Fast discharge circuit

Discussion in 'General Electronics Chat' started by cryosar, Dec 15, 2009.

  1. cryosar

    Thread Starter Active Member

    Apr 2, 2009
    40
    0
    Hi,
    I have attached a application note from ST Microelectronics. I have a doubt in this circuit. am not able to understand the output voltage fast discharge circuit. can anybody explain me this?

    Thanks in advance
     
  2. JDT

    Well-Known Member

    Feb 12, 2009
    658
    85
    If the D17 is not conducting (less than 17V across it). then Q10 does not switch on. If Q10 is not on, then Q11 is switched on - base current supplied by R77. The resistor R78 (560R) is connected across the capacitors, discharging them fairly quickly.

    I notice that on a lot of laptop power supplies, the power LED stays on for a long time after the power is disconnected due to energy storage in the capacitors. This circuit is designed to reduce this.

    BTW There is a typo in the text: "D17, R75+R78, Q10 and Q15" should read "D17, R75+R78, Q10 and Q11"
     
  3. cryosar

    Thread Starter Active Member

    Apr 2, 2009
    40
    0
    Thank you very much
     
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