amature capacitor question

Discussion in 'General Electronics Chat' started by babbock, Jan 24, 2011.

  1. babbock

    Thread Starter New Member

    Jan 24, 2011
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    Ok I will preface this with the following: I am pretty new to electronics. I took a class or two in high school, but nothing very advanced, so forgive me :)

    I do not fully understand how you can charge capacitors over a somewhat lengthy period of time, and discharge all at once. My understanding is that the timing constant was the same on charge and discharge.

    Here is the scenerio I am puzzled about:
    You have a low current low voltage power source, say a few AA batteries. You use them to slowly charge a capacator bank(to a few hundred volts), then discharge in a split second.

    If using a resistor to charge, i guess that would slow the charging, but would end up putting less in, or am I missing something?
     
  2. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    The resistor won't result in less charge getting in because as the capacitor fills with charge, its effective resistance rises, so the series charging resistor becomes so small in proportion to the total resistance that it can be ignored.

    It will slow the flow of charge in, as you've noted, but given enough time, the capacitor will reach the same charge, no matter what series resistor is used.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    An example of what you are curious about is a camera's electronic flash attachment.

    You turn on the flash, and you hear the whine of an inductive boost circuit charging up a capacitor to perhaps several hundred volts, which takes a number of seconds.

    When you then click the shutter, the energy stored in the capacitor is then discharged into a flash tube, which ionizes the gas in the tube for a brief moment in time; the capacitor is deeply discharged.

    Keep in mind that high voltage stored in capacitors can be quite hazardous to people, even lethal. Even small currents that pass through the torso may cause ventricular fibrillation, which quickly leads to unconsciousness and death.

    We like for our newcomers to stick with low voltage, low current projects (<50v) as they present a minimal shock hazard.

    As far as charging slowly via a resistor - there is some power lost in the resistor, but if the current from a power source such as a battery is unchecked (no limits) then power is dissipated in the battery itself, which (if the load is heavy enough) will cause heating of the battery and possibly rupturing it/them forcefully. All power sources have at least SOME internal resistance, except for "ideal" mathematical models in simulators.
     
  4. babbock

    Thread Starter New Member

    Jan 24, 2011
    3
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    thanks for your reply.

    I understand that thats basically what a camera flash does, it just doesnt totally make sense why it works/how to reproduce that on a higher or lower scale. I am guessing there is some additional component which steps the voltage up, a CW multiplier or something.

    So basically, you step the voltage up to match your capacators, charge with an inline resistor, then discharge w/o the resistor? or am I missing something?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    A transistorized oscillator circuit switching current through an inductor on and off (a "boost" circuit) charges the capacitor via a diode. In these circuits, battery life isn't very critical, as the battery only has to last for a single roll of film. Still, since the battery voltage is very low compared to the end of the charge cycle, not much in the way of battery current limiting is required.

    Someone did a pretty good write-up of a Kodak disposal camera flash on this site:
    http://www.increa.com/reverse/dc/

    If you're interested in experimenting with flyback converters and boost converters, check out Ronald Dekkers' "Flyback Converters for Dummies" page:
    http://dos4ever.com/
    It's the 1st entry below "(Nixie) Clock Projects"
    If you decide to actually build the boost (figure 3) or flyback (figure 16) converters, replace R4 with a much lower value (around 39k) to keep the output voltage within a reasonably safe range.
     
  6. babbock

    Thread Starter New Member

    Jan 24, 2011
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    so voltage is partly responsible for how fast you will discharge?
    IE a 1 farad cap at 5 volts wont dump as much energy as fast as lets say a .5 farad cap at 400 volts etc?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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  8. soda

    Active Member

    Dec 7, 2008
    174
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    A 1 farad cap is about the size of a milk bottle. So. i think you mean a 1uF capacitor. Promise me you won't play with farad capacitors and that is to say if you can afford it. [​IMG]
     
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