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Thread: Capacitor time formulas (for noobs)

  1. #1
    Maximum Bitrate KyleYankan's Avatar
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    Capacitor time formulas (for noobs)

    Hi everyone. Let me make it VERY CLEAR that I am not planning on using a capacitor as a tank circuit. I'm only writing this because I'm reading my electronics book, and ran across this. I thought I would point this out for the n00bs.

    Time = Farads * Ohms

    That means, that how many ohms your computer provides, times the farads of the capacitor is how many seconds it will last.

    Volts = Amps * Ohms

    therefore Ohms = Volts / Amps
    therefore Time = Farads * (Volts / Amps)

    Volts is usually 12.

    So, for my 120 watt computer, I can figure out the amps by dividing by the voltage.

    120 watts / 12 volts = 10 amps. I can plug that in my formula no problem.

    Time = Farads * (12 / 10)

    However, I need to know either
    a) how long the capacitor I have will last
    or
    b) how big a capacitor is needed to last x seconds.

    I said, I need a capacitor that can last 10 seconds.

    10 = Farads * (12/10)
    10/(12/10) = Farads
    8.3333 Farads are required for a "normal" car startup. Do you have any 8 farad capacitors laying around?

  2. #2
    FLAC
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    Kyle,

    You may want to go re-read a bit. Take a good look at the definition of "last". The capacitor voltage decays at an exponential rate. You need to define "last" as the point in time at which the voltage drops below the acceptable operating point for whatever is connected to it. The standard "RC time constant" curve defines that time at which the voltage reaches 63.2 percent of it's final value. If you are discharging from +12V, the RC time constant is the time at which the voltage decays to 4.416 volts (12 * (1-63.2)). There are very few DC-DC converters/inverters that will continue to operate at 4.4 volts...

    HERE's a pretty good explanation.

    Good start...keep reading...
    MikeH

  3. #3
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    Another capacitor thread


    Check it out here >>>

    1 Farad CApacitor Tank?

    I get a feeling noobies comes to this forum with a spare 1F capacitor on their hand.

  4. #4
    MySQL Error MatrixPC's Avatar
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  5. #5
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    Haha

    4.7F for 1 seconds protection

  6. #6
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    4.7 farad???

    Impossible. Has anyone thought about making a poor man's UPS, by buffering the 12v current with a gel cell battery and some relays or something?

    Be a lot cheaper than dropping $400 on capacitors.

  7. #7
    MySQL Error jcdillin's Avatar
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    yes, plenty have done that with gel cells
    肚子笑痛了
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  8. #8
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    Quote Originally Posted by slartiblartfast
    Has anyone thought about making a poor man's UPS, by buffering the 12v current with a gel cell battery and some relays or something?
    Not that Im aware off, no.

  9. #9
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    Quote Originally Posted by jcdillin
    yes, plenty have done that with gel cells
    Ah man you ruinned it

  10. #10
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    probably seems like the way to go.

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