Lab Power Supply - The Lost Current

[xavier60]

I did some testing with my bench supply.
Connecting a 12v battery to the output with the correct polarity and the bench supply powered off did not cause any problem.
Connecting a 12v battery to the output with reverse polarity and the bench supply powered on caused it to current limit.
I had the CC set to 1 amp. Higher current settings would cause a much higher dissipation in the MOSFET because of the added voltage across it.
When I made the last tidied up PCB, I made provision for a BC548 plus some resistors and diodes that sense reverse polarity and clamp Gate drive. I have now added theses parts. So  now when more than 2v reverse voltage is applied to the output, the MOSFET is turned off.
I guess eventually the output capacitor will explode. I could replace it with ceramics but the capacitance varies too much with changing voltage.

[radoczi94]

Hm, interesting. I don't know what will happen to the series pass element if it is a transistor. What will hapen to the circuit if it is on, and set to a voltage lower than the battery voltage? I guess, the CV opamp pulls down the gate, the mosfet closes and nothing happens.

[xavier60]

Hm, interesting. I don't know what will happen to the series pass element if it is a transistor. What will hapen to the circuit if it is on, and set to a voltage lower than the battery voltage? I guess, the CV opamp pulls down the gate, the mosfet closes and nothing happens.

The CV op-amp will turn off the TIP142's. There should be no problem.
Btw, I don't understand the terms "open" and "close" with respect to transistor conduction states.

[radoczi94]

Btw, I don't understand the terms "open" and "close" with respect to transistor conduction states.

Sorry, the cause is probably the lack of my english knowledge. In hungarian, we open (turning on, forward biasing) and close (turning off, reverse biasing) a transistor or a diode. Thanks for the correction, I did not knew about this, will not use again.