relays by using optocoupler. I have some optocouplers to isolate MCU's power and Power supply for the relays (to control LEDs, DC and AC drivers).

1) Can I control the relay according to the schematic below but without FET? Do I have to use FET. enter image description here

2) I have a 24V DC and 220V AC to control relays, can I use the same schematic for AC connections?

Thank you so much.

  • \$\begingroup\$ You want to replace the FET with optocouplers? Check the current the relay requieres to switch (look into the datasheet). Then check the current your optocoupler can handle (also, look into the datasheet). I'm pretty sure, that your optocoupler won't be capable of handling that current, so you propably will still need a FET. But unless you tell us which relay and which optocoupler you want to use, there is no way to tell for sure. \$\endgroup\$
    – jusaca
    Commented Jul 25, 2019 at 7:55
  • \$\begingroup\$ 1) this is the relay, ozdisan.com/elektromekanik-komponentler/role-role-soketleri/… 2) this is the optocoupler, ozdisan.com/optolar-sensorler/optolar/… \$\endgroup\$ Commented Jul 25, 2019 at 7:59
  • \$\begingroup\$ I dont want to replace the FET with opto. I want to learn if I can handle the control of the relays by this optocoupler. \$\endgroup\$ Commented Jul 25, 2019 at 8:01

1 Answer 1

  1. You can replace the FET with the optocoupler if it's maximum rated collector current is higher than the current drawn by the relay's coil and if it's collector-emitter breakdown voltage is higher than the voltage you are using to power the coil. But remember to keep the MCU ground and relay ground separate if you use optocouplers otherwise it defeats the purpose of the isolation.

  2. You cannot use 220V AC to power a dc coil relay. If you want to use AC you can use a triac instead of a relay and a triac-output optocoupler, but then the load has to be AC too, otherwise the triac will stay "on".

NOTE: If you use a relay, remember to add a flyback/free-wheeling diode reverse biased toward the 24V source to avoid inductive voltage spikes that will fry the opto.

  • \$\begingroup\$ Max rated current is higher than current drawn, and Vce breakdown is higher than coil powering voltage. So should I replace optocouplers, or add FET's ? \$\endgroup\$ Commented Jul 25, 2019 at 8:13
  • \$\begingroup\$ I see the relay currents are pretty close to the opto's max of 50mA. Rather be safe and add the FET after the opto like this -> link. Remember to add the diode and keep the grounds separate. \$\endgroup\$
    – BdT141
    Commented Jul 25, 2019 at 8:24
  • \$\begingroup\$ By saying separate, do you mean having different ground planes on PCB? \$\endgroup\$ Commented Jul 25, 2019 at 8:29
  • \$\begingroup\$ Not necessarily a plane for each, but the two grounds must not be connected to each other. \$\endgroup\$
    – BdT141
    Commented Jul 25, 2019 at 8:29

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