How to discharge the capacitor?

Discussion in 'General Electronics Chat' started by samy555, Oct 21, 2015.

  1. samy555

    Thread Starter Active Member

    May 24, 2010
    116
    3
    Hello
    From: http://www.talkingelectronics.com/projects/TheTransistorAmplifier/TheTransistorAmplifier-P2.html

    I've built the following circuit:

    [​IMG]


    In fact I used a 200K ohm resistor instead of 100K and a 1000uF capacitor and I had a delay time of about 3.5 minutes.

    The problem is when the main power off the 1000uF capacitor maintained its charge for long time and if the main power returned the circuit is ON immediately without any delay. How to discharge the capacitor?

    Thank you very much
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,287
    1,252
    You could try a schottky diode across the 100K resistor with the anode to the + of the cap.
     
    Last edited: Oct 21, 2015
    samy555 likes this.
  3. samy555

    Thread Starter Active Member

    May 24, 2010
    116
    3
    Yes
    excellent
    I try it and the cap. dischaged very fast
    thank you very very much
     
  4. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    This should work.
    Fig71j.png
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Even better; ditch the Darlington and replace it with a regular NFET (not logic level); That makes it possible to use a much smaller timing capacitor, and a much higher value for the timing resistor (now 100K). You can eliminate the 330K and the other resistor tied to the base of the Darlington, too.
     
  6. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    783
    Just put a diode in parallel with the 100k resistor - pointing upwards so its reverse biased when the capacitor is charging. When you switch off and the supply rail falls, any voltage left on the capacitor forward biases the diode and discharges through the other circuit elements.
     
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