Rudge wrote:
I need a T039 NPN transistor with an 80 volt BVceo rating such as a BC142.

My nearest stockist holds the BC141 which has a BVceo rating of 60volt.

If I buy a BC141 and test the breakdown voltage, what are the chances that
it will exceed 80volt?



I would also like to know how manufacturers make these almost identical
transistors with different voltage ratings.



Do they just make a batch of BC140s and those which exceed 80v get stamped
'BC142'?



Or do they make separate production runs for each type?



Also, do the higher voltage versions loose out on some other parameter such
as gain?



Many thanks,

Rudge



There are a BUNCH of issues here.
What's the actual voltage applied in the application?

Given volume discounts and inventory reduction, it's not unusual
for a vendor to use a part in an application that would do fine
with a MUCH lesser part.

Do you really care about Bvceo? That's relevant if e is really o.
Excellent metric, but sometimes not the metric you want.

If you're running it anywhere near 80V, you might wanna think
about secondary breakdown. Devices with similar text specs
can behave quite differently at high voltages.

Worry about temperature. If you're selecting breakdown voltages,
make sure you measure at temperature extremes.

One simple way to test breakdown is to put your voltmeter in series
with the power supply. IF the meter is 10megs and you have the reading
and the power supply voltage, you have enough info to calculate
the current at various voltages. If you need more current shunt
the meter with a resistor. If you try to use a current meter and the
device goes into avalanche, you've just toasted your transistor and
maybe your meter.

I don't know how they do it today, but back in the day, there were
a few "formulae" for transistors. For a given die, you ran it
across the test bed and put it in whatever bin the specs met.
If it didn't meet any, it went into the 2n3904 bin.

Whether you can up-spec a part by testing depends a lot on the
yield of that part. Some parts have high yield and they
have to ship better parts than the spec requires. But that's
not always the case. Back when I didn't know better, I tried
to select 1% resistors out of 20% for a test jig.
Turned out there was a BIG hole in the distribution that was
10% wide. 10% resistors had a big hole in the distribution
5% wide and so on. Somebody got there before me ;-)
But I fooled them, just recalculated the ratios to be
10-20% off the standard value.

mike

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