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Capacitor Charger | LED Illuminate when Complete

  1. Jul 21, 2009 #1
    I'm looking to assemble a capacitor charger in a nice, durable enclosure. I already have the DC to HV DC converter. I'm just looking, now, for a way to indicate when the capacitor is fully charged, practically identical to how a flash-enabled camera indicates when its capacitor is charged and ready for flash.

    I'm not an electrical engineer, as such, I've thoroughly searched the big G for schematics or diagrams of this circuit and have come up empty handed. I'm sure it's rather simple and involves the manner of which the capacitor blocks DC current flow once the capacitor has been charged to the voltage of the source.

    Any hints or help would be greatly appreciated! Thanks in advance for your time.
  2. jcsd
  3. Jul 21, 2009 #2
    Well if you know how to read an electronic schematic I can provide you with some very simple setups.
  4. Jul 21, 2009 #3
    That would be outstanding! :-)
  5. Jul 21, 2009 #4


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    You probably have something like this:
    HV supply........... resistor..............capacitor............ground.

    If so, you could lift the connection to ground and put a small resistor in there.

    Now, if you measure the voltage across this resistor, you could see when there was no longer any current flowing into the capacitor. This avoids having any delicate circuitry at high voltage.
    The start current would be about (HV supply) / (resistance of resistor) and you could use this current to work out a suitable resistor.
    For example, to use the 2 V range on a digital multimeter you might like to make the voltage across this resistor start at 2 volts and decrease as the capacitor charged up.
  6. Jul 21, 2009 #5


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    This is exactly the solution I was going to propose.
  7. Jul 21, 2009 #6
    So in that scenario, the LED (if placed past the resistor) would illuminate as long as the capacitor is charging at an appreciable rate, as such, it would discontinue to illuminate once the capacitor was relatively charged?

    If so, that is a very effective solution. However, I'm wondering what that actual "delicate circuitry" would be that could produce the effect of the LED illuminating once the capacitor has become "completely" charged. Due to my limited electrical knowledge, I simply can't conceptualize how this could be done. :confused:

    Thanks for your help so far guys; I appreciate the responses.
  8. Jul 22, 2009 #7


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    You could turn a LED on and off with an Op Amp.

    When the voltage across the resistor above drops to less than 0.1 volt when it started at 10 volts, you could say the capacitor is fully charged. It is better than 99% charged. An Op Amp can easily do this but it would need a dual power supply.

    Or if you would accept 0.6 volts across the resistor as a fully charged capacitor, you could turn on a NPN transistor which then removes the bias voltage from a conducting transistor which was passing current through a LED.

    So, the second transistor conducts and lights the LED unless the first transistor is turned on.
    The first transistor turns off when the capacitor is almost fully charged. This turns the LED on.

    See attached picture.

    A meter is a better way of indicating, though. One hazard with the LED approach is that you could have an open circuit capacitor which would light the LED immediately. A meter would show you it charging or not charging.

    Attached Files:

    • LED.JPG
      File size:
      4.9 KB
  9. Jul 24, 2009 #8
    Thanks so much! That's practically exactly what I wasn't able to think up myself. I wish these forums has plus rep options, I would have a lot of it to give out! ...and the schematic really helps, thanks again. :-)
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