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Mike & speaker funnybusiness..

Discussion in 'Electronic Design' started by Robert Baer, Mar 19, 2013.

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  1. Robert Baer

    Robert Baer Guest

    Well, my trusty AT&T phone died and a got a new one.
    So,i thought to take apart the handset..err..BASH apart,as it was
    *NOT* made to be taken apart in any nice way.
    Surprise!
    The electret mike had a _diode_ (signal) in parallel, reversed polarity.
    Say what? Waffer??
    And the dynamic speaker had _two_ diodes (1N4000 series),
    back-to-back in parallel across it.
    Say what? Waffer?
    Yes,i know that diodes are cheap, but since there is no useful
    function for them, what gives?
     
  2. Lightning IS an electrostatic discharge, and no, one cannot generally
    protect against a true lightning intrusion. They start at like 6MV.

    You would have a hard enough time stopping the EMP jolt they cause.
     
  3. miso

    miso Guest

    Possibly the diode across the speaker is for back-EMF. Drop the phone
    and speaker generates a pulse.
     
  4. Robert Baer schrieb:

    [...]
    Anti-parallel diodes connected in parallel to the handset speaker
    are very common for hearing protection purposes:

    <http://www.fernsprecher.info/html/w48_schaltbild.html>

    HTH

    Reinhard
     
  5. That is what he is talking about here. And ESD in an electronics context
    is considered to be something else. As in, say, human body model as
    opposed to struck-down-by-the-almighty model.
    Careful, you'll attract the real lightning nutter.

    Oh why not, lets try anyway: Lightning rod, mains whole house protector,
    lightning surge protector.

    See what turns up.
     
  6. John S

    John S Guest

    Actually, I had underground telephone service in the country. It seems
    that lightning will follow the tree it hit down the trunk, to the roots
    and into the nearby telephone cable. I have several pieces of equipment
    which testify to that fact.
     
  7. Guest

    This causes a common mode into the armored cable. The current flows
    along the cable shield in both direction, eventually spreading into
    the surrounding ground. The size of the common mode voltage depends on
    the total resistance from the point the cable was hit to neutral
    ground and the current (U=IR), typically hundreds or a few thousand
    volts.

    For a simple telephone set, the common mode is not much of an issue,
    since it is floating and relies of the differential signal between the
    conductors in the pair and hence not much need for diodes across
    microphone and headphone. However, there should be a large gap
    between the headphone and the human head, so that the human does not
    get hit by this common mode voltage in the head by a small spark (that
    is the U=IR losses, not the megavolt range lightning).

    However, when the "telephone" is also grounded locally or at a power
    substation, such as in mains powered phones, answering machines,
    modems and ADSLs, there is a big risk that the common mode voltage in
    the telephone pair will flashover to ground, through audio/mains
    transformers and optoisolators or simply flashover between too closely
    mounted PCB tracks, causing a lot of damage.
     
  8. Jasen Betts

    Jasen Betts Guest

    Yeah, underground phone service didn't protect me eiither, I had a few
    MC1488s and MC1489s in serial ports die before I learned to also
    unplug the modem
     
  9. Tauno Voipio

    Tauno Voipio Guest

    The 1488 and 1489 were called 14-leg fuses, for a reason.
    In the 70's, we had a standing order to set up good quality
    sockets for them when changed for the first time, to save
    the PCB's.
     
  10. oooh, that's gotta hurt Jim.... :)
     
  11. Guest

    Diodes are for transients created during normal phone operation. As onlyone other noted, parallel 1N4000 series diodes limit voltage to the speaker and protect your ear from excessively loud noise transients. Diodes conduct only when speaker voltage is excessive.

    Other speculated anomalies are made irrelevant by solutions required at the subscriber interface; where phone wires enter each building.
     
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