Transistor VCBO test setup

[[email protected]]

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
res.
and the knee is not well defined (rounded).
Perhaps that is normal for Ge .
Will appreciate any help.
Ed

 

[Robert Baer]

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
res.
and the knee is not well defined (rounded).
Perhaps that is normal for Ge .
Will appreciate any help.
Ed

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.

 

[[email protected]]

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.

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
Ed

 

[Chris Jones]

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
Ed

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.

Chris

 

[Robert Baer]

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
Ed

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).