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Help Component ID, TVS??

Discussion in 'Electronic Design' started by Stan, Feb 13, 2007.

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  1. Stan

    Stan Guest

    I'm trying to ID a component taken from the Degauss circuitry of a HP/
    Compaq monitor. (Background - This monitor has been exposed to approx
    900V instead of the normal 240V which has caused an issue or two. I
    have already replaced the 150uF smoothing cap which was getting very
    hot and venting.) The monitor is basically functioning OK but i'm
    concerned about a particular component. The device is an axial
    component with 2 leads, on first glance it looks similar to a 0.5W
    resistor with 3 coloured bands. However, i'm sure its not a resistor
    since the body is transparent, the PCB marking is Z801 (not "R") and
    the PCB symbol is what i would describe as two diodes joined cathode
    to cathode. (i.e. |>|<| ). The 3 coloured rings are Green, Grey,
    Grey. Within the body of the device there appears to be a small, dark
    coloured cylinder.

    Starting from terminal 1 of the isolation transformer secondary,
    (T1S), the degauss circuitry is basically a series circuit of :
    T1S > PTC thermistor > Pair 1 of NO Relay Contacts > Degauss Coil >
    Pair 2 of NO Relay Contacts > T2S.
    There is also a small capacitor across the degauss coil.
    So, when the relay is activated the 240V AC is applied to the degauss
    coil, the current will gradually reduce due to the PTC thermistor
    heating, at some point the relay is de-activated.

    The device in question is connected across the 1st pair of NO Relay
    Contacts. Based on this and the PCB symbol described above i think the
    device is a Bipolar Transient Voltage Suppressor. I guess its job is
    to suppress inductive kick that may occur when the relay is switched
    off. Does this sound feasible? I have tried to find similar looking
    devices on semiconductor websites with no success. They always seem to
    be a Diode style package rather than a "translucent resistor" style i
    see before me. This makes me question whether it really is a TVS.

    I'm concerned for the health of the component because during operation
    i can see a small blue spark across the dark cylinder contained within
    the translucent body. This seems to occur at random intervals. During
    operation the component has approx 70V AC across it although i suspect
    this is only noise picked up by the degauss coil. To test this theory
    i connected a 100K resistor across the suspected TVS and observed that
    the voltage drops instantly to zero.

    I desoldered the component and using my DMM found its resistance to be
    greater than 20MOhm. Probably because its knackered (i suspect!)

    To summarise my questions are:
    Does this sound like a Bipolar TVS?
    If not what is this component and are these blue sparks an indication
    of device failure?
    I imagine the coloured bands indicate a component rating/
    specification, (as with resistors). Any idea what rating these colours

    Many Thanks
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