I need to detect AC voltage with MCU for ensure relay health condition, the cheapest and smallest solution are most welcome. I have no hard time restrictions or noise canceling\debounce requirements (will processed inside MCU).
Most of cheapest optocouplers (OC) has adequate
CTR only with
If >= 1mA. But with
VAC 230V, the dropping resistor (
Rdr) must have decent power dissipation raiting, size and price.
To use SMD or cheapest THT resistors, in worst case
Vac == 270V, i must have
Prdr < 0.2W. It means, what
Rdr must be not less
510k for long life safety).
So in another wrost case
Vac == 185, i will have
If ~= 0.3mA with pretty low
For example, for TCMT110x what i used in project it would be near 0.2x.
Ok, that looks pretty clear, i just need to use:
- Double diode optocoupler (otherwise the
OCwill breaked by reversed 230VAC voltage).
- Amplifting transistor on the output.
Does my thoughts has mistakes? Does i not consider some effects, what i must consider?