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Charging Circuit

  1. Jul 30, 2010 #1
    Hello, would anybody please tell me how a charging circuit works, such as one for a capacitor in a disposable camera...how can the 1.5V battery fully charge a 330V capacitor? I believe transformers are involved to kick up the voltage, but how is DC converted to AC for that? I'd also like to know the components I would need to make my own rugged version of a charger like this...

    Thank you!
  2. jcsd
  3. Jul 30, 2010 #2


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    Staff: Mentor

    If you don't know how the circuit works, I'm not sure you should be trying to build one that boosts up to several 100s of volts... What do you intend to do with the circuit?

    You can learn more about these kinds of circuits by using the search terms "DC-DC Converter", "Boost Converter" and "Flyback Converter". You can search at wikipedia or National Semiconductor or Maxim Semiconductor, or just with Google.
  4. Jul 31, 2010 #3
    You are right.
    A transformer provides the step up ratio.But transformers only pass AC signals.
    So a transisitor driven by an oscillator "chops" the dc to make quasi-square waves
    that then are fed in to the xformer.

    Any flourescent lamp has a ballast that does this because the gas
    needs high voltage to arc. Buy one and take it apart and you can see
    the components. Cost = $3 - 6 or so.
  5. Jul 31, 2010 #4
    Not exactly true according to:

    Larger tubes may need a step up, but they don't need "high voltage" -- you may be thinking of "neon" sign tubes. The main task of the ballast is to limit the current, thus the name "ballast".
  6. Jul 31, 2010 #5
    Au contraire .....
    High voltage is officially reached at 50 volts.
    That my not seem high to many, but the power industry marks it so.
    I worked in high energy physics for many years and I know this to be true.
    The lamp operating point will more often than not have a drop grater than 50 volts.
    How do i know that ? I designed one of the first electronic ballasts at
    Westinghouse for an HID lamp back in 1983.

    Furthermore, even with a starting gas, the initial strike voltage for a gas tube
    will be typically around 1.5 to 2.5 Kilovolts. That is clearly high voltage.

    To the OP ..... here is a link where you can see a non magnetic electronic ballast
    and its components.


  7. Aug 1, 2010 #6
    I made a coilgun out of a disposable camera circuitboard hooked up to five 330V 80uF caps found in the cameras. It fires a nail at a very low speed (not enough to penetrate a paper towel), but I was not going for anything crazy since this was really the first electrical project I've ever conducted...

    I used the same AA battery to charge the capacitor bank and that took a while - I wanted to speed that up or at least be able to demonstrate how it works.

    Thank you, these were helpful.

    Thanks a lot, I understand how this works now and took apart a lamp I found lying around. Had some extras at home.

    And that's pretty damn cool.
  8. Aug 1, 2010 #7
    Ah, OK. I think of "high voltage" as above your typical mains voltage... say 100's, not 50. This is something I've not worked with, how is the strike voltage generated? I thought it was the little starter thingie in your typical home fluorescent bulb, and that the ballast was there simply to add a load for AC current limit. It appears from the wiki page that ballasts for larger tubes can also be step up xfmrs. So what's the actual operating voltage across the range of sizes then?
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