How can I provide a C wire to my thermostat from the transformer?

Question

I have a Rheem handler with a 46-23115-02 transformer.  I do not have a C-wire connection that I can see.
I have a 5 wire cable, so I do have a blue wire available that I can connect as the common wire to operate the Honeywell wireless thermostat.
The transformer in the air handler is:

24 V red connected to the red wire on the thermostat
24 v brown wire that connects to the condenser unit
other brown wires that go to a 24 v relay or a secondary transformer
green wire which is the fan is connected to this secondary transformer or relay... not sure..

Should I connect the C common wire from the thermostat to this brown 24 V wire?
Or should I connect the C common wire from the thermostat to this orange comm wire?
the transformer also on the label says...
black wire 240 v
blue wire 208 v
orange-comm is this the common terminal wire?
Thanks for your help and advise.

Michael Karas · Answer

Do not connect the thermostat common wire to the transformer primary. The C for the thermostat needs to connect to the transformer secondary, which from your description sounds like the red and brown wires. Since the red of the transformer secondary is already connected to the red going to the thermostat then the C terminal for the thermostat has to connect to the opposite side of the transformer secondary. This would be the brown wire according to your description. It is fully expected that the brown wire already connects to some other things also powered by that 24VAC transformer secondary.

The COM terminal (orange wire) you saw related to the transformer primary is connected to one side of the AC mains voltage. The other side of the AC mains voltage attaches to the transformer primary through either of the black wire or blue wire depending upon whether the AC mains is supplying a voltage of 240VAC or 208VAC. This is needed so that the transformer can produce the proper 24VAC on its secondary based upon which mains AC voltage is available.

How can I provide a C wire to my thermostat from the transformer?

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