I'm doing a project to replace my thermostat with a DIY one.
I'm trying to figure out how to switch the heating/cooling/fan/humidity wires. I'm inclined to go with an optocoupler like the 4N35. It can switch 24VAC with ease, but has a low threshold for amps (150mA).
Most older thermostats that I know, give the familiar clicking sound when heat is called, implying that they use a mechanical relay. But when looking to more modern thermostats, like the Nest or Ecobee, there is no clicking, either using an SSR or an OC.
The problem is that I can't find anywhere any "specs" of what a thermostat should be able to switch. As the wiring is only 24-28 gauge wire it should never use more than 225mA – 580mA.
Is my assumption correct? Should I opt for using relays or would a simple optocoupler do?
Best Answer
The 4N35 CANNOT switch 24 VAC. It is a transistor output rated at a maximum of 70 VDC and 50 mA.
Providing your switch circuit can tolerate the voltage drop you could use a bridge rectifier and then a 4N35.
However it you could use an MOC3020 or similar. This optocoupled Triac could be directly connected to the switch lines.