Hmm, if I understand correctly, you mean the fuse is blown when you power the controller by USB? Since the fuse is on the 24V AC line and not on the USB power (5V) line, I can’t think of any obvious reason why this would happen. Did you measure the resistance between VIN-GND, as well as VCC-GND? What are the resistance values you are getting?
OK, that’s good to know. I suggest doing the following:
1. Measure the resistance between VIN-GND again, and the value should be somewhere around 4.5Kohm to 9Kohm. If it falls below 4Kohm, it’s likely that either there is a short somewhere, or a component is burned (most likely the zener diode, or the LM2596 switching regulator).
2. If the resistance is correct, then something on the 24VAC input line is shorted. There are only a few components on the 24VAC line (fuse, diode D1, capacitor C1, LM2596, so it shouldn’t be too hard to narrow it down.
Coming back to this thread: it occurred to me that it could also be that the fuse just got broken due to mechanical reasons. As long as the VIN-GND and VCC-GND resistance values are acceptable, you can replace the fuse with a wire and test again. If you need a replacement fuse, contact [email protected] and we can arrange to send you a replacement fuse.