Electronics > Projects, Designs, and Technical Stuff
Lab Power Supply - The Lost Current
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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:

--- Quote from: radoczi94 on October 30, 2018, 06:12:35 pm ---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.

--- End quote ---
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:

--- Quote from: xavier60 on October 30, 2018, 09:02:14 pm ---Btw, I don't understand the terms "open" and "close" with respect to transistor conduction states.

--- End quote ---
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.
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