We don’t have a C wire, and the 4 wires coming into our furnace/ac unit from the current thermostat are each wire nutted to another wire before going to the control board. Too complicated! Has anyone used a 24v wall plug power adapter/transformer?
Did it work?
according to @speadie if u use this external 24v transformer, your a/c will not work. It’s only suitable for heat only systems. If u post pix of your current wiring at your tstat & the wire nuts, we can help u wire the C wire adapter. It’s really not too difficult.
see his explanation here:
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Top pic is the thermostat, next are the wires from thermostat wire nutted with other wires in the furnace a/c unit, last is the combined wires attached to the control board. Our confusion is the instructions for the adapter doesn’t include what to do with combined wires as in 2nd pic.
that 2nd pic is not very good. can u straighten the wires & post a better pic? All I could make out is the blue Y wire connecting to the yellow wire and the red wire. The wiring at the tstat is pretty straightforward. At the wire nuts you want to find the 4 wires from the thermostat & connect those wires to the C wire adapter. Make sure to label those wires on the furnace side 1st so when you wire nut the wires from the adapter you’ll know which wires connect. The adapter does need to connect to the C wire, so you need to find the 24v transformer. The easiest way is to trace the red wire from the tstat, that wire should connect to the power side of the transformer, then the other wire connected to the transformer is the C wire. Trace that wire, hopefully it goes to a wire nut where you can add the C wire from the adapter. good luck
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You need a wire with an insulated female spade connector on one end to connect to here 
That wire will be your C wire.
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Thank you! We got scared when we saw the thermostat wires combined to other wires with nuts, and the control board had those combined wires attached to the control board with spades to xfmr-r and xfmr-c. We used the adaptor by taking the thermostat wires out of the nuts and putting in the adaptor, and the adaptor wires got nutted in there place. Added a spade to the adaptor C wire and plugged it into control board. It works!!! So happy! 
Thank you! We got scared when we saw the thermostat wires combined to other wires with nuts, and the control board had those combined wires attached to the control board with spades to xfmr-r and xfmr-c. We used the adaptor by taking the thermostat wires out of the nuts and putting in the adaptor, and the adaptor wires got nutted in there place. Added a spade to the adaptor C wire and plugged it into control board. It works!!! So happy!