See attachments Power1 and Power2. I know it's kind of weird, but I don't have a decent schematic capture software right now. I'm doing some reverse engineering and I'm trying to understand the functionality. The two schematics are located in different PC boards that are interconnected. As you can see, for example, the +27V point comes out from Power1 schematic and connects with Power2 schematic where it's feeding a LM317 regulator. I'm not showing the whole circuitry. There's another control circuit which processes a signal coming from a current transformer. Additional information: the input line to line voltage ranges from 220 to 480 VAC. The circuit is used as earth leakage protection. So based on the schematics:
1) How are the +27 and -13 voltages generated? I'm kind of confused as it seems like in Power1 schematic the reference is NOT zero volts. I know there should be a trick with the zener diodes.
2) How is the FET turned on? Is it through the +V point which connects with -13V through the SCR in Power2 schematic?
3) At the drain of the FET, there is a freewheeling diode for the coil and a stack of three TVS diodes in series (not sure why three).
4) At last, I want to understand better the setup at the source of the FET
I'm actually not asking so much, just few hints that help me interpret the circuit in a better way.
What improvements would you make to this circuit?
1) How are the +27 and -13 voltages generated? I'm kind of confused as it seems like in Power1 schematic the reference is NOT zero volts. I know there should be a trick with the zener diodes.
2) How is the FET turned on? Is it through the +V point which connects with -13V through the SCR in Power2 schematic?
3) At the drain of the FET, there is a freewheeling diode for the coil and a stack of three TVS diodes in series (not sure why three).
4) At last, I want to understand better the setup at the source of the FET
I'm actually not asking so much, just few hints that help me interpret the circuit in a better way.
What improvements would you make to this circuit?