Can someone help me understand how this contactor works?

A1 (Yellow) Always has 120v and is connected to a high limit switch. A2 (black) Always has 120V and is connected to a line voltage built-in thermostat.

Both sides of the thermostat also read 120v at all times and doesnt change with the contactor closes and the space heater is running.

The root problem I'm trying to solve is how do i get a low voltage thermostat to control this 31a 240v heater?

240v 3 pole contactor

Space heater diagram


Not with that contactor

The contactor in your illustration is internal to the heater. It is a 240V contactor and should never be connected to a 24V thermostat.

You are seeing 120V because your electronics training is in automotive or hobbyist DC electronics and you are stuck in the notion of a GND/common from which all voltages are measured. You have permanently clipped your black voltmeter lead to the machine's chassis. If you want that to work with mains power, try moving to the UK. Here, everything is relative, there is no common, and you must measure between opposing terminals and leave safety ground out of it.

That contactor coil is 240V.

Switch the T-STAT line instead.

The T-STAT circuit is a pilot signal for the contactor whoch carries 240V but at very little current (limited by the contactor coil). We can use that. Note that it goes through a couple of safety interlocks, those should not be defeated.

I would put a much smaller contactor (i.e. Relay) to replace (or in series or parallel with) the "T-STAT" switch shown on the diagram. That would be a relay with a 24V coil, 240V contacts and insulated for 240V.

Get a 24V supply transformer (they are $12 and attach to any junction box). The transformer supplies R and C. Run R, C and W to the thermostat, run C and W to the coil of the aforementioned relay, and you should be all good.

If you put this relay in series with the factory Thermostat, the heater will fire only when both thermostats call for heat, so set the factory 'stat to 90F setting so it is always calling for heat.

If you put this relay in parallel with the factory thermostat, the heater will fire when either thermostat asks, this can be a great way to make the thermostats be backups for each other. Set the latter to 40F (or whatever you consider your don't-freeze-pipes minimums) so it is rarely calling for heat.

  • thank you. I have this, amzn.to/2NO7cMK which was previously installed in the first heater I purchased. Subsequently, that unit failed and was replaced and I'm questioning if the installation of the Aube caused the failure. This is a question i previously posted related to my OP. diy.stackexchange.com/questions/148527/… Oct 18 '18 at 13:22
  • Probably because your last setup interrupted the power through the contactor. I looked at that, but it's the wrong way to do it because it will defeat some of the thermal safety interlocks in the heater. I suspect that explains the overheat. Do it as I suggest, have it interact with the T'stat instead of going anywhere near the relay. Oct 18 '18 at 13:32
  • @Harper -- I think he was switching the T-Stat line from the looks of how he had the Aube installed -- the safety interlock shouldn't have been defeated by that. Oct 21 '18 at 5:37
  • @ThreePhaseEel Yeah, OK, I see what he did with the Aube, making the best compromise out of a very limited option for wiring that. it technically should work, but it isn't. "doesn't" beats "should" every time. Probably a big part of it is the Aube transformer is only sized to power itself, so he's running it way out of spec to power itself and the Nest. I would be much happier if OP went with a common 40VA transformer ($12) with loads of power for the Nest, then any electronics-supply 24V relay to shunt the internal T-Stat. Oct 21 '18 at 6:11
  • @Harper, can you help me understand why the Aube RC840T-240 On/Off Switching Electric Heating Relay with Built-in 24 V Transformer isnt working in my case? Reading through your suggestions, it seems this should accomplish the requirements you specify. Also, I'm a bit confused when you indicate either to wire a relay in series or parallel!! Oct 23 '18 at 13:55

Contactors work by using a lower voltage coil (normally lower voltage or the same voltage but very small current) to pull in a set of contacts (switch) that connects the much larger load. For example many control systems use 12v or 24v at a few watts to pull in contractors that are controlling much larger loads in residential think of 240v at 50 amps being controlled by 24v at 5 watts. In your case your high limit is closed thus 120 v on both sides of the limit switch, if the switch opens the current path is opened and the contactor cannot pull in. The 120v control voltage in this case comes from low current controlls to operate the much larger load.

  • great explanation and for the most part, this was my understanding going into the question. So the thermostat is a SPST that is normally open right? And as I understand it when the stat closes calling for heat it connects both L1 and L2 through the A1 and A2 terminals. Sorry for the newbie questions, I know just enough to be dangerous. Oct 17 '18 at 15:21
  • Yes a1-a2 is the control voltage to energise the contactor (a contactor is just a big relay) when it pulls in it connects l1 & l2 to the load.
    – Ed Beal
    Oct 17 '18 at 16:30

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