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Electric pen tester - how do they work??

Discussion in 'Electronic Design' started by Colin, Dec 26, 2006.

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  1. Colin

    Colin Guest

    Hi all,,

    Can anyone expalin how those pen type testers work, the ones that glow
    red when a Live feed is detected through the PVC coating of cables?? I
    took one apart and all that was inside was a couple of resisters and a
    coiled spring with a thin metal rod down the center. I am wondering how
    it actully works?

    Seems very simple but cannot explain what the coil is for??

    Thanks
    Col
     
  2. Guest

    Look up "induction".
     
  3. Colin

    Colin Guest

    Blimey that was quick for boxing day!!!

    Thanks

    Col
     
  4. Guest

    There's bugger all on the telly. I think you'll find there's a bit more
    than a couple of resistors in a pen tester. The coil gets a small
    current 'induced' in it when it is brought near to a cable which is
    carrying current. There will be a chip which detects this small current
    and turn on the lamp. There'll be a battery too, to provide power I'd
    have thought?
     
  5. Colin

    Colin Guest

    So after a quick google search, it would seem that the coil spring
    would be pulled by the magnetic field and contact the center thin rod
    making the circuit?? Is that right?

    Didn't think there would be that much magnetic fied around a live
    wire...

    As you can tell I am new to electronics..

    Thanks
    Col
     
  6. Colin

    Colin Guest

    Thanks Mike ignore my last post.. I will go back and have a look at the
    components again. I need to identify the components as I need to
    recreate the whole setup within a different structure for a project I
    am doing.

    I will need to study induction a fair bit then...

    The pen I took apart was from china and I sourced the resisitors but
    were difficult to get hold off so was wanting to redo the whole thing
    with readily available parts.. It does have a single 1.5v battery for
    the LED... I was also wanting to change the LED for a buzzer.. which I
    hope will be feasible.

    Thanks very much for your help today... emmerdale and corrie are
    finishing soon :)

    Col
     
  7. Greg Neill

    Greg Neill Guest

    No, the alternating field around the wire induces a
    current in the coil (transformer action). This
    current if it passes a given threshold value, causes
    a circuit in the tester to light the indicator.
     
  8. Palinurus

    Palinurus Guest

    -
    Nope. It's electrostatic induction, not magnetic. There are also AC/DC
    versions of these gadgets, though the DC ranges are more complex and a
    lot less sensitive, for obvious reasons. Here's a rather cute example.

    http://www.electronicsforu.com/efylinux/circuit/cir114.htm

    As described, it only works on AC, but a little delta-C can be applied
    by hand motion. A manual chopper, if you will.
     
  9. Luhan

    Luhan Guest

    The coil is for inductive pickup (probably).

    Look for a small black blob on the circuit board, that is the actual
    circuit epoxied in.

    Luhan
     
  10. Greg Neill

    Greg Neill Guest

    The OP indicated that his unit employed a coil. I am
    aware that there are electrostatic models.
     
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