Since the advent of the many microcontroller development boards, like Arduino, there have been a number of relay modules sold to drive mains AC loads.
A lot of these seem to use an optocoupler, driver transistor and a relay to drive the load (example on Amazon)
Why are they implemented like this?
Some of my thoughts:
- Relays provide as good or better isolation than most optocouplers
- There is still a driver transistor present, so it is not component saving
- There is still inductive kickback protection, so it is not component saving
- Optocouplers are not as cheap as transistors, so additional cost compared to just a driver transistor
- There is no need to meet any regulatory requirements as these are DIY products
- I have never seen small mains relays driven by optocouplers in commercial equipment
- A number of these boards don't seem to be designed brilliantly (no regard to clearance or creepage), so even if the optocoupler is simply to provide two layers of isolation, the board fails at this.