Discharging Cap

Discussion in 'General Electronics Chat' started by bug13, Sep 24, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    I came up with the following design, the idea is, to minimum current consumption when the power is on, and discharge the cap (C1) seasonal fast when the power is off.

    The cap will be discharge to about ~1V in 0.5s, and continue to be discharge by the 100K(R3) to 0V over time.

    The R4 is the internal resistance of two CR2032. the M1 is just a random PMOSFET I picked from LTSpice.

    The simulation in LTSpice looks fine, but is there anything else I need to worry about in practice?

    [​IMG]
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
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    What are you trying to accomplish here?
     
  3. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    • Discharge C1 seasonally fast (0.5s) when V1 is off (V1=0V)
    • when V1 is on (V1=6V), keep C1 charged to V1-Vd, and keep the discharge circuit using minimum current.
    It's part of a rocket ignite circuit, the aim is discharge C1, just in case someone remove the power without turning off the circuit properly. (or a broken wire to the power supply)
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Just a couple of observations. You picked a logic level FET so it stays on to about 1.2 volts in the simulation. In real life it may shut off a bit higher.
     
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  5. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Do you mean if I don't use a logic level mosfet, it may shut off a bit higher, or do you mean I may shut off a bit higher even with a logic mosfet?
     
  6. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Both. If you look at the data sheet you can see the variation of gate threshold voltage from part to part which is the voltage where it is just barely turns on.
     
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