Capacitor Bleed Off

Discussion in 'General Electronics Chat' started by craigfoo, May 25, 2013.

  1. craigfoo

    Thread Starter New Member

    May 16, 2013
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    I have a circuit where I need to bleed off the capacitor as soon as the circuit loses power but I can't just put a resistor across the cap because it turns my circuit into a voltage divider where I don't want it to. I'm controlling the charge time. Any thoughts on how I can bleed off the cap? See attached circuit.
     
  2. craigfoo

    Thread Starter New Member

    May 16, 2013
    22
    0
    Last edited: May 25, 2013
  3. JohnInTX

    Moderator

    Jun 26, 2012
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    1,028
    You might need an analog switch with a higher voltage rating..

    You could also use a PFET across the cap with the gate connected to the applied power and a resistor from gate to ground (it can be big). When power is applied, the PFET will be biased off. With no power, the gate resistor will pull the gate low and turn on the PFET. Check the max current and use a limiting resistor in the discharge line if the ON resistance is not big enough to keep from over-current in the PFET.

    Take a look at a Fairchild MMBFJ177. I've used it in just such a situation. The attached circuit is an excerpt but shows the general idea.

    Have fun.
     
    Last edited: May 25, 2013
    PackratKing likes this.
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    5,448
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    Does it really matter if you do put a resistor in parallel? Presumably you could redesign the circuit by changing R4 having regard to the required charging time(s) and the added parallel resistor. The op-amp will switch at around 6V on the POS terminal. So the caps only need to charge to something slightly greater than 6V. With a bit of thought you could obtain the desired result without resorting to a special discharge circuit.
     
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