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Toy circuit safety question

Discussion in 'Electronic Design' started by [email protected], Dec 14, 2008.

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  1. Guest

    My cousin's husband asked me about an electric shock circuit. He
    remembers at school some kid made joke suitcase, that if picked up
    zapped the recipient.

    I know the circuit, and I strongly expect it to be the same one used
    in the practical joke pens and staplers. It just involves a DPDT
    relay, a 100uF capacitor, a small transformer and a 9v battery.

    The basic idea is that the capacitor is charged up (to 9v) triggering
    the coil which then disconnects the battery and discharges the
    capacitor through a transformer which can step the voltage up. The
    transformer has primary/secondary impedances of 1.2kOhms / 3.2Ohms
    (leading to a step-up factor of about 20).

    My question is just - how safe is this circuit? He wants to make it
    to demonstrate some electronics to his son (and I agree with him that
    there is a lot in it for a boy to learn).

    I calculated the output energy of each pulse:
    E = 0.5 * C * V^2
    = 4 mJ
    which is very low, so I told him it's fine if he's sensible, and keeps
    the output away from the face etc. I just want to check here that my
    logic is sound.

    Thanks.
     
  2. Bob Eld

    Bob Eld Guest

    It should be safe enough. The output voltage is very much higher than a 20:1
    step up would indicate. It's probably several thousand volts and is due to
    the collapse of the magnetic field in the transformer when the switch (or
    relay) is opened. That is a different mechanism than simple transformer
    action.

    You did not mention it but the circuit can be connected to continuously
    interrupt the current or buzz putting out a continuous train of high voltage
    pulses, fifty or so a second. Sounds cool and "shocking" but not dangerous.
    Bzzzzzt! Yikes. There's not enough energy in the system to do damage.
     
  3. Charles

    Charles Guest

    Certain medical conditions can make this a dangerous road to travel.
     
  4. Guest

    Well that is quite a vague statement. Obviously they won't be using
    it on people with pacemakers, or anybody they don't know well.
     
  5. Guest

    It should be safe enough. The output voltage is very much higher than a 20:1
    I am interested in this. Both from the perspective of "how this thing
    works" and also because I am a physical scientist by training and by
    profession. Even if you know of a paper which explains this effect?
    I have a feeling it is to do with the inductance of the transformer,
    V=LdI/dt,
    and since with a square wave the dt is very small then the voltage
    across the inductor is very large?
    Actually this is what the circuit does - when the capacitor has mostly
    discharged the relay switches off from the transformer to connect the
    capacitor to the battery again. I'm not entirely sure, but someone
    told me this is the "electric fence circuit".
     
  6. Bob Eld

    Bob Eld Guest

    `
    High voltage generated from a collapsing magnetic field is very common.
    That's the way induction coils, ignition coils, fly back transformers, and
    many other inductive circuits work. Look up induction coil, fly-back
    circuit, inductive kick and so on.

    Yes V = L di/dt and V = Nd(phi)/dt. When current flows in an inductor it
    creates a magnetic field (phi). When the current is interrupted the field
    collapses trying to maintain the removed current flow. Since there is no
    load when a switch opens other than stray capacitance in the windings, the
    rate of current collapse, di/dt becomes very high. This creates the high
    voltage pulse, Ldi/dt.

    The stored energy in the inductor is 1/2 LI^2. This energy comes out both in
    the high voltage pulse but also in the spark that occurs at the switch when
    it opens. A capacitor across the switch can recover some of this energy and
    by resonance re-apply it to the coil for a number of high frequency cycles
    enhancing the output. Automotive ignition works this way.
     
  7. Jasen Betts

    Jasen Betts Guest

    It's a "flyback converter" you can look that phrase up and get a
    better description than I can give.
    yeah that's it exactly.
     
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