Sizing transformer for Nest Thermostat and relay

[AMacD]

I tried to create a circuit that would provide a C wire to Nest Thermostat and fire a relay. Thermostat works but relay wouldn't. The transformer I used is 24 Volt 500mA and the relay is a 24V Dpdt YJ2N-LY by TWTADE. I tried many different wiring configurations but the relay won't fire. If I hook the transformer directly to the relay it fires but once I wire the Nest into it it won't. My setup is essentially this but for heating only so I use the W wire. I also tried both Y & W wires and no go. Also tried reversing the VAC feed which shouldn't matter unless there was some find of diode inside the wall wort transformer but still nothing.

Is there specific minimum transformer amperage needed to power both the NEST and fire the relay?

[Markjosephp]

Hello AMacD,

It sounds like you've put in a lot of effort into setting up your Nest Thermostat and relay circuit. The fact that the relay fires when connected directly to the transformer but not when wired through the Nest Thermostat suggests there might be an issue with the power supply or wiring configuration. Troubleshooting can be a bit tricky, but let's see if we can figure this out.

• Ensure that your Nest Thermostat is properly connected. Double-check the wiring to make sure all connections are secure and correctly placed.
• The 24V transformer should be sufficient for both the Nest Thermostat and the relay. However, ensure that the voltage doesn't drop significantly when the relay is connected. Measure the voltage at various points in your circuit to identify any drops.
• While your transformer is 500mA, the current draw depends on the Nest Thermostat and the relay. Check the current specifications for both devices and ensure that your transformer can provide enough current.
• Some relays might have specific requirements. Check the relay's datasheet for any special conditions or limitations. It's also worth trying a different relay to rule out any potential issues with the specific relay model you're using.
• Ensure that your relay is compatible with your heating system. Some systems may have specific requirements or need additional components to work with a smart thermostat.
• Verify that the Nest Thermostat is not causing any short circuits or unintentional feedback in your circuit. The thermostat may have protective features that affect the behavior of the relay.

Let us know if you have more questions.

Best regards,
Mark

[AMacD]

I had used 1/2 of the 25ft wire that came with transformer to also wire back to the relay. I think it was 22 gauge and I changed it out to heavier gage solid wire and it works. Could the tiny wire have been throttling the voltage or did I have a bad connection I don't know but it works now. Thank you Markjosephp for your troubleshooting tips.

[Markjosephp]

Hey AMacD,

Thanks for the update! I'm glad to hear that you got things sorted out with your transformer and relay. Switching to a thicker solid wire was a good call, as it likely helped with reducing resistance and improving voltage flow. Those smaller-gauge wires can indeed cause voltage drops, especially over longer distances.

It's plausible that the initial 22-gauge wire was limiting the current flow, leading to a voltage drop and affecting overall performance. Also, poor connections might have played a role in the voltage issues. By addressing both the wire gauge and ensuring solid connections, it sounds like you've tackled potential issues on both fronts.

If you've got more questions or need further assistance, let me know.

Warm regards,
Mark