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Doorbell button spark

Discussion in 'Electronic Repair' started by Kroma, Sep 23, 2005.

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  1. Kroma

    Kroma Guest

    Hello,

    I recently installed a wired doorbell and it works very well indeed.

    However... I was leaving the house in the dark (and it was VERY dark)
    this evening and pushed the bell push in order to hear how loud the
    doorbell would be to callers. I was shocked to see that the bell push
    briefly lit up as I released the button. As it is not an illuminated
    bell push (I have the unit installed with batteries) and owing to the
    fact that the light had a blue tint, I am assuming that the light I
    saw was in fact a spark within the bellpush. There was no spark as I
    pressed the button - only on release. In fact the spark isn't visible in
    daylight - it has to be VERY dark!

    In all other respects the unit appears to be working perfectly well
    and the spark isn't apparent during daylight but can anybody tell me
    what could be causing the spark or is this normal? I had checked the
    bell push very carefully before securing everything in place and the
    connections appeared to be good.

    Thanks in advance,

    Daz
     
  2. Funfly3

    Funfly3 Guest

    perfectly normal
    perfectly normal for a battery doorbell its caused by back EMF
    http://www.physics.brown.edu/physics/demopages/Demo/em/demo/5j1023.htm it
    can give you quite a shock
     
  3. Ross Herbert

    Ross Herbert Guest

    When asking questions like this it is best if you give make/model
    and/or operating principle (or the sound it makes).

    Here's my guess. If the doorbell is battery powered and uses a
    solenoid to strike one metal gong or bar when the button is pressed
    and then retracts under spring tension to strike another metal gong or
    bar on releasing the press button (produces the ding - dong sound),
    then the sparking at the button is most likely due to back emf from
    the solenoid. Nothing to worry about....
     
  4. Kroma

    Kroma Guest

    I'm sorry my info was a bit scant.

    It's a Byron wired doorbell of the 'ding dong' chime bar variety. It's
    battery operated. Does that still sound normal?

    Thanks,

    Daz
     
  5. Funfly3

    Funfly3 Guest

  6. Perfectly normal with DC. That's why an AC switch has a lower rating
    current wise on DC.

    The effect is rather like how a basic car ignition works. Build up a flux
    in an inductor, which a doorbell is to a certain extent, and then switch
    off the current. The flux collapsing produces a high voltage. In an
    ignition system you use a transformer (coil) to get an even higher voltage.
     
  7. Yes - the solenoid is almost a pure inductor.
     
  8. If it worries you, you can put a diode ( 200-400V PIV) across the
    solenoid coil so that it is reverse biased when the solenoid is
    energized. When you release the button, the diode will be forward
    biased and will absorb the back EMF. There is a slight possibility this
    mod might slow the solenoid release, interfering with the tone, You
    have to try it to see.
    --
    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"

    The Lost Deep Thoughts By: Jack Handey
    Before a mad scientist goes mad, there's probably a time
    when he's only partially mad. And this is the time when he's
    going to throw his best parties.
     
  9. Every time you interrupt a current flowing with a mechanical switch you will
    have sparks. Even when you switch on you can have sparks due to the
    contactbounce. During daylight you will not see them but you can often hear
    them through an AM radioreceiver. Your lightswitches in the house will also
    spark although you may not see them. They can can for instance ignite
    gasexplosions (if there is gas in the room of course.) Doorbellswitches will
    spark even more because of doorbells contain coils and the back EMF will
    give a high voltage pulse when you switch off. You can try to reduce the
    sparking by placing a capacitor over the switch. But there's no reason to
    worry. Millions of doorbellswitches are sparking every day and you only
    realize it because you saw it in the dark.
    BTW. Did you ever put off a shirt or a pully (wool or synthetic, cotten
    hardly sparks) and hear the crackling of *that* sparks? You can even see
    them when its dark enough.

    petrus bitbyter
     
  10. Kroma

    Kroma Guest

    Thank you for your in-depth replies everybody! I have now stopped panicking
    and will leave the doorbell as-is!

    Thanks again,

    Daz
     
  11. Arfa Daily

    Arfa Daily Guest

    To add to Dave's excellent analogy of the car ignition system ( or at least
    the old Kettering ignition that used to be fitted, rather than the new
    fangled electronic variety that they now have ), you can suppress the
    contact arc by fitting a capacitor of somewhere between 0.1 and 1.0uF and
    rated 200 to 400v, across the bell-push contacts. The sparking that occurs
    is the exact reason why such a capacitor ( also known in the motor trade as
    " The Condenser, mate " ) is fitted across the points in the distributor
    head. There is a school of thought, however, that with intermittently used,
    unplated contact sets, such as are to be found in bell pushes, it's a good
    idea to let them spark, as this keeps the oxide layer, which inevitably
    builds up on the contact faces, ' punched thru ' to clean contact metal.

    Arfa
     
  12. Still the same principle of cutting off the current flowing in an
    inductor, though, - it's just that the mechanical switch in the form of
    points has been replaced by a more sophisticated electronic one.
    IIRC, it also formed a tuned circuit to improve efficiency.
     
  13. Arfa Daily

    Arfa Daily Guest

    And the coil has been replaced by a quad unit that doesn't require a
    distributor ...

    ;-)

    Very probably. Years ago, I built many electronic ignition units for
    friends, based on a circuit published in Practically Witless magazine. That
    had a CTV 0.47uF 1000v boost resevoir capacitor in the output, effectively
    placed in series with the coil. It was charged to a high voltage, then
    dumped into the coil by a thyristor, controlled by the points. This caused
    the coil to ring as an LC tuned circuit with the cap. This resulted in a '
    long ' spark, which was basically AC, thus having the effect of making the
    plug electrodes last much longer, as no metal migration took place. The plug
    gap was increased to around 50 thou as I recall, to take advantage of this
    massive long AC spark. I also seem to recall that this circuit still pushed
    a comparitively significant current through the points, via a resistor, to
    overcome the oxidation insulation problem that I mentioned. It was essential
    that a new set of points was fitted at the same time as the ignition unit,
    otherwise, the engine would soon come to a stop due to the quick oxidation
    of the ( already ) burnt points surface.

    These ignition units were so good ( remember the commercially available "
    Sparkrite " models - three of them as I recall ) that when I took one off of
    a car that I was about to sell, the engine, which had been running
    perfectly, turned out to be so far out of tune, that it wouldn't even start.

    I guess all modern cars now have something along these lines, which accounts
    for why, in conjunction with the automatic self tuning engine management
    sytem, modern engines can go 20,000 miles between services, and start first
    flick of the key, under all weather conditions.

    Arfa
     
  14. But still coils. So the same principle. ;-)
    Capacitive discharge.
    It got round one problem but caused others? At least you could switch it
    back to points if the electronics failed. As they often did.
    Yes - dwell angle is important with points to give the coil enough time to
    build up the correct flux. Not so with CD.
    Well, many modern cars will do 100,000 miles without the ignition system
    being touched in any way - long life plugs. If it doesn't break first, of
    course. Servicing consists more of just changing some fluids and filters.
     
  15. Asimov

    Asimov Guest

    "Kroma" bravely wrote to "All" (23 Sep 05 10:45:42)
    --- on the heady topic of "Doorbell button spark"

    Kr> From: "Kroma" <>
    Kr> Xref: core-easynews sci.electronics.repair:342888



    Kr> Hello,

    Kr> I recently installed a wired doorbell and it works very well indeed.

    Kr> However... I was leaving the house in the dark (and it was VERY dark)
    Kr> this evening and pushed the bell push in order to hear how loud the
    Kr> doorbell would be to callers. I was shocked to see that the bell push
    Kr> briefly lit up as I released the button. As it is not an illuminated
    Kr> bell push (I have the unit installed with batteries) and owing to the
    Kr> fact that the light had a blue tint, I am assuming that the light I
    Kr> saw was in fact a spark within the bellpush. There was no spark as I
    Kr> pressed the button - only on release. In fact the spark isn't visible
    Kr> in daylight - it has to be VERY dark!

    Kr> In all other respects the unit appears to be working perfectly well
    Kr> and the spark isn't apparent during daylight but can anybody tell me
    Kr> what could be causing the spark or is this normal? I had checked the
    Kr> bell push very carefully before securing everything in place and the
    Kr> connections appeared to be good.

    Kr> Thanks in advance,

    Kr> Daz


    When the button is closed some energy is stored outside the wire just
    like an electromagnet. When the button is released that collapsing
    magnetic energy induces charges in the wire. However the switch
    contact is open leaving only air which has a very high resistance. The
    induced charges generate a high voltage which ionizes the air path and
    allows a current to flow. The longer the wire the greater the voltage.
    The resulting spark is like a mini thunderstorm.

    A small network called a snubber, which consists of a resistor and
    capacitor in series, can be placed across the arcing contacts to
    safely dissipate the stray inductive energy and eliminate sparking.

    A*s*i*m*o*v

    .... Chico of Borg: "Resistance? Atsa no good!"
     
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