Capacitor discharge current and time.

Discussion in 'General Electronics Chat' started by authorjim, Sep 30, 2014.

  1. authorjim

    Thread Starter New Member

    Jul 15, 2013
    3
    0
    First off I'm not an electronics guy so here goes.

    I was trying to calculate the discharge time for a capacitor and found the following
    CV=IT so if I have a 5f capacitor and its charged to 10 volts.

    So the discharge time for 250 ma draw is 200 seconds. Right?

    5x10/.25=200 seconds

    Here is where I get confused unless I messed up the first part.

    If I have 4 capacitors. All 5 volt all 5 farad caps and i put them in a series parallel arrangement
    Two together in series which gives me 10 volt 2.5 farad equivalent. Then I take the other two and do the same.
    Then I put both these series loops in parallel I get 5 farad and 10 volts. right?

    So these 4 capacitors together only give me the same 250 ma current draw for 200 seconds?
    I know this is a stupid question for you guys, but it does not seem right.

    Thanks
    Jim
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    The difference is that the 4 caps are charged to only 5V so each cap contains less energy then the single cap charged to 10V. In either case you have 1F of capacitance charged to 10V, so why would you expect a difference?
     
  3. authorjim

    Thread Starter New Member

    Jul 15, 2013
    3
    0
    As I said, I'm a newbie and I expected that the four caps would provide more time at the same discharge rate. After thinking about it what I forgot is that the 5 farad 10 volt cap will be alot bigger(i think then the 5 volt 5 farad caps so that makes sense.
    What I don't understand your last comment that in either case I have 1 farad charged to 10 volts. Why 1 farad?

    Thanks in advance
    Jim
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,451
    3,370
    Just a typo. He meant 5F at 10V.
     
  5. MrCarlos

    Active Member

    Jan 2, 2010
    400
    134
    Hello authorjim

    Since you're a newbie must learn something.
    These Are laws:
    If you connect two capacitors in parallel, the total capacitance is the result of adding the value of those two capacitances.
    If you connect two capacitors in series, the total capacitance is the result of multiplying the value of the first capacitance by the value of the second capacitance divided by the sum of both capacitances.

    Easier way:
    (A) Parallel: RT = C1 + C2
    (B) Series: RT = (C1 x C2) / (C1 + C2).

    Now:
    If you have two 5 F. capacitors charged to 5 volts and plug them in series they'll have at the ends of the series 10 Volts. but the capacitance is exactly half because the formula (B).
    But you have another circuit as here above, you will have a 5 F. capacitor @ 10 volts, again, because the formula (A).
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Yup, you beat me to it.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    In practical, whatever you want to charging +5V or +10V, you may always need to in series with two or four caps, as two 1F/2.7V in series to 5.4V for charging +5V or four 1F/2.7V in series to 10.8V for charging +10V.

    If you need to charge 5F/10V then you will need 20 caps.
     
  8. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,132
    267
    Does one need to worry about charge balancing with series connected super caps?
     
  9. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    If you care about that then you can in parallel with resistors or zener diodes for each cap to do the job.

    For a higher voltage as over 20V or more current then what your concerned will make sense and should be care about that.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    The attached picture was what I tested for two 1F/2.7V supercaps in series and charged for 5V about 3 seconds, it can be keeping brightness about 30 seconds for ten 3V/20mA leds, and 5 minutes for one 3V/20mA led.

    TwoCpas-1F2p7V-10-LED20mA.jpg
     
    Last edited: Oct 1, 2014
  11. authorjim

    Thread Starter New Member

    Jul 15, 2013
    3
    0
    Thanks to all, I did know the capacitance rules that is why I said I had two 5f 5 volt caps in series for 10 volts and 2.5 f
    and two more the same way for 10 volts and 2.5 farads. THen I put bought of those loops in parallel for 10 volts and 5 farads.

    Thanks for all the help.

    Jim
     
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