Twice a rat has gotten into the garage and chewed the control wires that go to the outside compressor, destroying a transformer. I’m thinking that if I put in line fuses on the wires, the rat would blow a fuse. I could repair the wires myself, rather than paying the A/C guy to replace the leads and the transformer. These are 24v, not the 220v that run the compressor.
I don’t know what current the 24v wires from the control circuits to the compressor carry. I guess I could start at half an amp and increase the size until fuses do not blow with normal operation.
Do you have a DMM?
Measure the AC current when the compressor is on, that should be the maximum draw.
I’m surprised that blew the transformer. I was trying to use my thermostat line (24 VAC) to drive a relay, only to find that the relay coil took too much current. I thought I blew the transformer, but it turned out to have a thermal fuse built in that reset itself after several minutes.
Maybe yours doesn’t have this, but transformers with built-in thermal fuses seem to be pretty common.
Yes, good idea.
One would think so. If the rodent crushed the two wires together, it would not be able to reset.
Does anyone have a link to which wires do what in the link to the compressor? I’ve only been able to find vague diagrams on Google.
My compressor unit actually had a nice schematic inside. Maybe you can find one for your model online. But most likely, the wires drive a relay that actually controls the compressor motor.
If the relay did have a (resettable) thermal fuse and the rodent crushed the wires together, then it would trigger the fuse, with the residual current keeping the fuse from resetting. Only when you cut the supply or opened the short would it reset (after some minutes).
I’ve bought two transformers…a guy working on another electrical problem replaced the transformer, and the replacement failed.
I was hoping that an HVAC guy would post. I’ll start at 1A and see what happens. Relays surely don’t draw much current. Winter is a good time to mess with the A/C.
I’ll get the model number and google it. Good idea, thanks!
Big relays can draw >1A, at least initially. Remember that it’s only 24 VAC. Here’s the model I initially tried to use for my project:
Note the coil resistance for the 24VAC coil model: only 12 ohms. So a coil current of 2 A. More than my transformer could take, at least combined with the load it was already carrying. (the PDF also says the power consumption is 10 VA, but I assume that’s in the steady state)
But yeah, fuses are cheap and you can work your way up if the 1 A one blows.
The square root of two comes in there somewhere. I’ll begin at 2A. I’m just protecting against a short, so I can over rate the fuses.
Or something even more complicated 
. Presumably there’s some reactive power here… complex impedance… phasors. It’s been a while since I’ve had to calculate this stuff.
I’m thinking the resistance value listed in the datasheet is simply the DC resistance of the coil. If this is the case, then the magnitude of the impedance of the coil at 60 Hz will be significantly higher. (The magnitude of the impedance of the coil is √(R² + ω²L²), where L is the inductance of the coil and R is the resistance at 60 Hz. Also note that resistance of the coil at 60 Hz will be higher than the resistance value at DC due to the skin effect.) So I would use the apparent power number (VA) for calculating current, not the DC resistance of the coil. At 24 VAC the steady state current would be around 0.42 Aᵣₘₛ. I don’t know what the short circuit current would be. If it’s well over 1 A, then I would think a 0.75 A or 1 A fuse would suffice. May want to make it a slow-blow due to the inrush current.
Of course, this is for yours. Not sure about @carnivorousplant.
If I remember right from 55 years ago many HVAC units come stock with a 20 VA transformer. at 24 volts that would be 1.2 amps. If I replaced the transformer, I always used a 40VA transformer. It cost about 10 cents more but would have a much longer life. So 1 amp should do.
I would consider replacing the wire with a metal clad control wire.
Here’s what the schematic from my A/C unit shows:
So yeah, 20 VA minimum, which means they probably put a 20 VA unit on it.
I ended up building a 2-stage relay system instead. The small relay drives the coils on the large one. Maybe I should replace the transformer too, though.
The only part number I can find is MC43B3XC1A, but that is the evaporator coil.
The A/C furnace is made by York. Will there be part numbers inside?
Thanks, everyone.
the transformer normally is outside the unit on older heaters. It is just a 120/24 volt transformer. the 40VA will cost pennies more but will last years longer. Now some of the newer ones have the transformer built into the PC board and you would have to change out the whole board to get a new transformer.