I'm doing some kind of reverse engineering to this very old power supply design. I'm having hard time trying to understand it all. I'm asking for some hints that help me read this schematic. From this circuit, you should know:
1 - The line to line voltage is 480 VAC.
2 - The input to the LM317 is +27 V as marked. Don't understand how +27V is generated in that arrangement.
3- The reference is not 0V for the bridge rectifier. It's -13V coming from the regulator.
4- The output of the LM317 (+13.25V) feeds another control circuit that I'm not showing.
5- When the coil is activated, it shut downs the main power.
What I really want to understand is how the +27V is generated, and how the FET is turned on. My guess is that when the control circuit gives the signal to the SCR, it activates the coil, but don't understand how. You can see this design is strongly dependent on the zener diodes. There are 4 zener, 43V, 10V, 12V, and 13V. How would you improve this circuit?
EDIT: The three diodes in series below the coil are TVS diodes. Each one is 300V, so the total reverse breakdown voltage is 900V.