how does this work?? 1

[2dye4]

I am reverse engineering a circuit that switches
a 200 volt peak to peak ac voltage with a single bipolar
transistor rated with a Vce breakdown of 400v.
In the off position the base can be considered tied
to the emmitter by external circuitry.
When turned on by a base current the transistor
conducts current in both directions , collector
positive for a half cycle and then collector negative
for a half cycle.
A constant current through the base emitter junction
is maintained when on.
The current is low level, approx 20ma
Can a bipolar transistor live with this type of
use ????

 

[IRstuff]

So long as breakdown does not occur, there should be no problem.

The low current is what allows it to work, since the reverse beta is usually quite low, compared to the forward beta.

TTFN

 

[nbucska]

The usual base-emitter breakdown voltage is about 6V max.
I can' believe 200V.

Most likely the transistor is in the diagonal of a diode bridge which may be as small as a few mm. square (
dissipation <<30 mW)



<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[2dye4]

Thanks IRstuff
I forgot about transistors still having gain in
opposite polarity.
If the load were more it probably would die.

 

[sreid]

I just got to ask; do you intend to call the transistor manufacturer and ask what they'll guarantee for a breakdown voltage, emittter to collector, base shorted to collector.

 

[itsmoked]

I have used transistors that had only minor reversed gain reduction. Of course they typically handle 10-20% less power when reversed.

 

[IRstuff]

It depends a lot on the design of the transistor. In order to prevent "reach-through," the base depth would need to be substantially increased as well as potentially having the dpoing profile modified. Such actions would tend to drastically reduce the forward beta, while probably not affecting the reverse beta by much.

I haven't found any high voltage transistors that have V_EBO much more than 6V, but that's dictated by the configuration of the low resistivity emitter diffusion. If the emitter junction was designed to mirror the doping profile of the collector-base junction, you'd get symmetrical breakdown characteristics as well as symmetrical, but pitifully low betas.

TTFN

 

[itsmoked]

I have run across circuit with the transistors in backwards
that worked fine. Several times. Not that I would do that!!

Why are we talking about the VBE??? Just because nbucska miss read the original post??

 

[nbucska]

Sorry, itis Veb0.
I have used one transistor AC switch AC but for only a few Vpp.

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[2dye4]

One hint to why this works is a load that is
modeled as a capacitor in series with a
resistor.

The transistor does break Vbe in the reverse direction
but for only one cycle.

 

[nbucska]

A star 4 2dye !!!

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032