Charging capacitors with too high of a voltage

Discussion in 'General Electronics Chat' started by shaqywacky, Feb 4, 2011.

  1. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
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    I bought some large capacitance, low voltage capacitors, 1300F and 5V. I also bought a AC/DC converter for charging them. I tested the converter and it seems to be giving about 7V. I have two questions concerning this:

    1. I am just taking the two leads from the converter and connecting them to the voltmeter but while I was reading through VOL 1 here I think I remember something saying that you had to put a load or something between the terminals so that it could accurately read the voltage. Is this true or did I misremember something?

    2. Say this converter is really giving 7V, if I just watched the capacitor charge and stopped it around 4.5V would there be any damage or any reason to not do this?

    Thanks guys!
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Why don't you just put a resistor between the converter and the capacitor? It will limit the output current of the converter to something it can handle when the output is shorted, ie. 0V on a big capacitor. It will also help you protect the capacitor. You will still have to watch the capacitor and stop the charging!
     
  3. tom66

    Senior Member

    May 9, 2009
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    Not necessarily a good idea - a converter designed to charge a very large capacitor could output many amps, and most resistors would overheat, you'd probably need about a 10 ohm resistor, according to I^2*R that might dissipate 10W at 1A, not to mention higher currents, or short circuit current!
     
  4. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
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    Because of what Tom said is the reason why I didn't try the resistor, I could have gone a bought one but I'd like to hear the answers to the first two questions also.

    Actually I just checked the current and its about half what it says on the converter(and the voltage is about twice what it says, I feel like there's a connection here). The current is about 86mA which seems really low to me.
     
  5. tom66

    Senior Member

    May 9, 2009
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    86mA will take a few weeks to charge a 1300F cap. Sounds like you have a faulty unit.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If the voltage of the supply is 7V, even into a short-circuit the maximum current into a 10 ohm resistance would be 700mA, with a dissipation of 4.9W; P=V^2/R. You could probably get away with a 6Watt wire-wound.

    More of an issue would be the time constant; 10 ohms with 1300F gives RC = 13000 seconds, or 3.6 hours - it would take a bit longer to reach 5V. You would hardly want to be vigilant all that time, so I think some kind of automatic limiter is called for.
     
  7. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
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    I found another converter which says .7A but I think it's a little lower than that. It seems to take about 2 hours to fully charge, its a little hard to tell because I've been testing what it can do everyone in a while and it looses some charge during that.

    I have another question. When I have it charging and I check the voltage, it is about one volt higher than when I check the voltage of the capacitor not while charging. Which one should I use as my fully charged to 5Vs limit?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Don't charge it to more than 5v.

    Be careful with it when it's even partially charged, as it's capable of putting out a large amount of current. If you accidentally short the terminals, it will do some damage, and you may get burned.
     
  9. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
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    So when it's not charging should be the time I measure it?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    No, measure it when it's charging.
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Use a few diodes in series with power supply to ensure the voltage doesn't go over 5V. Otherwise:

    [​IMG]
     
  12. shaqywacky

    Thread Starter Active Member

    Apr 1, 2009
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    @Sargent What's weird is that the difference between the charging and non-charging voltages varies sometimes. Like before I said there was about a one volt difference(IE charging was 5V and not charging was 4V) then I messed around with it and it lost about 2 volts. Then I started changing it again and the difference is now only about 1/4 of a volt.

    @thatoneguy So I should just put a between the positive terminal of the capacitor and the current source and another between the negative terminal and the ground?
     
  13. tom66

    Senior Member

    May 9, 2009
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    This will dissipate the same power as a resistor.
     
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