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screwdriver tester

Discussion in 'Electronic Basics' started by [email protected], Oct 8, 2007.

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  1. Guest

    when i stand on a wooden stool and touch a live wire i dont get a
    shock because circuit is not complete with ground as wood is a
    insulator but when i touch a screwdriver tester to that live wire the
    bulb lights up even while i am standing on the wooden stool how can a
    bulb light without getting earthing?
     
  2. Jamie

    Jamie Guest

    It's magic dust!
     
  3. Even with no ground connection to complete a resistive
    circuit, your body has capacitance to the rest of the
    universe, so charges and discharges each half cycle of the
    AC with a small capacitive current. This small capacitive
    current (sub milliampere) is well below your threshold of
    feeling, but is above the threshold to light a small neon bulb.

    If the AC voltage were much higher (say, hundreds of
    thousands of volts, like what is carried on the high voltage
    transmission lines that cross the continent) the capacitive
    currents they would drive into your body capacitance would
    reach the threshold of feeling, even if you were suspended
    from a helicopter.
     
  4. That, plus the very low (< .1 ma) current required by a small neon lamp,
    plus the unipolar capacitance of your body. The tester has a large
    resistance in series with the lamp, so you won't get stung even if you are
    grounded.
     
  5. Eeyore

    Eeyore Guest

    You're acting as a large value resistor. Enough current passes to light the neon
    bulb without giving you a shock.

    Graham
     
  6. Jamie

    Jamie Guest

    really, I thought it had something to do with capacitance with
    AC combined.
    Oh well, back to school.
    snort snort..
     
  7. Eeyore

    Eeyore Guest

    Are you saying that the body (or wood) is incapable of making a resistive connection ?

    Graham
     
  8. John Fields

    John Fields Guest

     
  9. Your body is one plate of a capacitor, and the earth is the other
    plate. It has nothing to do with resistance, in this application. it
    is like running your hand along the glass of a fluorescent lap that
    doesn't come on when you apply power.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  10. Rich Grise

    Rich Grise Guest

    Not at all, merely that in this case the conductance is negligible
    compared to the capacitive reactance.

    Hope This Helps!
    Rich
     
  11. Guest


    If you placed one 60 hz 120v wire upstairs in a wooden house with no
    other wiring or metal objects and the neon lamp still lit I would
    guess that trickle energy and the wire voltage that made the neon bulb
    light converted into trickle radiation into the air space surrounding
    our candidate holding the other end of the wire.

    I think radiation resistance through our candidate is as good an
    answer as capacitance on our candidates skin to some earth grounding
    surface ten feet away.

    At higher frequencies possible, but 50/60 hz, I would need to ask
    Radium.



    Usenet was a wild frontier 10 years ago. I remember ;-)
    Then Google invited itself to our campfire.

    * * *
    Christopher

    Temecula CA.USA
    http://www.oldtemecula.com
     
  12. sparky

    sparky Guest

    The wood is an insulator. The current flows through you because of
    capacitance.
     
  13. craigm

    craigm Guest

    Depending upon the moisture content, wood may be a poor insulator.
     
  14. So the screwdriver comes with a waterlogged wooden stool?
     
  15. zhafran

    zhafran Guest

    A beginner question here:
    Since it is due to capacitance, does it mean that the neon lamp will
    be turned ON for only a few second when we're testing it with DC
    voltage? Because capacitance has the characteristics to block DC when
    it's fully charged right?
     
  16. Yes. One flash, till you discharge the DC voltage on your
    body and the plastic surfaces of the screw driver handle.
    again.

    But don't take my word for it. Series connect a stack of 12
    each 9 volt transistor radio batteries, with one terminal
    grounded. Touch the other terminal that with the screw
    driver, and it should produce a single faint blink. Have
    someone else reverse the end grounded and tough the other
    end and you should get another blink.
     
  17. Rich Grise

    Rich Grise Guest

    That would be true if you were testing a DC circuit. Usually, those
    things are used on AC mains, and it's that AC that flows through
    the capacitive reactance of your body capacitance to ground. It's
    not much current (probably way less than one mA), but neon bulbs don't
    need much.

    Hope This Helps!
    Rich
     
  18. Rich Grise

    Rich Grise Guest

    zhafran, if you do this, use EXTREME CAUTION. 108 volts is enough to drive
    lethal current through your skin. If you insist on doing this test, then
    keep one hand in your pocket.

    Good Luck!
    Rich
     
  19. Nobody

    Nobody Guest

    More likely a few {micro,milli}seconds; the capacitance involved is a few
    picofarads.
    Right.
     
  20. zhafran

    zhafran Guest

    Does it also applicable to high AC voltage?
     
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