Picofarad capacitor

Discussion in 'General Electronics Chat' started by birdburdy, Mar 11, 2008.

  1. birdburdy

    Thread Starter New Member

    Mar 11, 2008
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    One of my mates jokingly jabbed me in the arm with two paperclip-prongs, which seemed to be connected to a capacitor. There was a spark and it made me jump quite a bit.

    After nearly trying to decapitate the twit, he explained that it was a very small capacitor (500pF I think) at very high voltage (several kilovolts). I told him that was completely dangerous but, rather annoyingly, what he then said made sense:
    the capacitance of the human body is around 300-500pF, and static electric shocks are between 3000 and 50,000V - so really it's no different from a static shock.

    Is this actually true, or am I really lucky to still be alive?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Such small shocks are nothing to be worried about ;)

    I've played far worse tricks upon my fellows in the past :)
     
  3. mik3

    Senior Member

    Feb 4, 2008
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    The think that matters for an electric shock is current and not voltage. In your case the 500pF capacitor carries very little stored energy. The voltage is high and sets a high current (for your body) but it lasts very little to harm you.
     
  4. erlend

    Member

    Feb 8, 2008
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    Actually I've always wondered this... people say current not volts, but I think it's a massive simplification.

    The current in the above is actually very large! In order to even feel such a small number of electrons, the voltage must be very large: at least 2kV-5kV. Using Ohm's Law now,
    Current = Voltage / Resistance ...
    assuming a resistance of about 5000 Ohms (quite low - e.g. wet skin)
    Current = 5000/5000 = 1A!

    However this large current is very very short lived. The usual rule for determining the time is takes a capacitor to discharge is 5RC, which here is
    5*5000*500E-12 = 12.5 microseconds.

    Looking at the energy,
    Energy = 0.5 * C * V^2 = 6.25mJ ... which is tiny.

    What did your friend use to power the capacitor though? I'm assuming he used batteries, but I can't see how he would get 5kV out of a battery!
     
  5. antseezee

    Active Member

    Sep 16, 2006
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    Q = 0.5*C*V^2 (energy in joules)

    You'd want to be worried about larger caps with high voltage ratings. Aka high uF caps in the kVs. They hurt quite a bit. In our college, there was this huge rheostat shaped cap only rated for 800V. One of the techs was like watch this. Hooked it up to a voltage source, charged it up, then took a shot through the forearm. It put two burn marks on his forearm.
     
  6. erlend

    Member

    Feb 8, 2008
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    How on earth did your friend get a 5kV capacitor?! Do they even exist?
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    Yes, of course. Our shop had them up to 50KV. Not much capacity in a body about 1 1/2" diameter by 4" long. They were essentially glass cylinders with the plates rolled inside separated by some material and also filled with mineral oil. The metal caps on the ends had threaded studs for mounting the wires.
     
  8. erlend

    Member

    Feb 8, 2008
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    Actually, can you not just put (for example) 5 1kV 2.5nF capacitors in series to make a 500pF capacitor, and since they are in series the total voltage they can take is 5kV? Is this wrong?
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    Many hobbyists do this very thing to build Tesla coils and similar HV experiments. It is important to have a high value resistor across each capacitor to balance the charges. Capacitors of a given value vary quite a bit from one to the other.
     
  10. birdburdy

    Thread Starter New Member

    Mar 11, 2008
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    That's the circuit he used... 15 capacitors in series. There were no resistors in parallel with the capacitors though? How exactly do they help?
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    They more evenly distribute the charge. It is possible to have a 2.5nF capacitor that is actually 3nF, and another which is actually 1.5nF. Both would be within specified 20% tolerance. Resistors, even those of several megOhms, can have much tighter tolerance.
     
  12. birdburdy

    Thread Starter New Member

    Mar 11, 2008
    3
    0
    He used two AA batteries, in series.
     
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