Relay not Triggering on 24 VAC Furnace Circuit

Other Hardware General Electronics
1

I bought a RT424524 relay from mouser and inserted in into my thermostat>furnace 24 VAC line. When I did that, the furnace won't turn on. I am thinking the resistance of the relay is stopping the circuit. But I don't know. I used my multi-meter to bypass the relay and measure the current in the 24 VAC. When I did this, the furnace turned on, but the multi-meter showed 0 AC current after a instantaneous current reading. I don't know why it is reading 0 current.

So I started reading other ways of doing this and an optocoupler keeps coming up. One circuit I saw had a 3K ohm resistor in series with the optocoupler input. I don't know if this will work. If the ~300 ohm resistance of the relay is stopping the circuit, 3K doesn't sound better. I could go without a series resistor, but since I don't know the current, I don't know if I will fry it. If the current really is close to 0, won't the optocoupler stop that too?

Any ideas?

I appreciate any help.

2

I guess I must ask where are you putting the relay and what is your goal.

3

I inserted the coil of the relay into the thermostat line about 3 ft from the furnace. When I say I inserted it, I mean I cut the line and soldered the two ends of the wire to the coil pins. The contacts are connected to an arduino uno. I have the uno hooked up to two i2c temperature sensors, an i2c RTC and SD logger. I am trying to be able to read when the furnace is one. Everything is working fine (software too) except this relay.

4

Improper implementation.
The coil should be in parallel with the furnace circuit, not in series with it as you have done.

5

Thank you! That worked. I had to inverse my signal after I did that.