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Resistor in PSU's input circuit

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garg29

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Resistor in PSU

I have many times seen a resistor on input of LM78xx serires regulator to the ground, (as I have shown in diagram). Sometimes they have put 2k2 resistor sometimes its 10k ohm . Can any of you guys please tell me what's the use of this resistor?

Thanks
 

Re: Resistor in PSU

Usually, it is there to ensure the power goes off quickly if the AC is disconnected. Without it, the DC rail may fall slowly, particularly as the voltage decays to a low level. Slow dropping DC could have bad consequences, particularly for logic circuits. Microprocessors may also need a rapid rise in voltage from zero to initialize properly. The resistor just bleeds some current to ground to make sure the starting point really is zero.

Brian.
 

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