Burning Up Control Transformers

Old 06-12-18, 06:11 PM
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Burning Up Control Transformers

Hi all,

So my wife calls me while Iím at work to tell me that the air con has shut down and the thermostat screen has powered off. Which is perfect since we live in South Florida and my mother-in-law is visiting from out of state. So I head home.

I trace the issue to a failed 220-24VAC 40VA control transformer which I replace. My wife is all proud of me as the thermostat lights up and starts to boot. But then it powers off and the control transformer starts to smoke.

Itís now 8:30pm and I canít buy a new control transformer without paying a $100 surcharge. I ended up going to Home Depot and buying a 120-24VAC 20VA doorbell transformer. I then went through the system and attempted to locate the fault that was causing the transformer to burn out.

I tested all the control wiring, ohming out each wire to the others and to ground and found all the wires to be in good condition. I ohmed the compressor contactor and got 20 ohms. Then I hooked the doorbell transformer up to each individual portion of the circuit along with an amp probe. I tested the compressor contactor, that was good. I tested the fan relay next, and that was good. Finally, I tested the thermostat and that appeared to be good.

All Iím after is air conditioning so I left off the heater control wire and anything that was connected to the heater I disconnected.

I turned on the unit and everything has been great ever since. But nothing has been fixed because I know that if I reconnect the heater side Iím going to fry that control transformer.

So my question is: where is the fault? Is it in the thermostat or in the heater?

Evaporator: RBHC-21J11SFG with electric heat

Condenser: RAMC-042JAZ

Thermostat: First Alert TERM-500

Thanks in advance,
Old 06-12-18, 06:30 PM
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Welcome to the forums.

When you reinstall the correct 40va transformer...... put a 3A fuse in one of the 24vac transformer leads to protect it in the future.

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