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If circuit have power on. Vin ( +20V) always higher than Vout (+12V). Vin was 8V higher o than Vout. That is diode will be reverse biased and no current will flow on diode.
Once power off. Vin voltage will become lower. Vout without load will have +12V. Vout therefore was higher with respect to Vin . Diode will be forward bias and Vou current will not dangerously flow to IC regulator . But flow on diode to Cin.
Once power off. Vin voltage will become lower. Vout without load will have +12V. Vout therefore was higher with respect to Vin . Diode will be forward bias and Vou current will not dangerously flow to IC regulator . But flow on diode to Cin.
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Switch resistor? You mean potentiometer?
Well, a potentiometer is a resistor, but if I wanted 4 fixed voltages I would switch 4 resistors to give 4 voltages. Maybe there might be trimmers in there to trim off the voltage, but essentially I'd be switching fixed resistances.
You mean I have to calculate the value of resistancce needed to give me a fixed voltage ?
If you still want to do three supplies here is how you would emulated a 2P4T switch with 3 DPDT
Switches from top to bottom, left to right...
Switch one when in the up position would be circuit one
Switch one in the bottom position and switch two in the up would be circuit two
Switch one in the bottom position and switch two in the bottom position while switch three is in the up position for circuit three
Switch one in the bottom position and switch two in the bottom position while switch three is in the down position for circuit four
Again fool proof switching only one circuit is on at any given time... Although I would probably go with a variable as has been suggested or even just use a 1P4T on just the outputs as suggested, not a huge waste to have them all on at once...
Switches from top to bottom, left to right...
Switch one when in the up position would be circuit one
Switch one in the bottom position and switch two in the up would be circuit two
Switch one in the bottom position and switch two in the bottom position while switch three is in the up position for circuit three
Switch one in the bottom position and switch two in the bottom position while switch three is in the down position for circuit four
Again fool proof switching only one circuit is on at any given time... Although I would probably go with a variable as has been suggested or even just use a 1P4T on just the outputs as suggested, not a huge waste to have them all on at once...
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If you want, just get a cheap digital multimeter, if you try I'd bet you could get the entire LM317 circuit inside the multimeter housing and have a multimeter looking power supply
the LM317 seems like the best idea. Switching fixed resistors by a Single Pole - multiple throw rotary switch does the trick. If you are lazy calculating the values, just use a potentiometer to adjust a voltage (while reading a DVM), remove and measure the resistance then use a 1% fixed resistor as close to the measured value as possible. Repeat as many times as the number of desired voltages.
