S: Schematics from the PSU TSZ07(Cipher M995S)

Dorry, I was using the term in a 'logical' rather than 'physical' sense. Computer PSUs always (I think that's true) have the outputs electrically isiolated fro mthe mains input. There is no direct electrical connection between them. Pwoer is transfered from the mains to the outputs by a double-would (separate priamry and secondary windings) transformer, contorl feedback is passed fro mthe outputs to the chopper transistor either by double-wound pulse transoformers or by optoisolators. I regard that electrical 'gap', bridged by the transformers and optoisolators as the 'isolation barrier'. It's often convenient to regard an SMPUS ans being in 2 halves, one on each side of this. The maisn side cotnains the mains rectifier, smoothing capacitors and chopper cirucit. The otuptu side cotnains the putput rectifiers, smoothign capaictors, crowbar, ay aditional regualtors, and the voltage feedback circuit. The chopper drive cirucit could be on either side of the barrier, depeneding on the design of the PSU. My guess is that this is the centre-tapped primary of the chopper transformer and that you have a push-pull chopper circuit (not unusual). What is this resisotr, etc, connected to? If it really is a 4700 ohm one, it must have quite a voltage acorss it to burn it out. Is the FET associated with this resistor one of the 2 you have just mentioned as being connected to the chopper transformer (I susepct these are the choppers)? -tony