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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?

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  • \$\begingroup\$ Before you even warm up the soldering iron, I suggest you first determine the load requirement presented by the switching element. It could be a contactor in your condensing unit that pulls a couple amps. Trying to judge this by the wire size is not good design practice. Measure it or look in the service panel for the diagram. \$\endgroup\$ – AlmostDone May 22 '18 at 1:17
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The 4N35 CANNOT switch 24 VAC. It is a transistor output rated at a maximum of 70 VDC and 50 mA.
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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.

enter image description here

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  • \$\begingroup\$ You are right, I had the MOC3020 on my shortlist. I'm new into reading the datasheets of Optocouplers and especially the differences between all the MOC types is a bit confusing. I probably would go for something that has at least zero-crossing circuitry. \$\endgroup\$ – Roger Far May 22 '18 at 1:28
  • \$\begingroup\$ Zero crossing provides no advantage for this application. \$\endgroup\$ – Jack Creasey May 22 '18 at 1:45
  • \$\begingroup\$ Can you elaborate? I assumed that zero crossing was needed for any AC switching just that you would have to do it yourself by detecting zero crossing and manual pulsing. \$\endgroup\$ – Roger Far May 22 '18 at 1:57
  • \$\begingroup\$ Since you are turning on relays in the heater/aircon you don't need to sync with the mains. You will be turning the unit on for a long time (many minutes at least). \$\endgroup\$ – Jack Creasey May 22 '18 at 2:39
  • \$\begingroup\$ Makes sense, I read a bit on it. The only thing I can't find much about is the Input trigger current. If I take the MOC3021 it's typ 8mA. Does this mean it will draw mA to stay latched? \$\endgroup\$ – Roger Far May 22 '18 at 3:47
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Typical maximum power for a household thermostat system is 15 VA at 24 Vac.

You must use a device that can switch AC current. It should be sufficiently robust to withstand a short circuit at the HVAC equipment end.

2A photo-MOS should work well. Personally, I'd be using JS-1 relays but that's mostly because I'm old-school.

You can use Triacs but driving them reliably is non-trivial.

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