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Transistor VCBO test setup

Discussion in 'Electronic Design' started by [email protected], Feb 20, 2007.

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  1. Guest

    I used to test transistors VCBO using a high voltage transformer,a
    varic, a limiting resistor, a diode and a current sampling resistor.
    An oscilloscope in XY mode was used, with the X calibrated in peak
    volts and the Y to detect the breakdown current.
    I don't remember the criteria to set the value of the limiting
    resistor. I am testing 100 W TO3 Ge transistors with 100 K as limiting
    and the knee is not well defined (rounded).
    Perhaps that is normal for Ge .
    Will appreciate any help.
  2. Robert Baer

    Robert Baer Guest

    It has been ages since i have done that with any transistor, and ages
    since i had any "flower power" (Ge) transistors to mess with (gave them
    all to a museum for use as replacements).
    But i do remember that they were rather leaky compared to any "sand
    power" (Si) transistors - so i am not surprised about that rounded quality.
    In fact, i would be surprised if it was sharp.
  3. Guest

    Thanks Robert, I am not doing this testing for fun; a friend has an
    EDM machine that uses 16 Ge PNP set in parallel in groups of 4 that
    can be switched so all 16 can be in parallel. One or more of 8
    limiting resistors got shorted by
    wiringpvcinsulationmeltingbecausebearingfrozeinfan what resulted in
    all 16 going back to the foundry state. The suggested replacement is
    the 2N1908 that there is no more,
    I found 10 Motorola and then went to subs from NTE (179), now comes
    the interesting part, the Motos and 2 ECG (179) test OK for BVCBO but
    the NTE's start going south at a very low voltage. The good ones (MOT
    & ECG), with 47K RLim clamp at 127 to 233 RMS, the NTE's clamp at 47
    to 80 RMS. The machine runs at 80 VDC and the transistors are pulsed.
    I would like to go Si but the mod might be over my head.
    PS: NTE is Nam Tai Electronics
    Thanks again
  4. Chris Jones

    Chris Jones Guest

    If you post a schematic (on the web or in ABSE), someone may be able to tell
    you whether substituting silicon transistors would be easy or not.

  5. Robert Baer

    Robert Baer Guest

    Unless the base drive is insufficent, there should be no problem in
    using a silicon transistor.
    The trick is in the paralleling, whether Ge or SI.
    First the use of an emitter resistor for each transistor - going from
    Ge to Si one would nominally double the resistor value previously used;
    i would say the drop should be between 300mV on the low end and 900mV on
    the high end.
    Next is to add resistors in series with the base; maybe calculate on
    a forced beta of 2 on the low end and 10 on the high end.

    Ther was one advantage with flower power over sand power; the voltage
    drop was lower, the power handling was larger, and higher temperatures
    were allowed (that is, if made after Indium was not the dopant).
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