>If you do what you say you did the transistor will die.
>HOWEVER, you probably omitted to say that there was a resistor between the +
>or - 12v supply and the base.

Right, a 10 Kohm resistor, to limit current (but I don't know what the
base-emitter impedance of the transistor is, so I don't know yet if the
value is a valid one. Clearly, I'm no engineer.. just a hobbyist beginner.
However, I just ordered the book "The Art Of Electronics", hoping I can
make any sense from it, of course, which I'm not betting too much on).


>If so, the transistor will possibly survive.
>
>For +12v applied via a resistor, the base will "clamp" at about 0.7v above
>ground. The resistor will drop the rest of the voltage.
>The current will be APPROXIMATELY (12-0.7)R.

So (assuming it's /R, not *R) it'd be about 1.13 mA.. which from the
datasheet and my very limited understanding abilities *seems* enough to
saturate the transistor.


>eg for a 1k resistor you will get about 11 mA.
>Whether the transistor survives depends on the transistors rating for base
>current.
>See spec sheets to see.
>
>For -12v applied via a resistor.
>For lower values of voltage the transistor acts like a reverse biased diode.
>Above a certain critical value the reverse biased junction acts like a zener
>diode.
>This is typically around 9 volts but varies widely with transistor type.
>If you apply -12v via a resistor and the base reverse breakdown is -9v the
>resistor will drop the difference.
>here Vresistor = 12-9 = 3volts.
>Again, whether the transistor survived depends on the value of R and
>transistor reverse bias base current rating.
>
>Were you using a base resistor?
>If so, what value?

10K

It was my "idea" that the value is high enough to make the BC548C survive
under all conditions, but of course I'm not sure and I wanted to ask.. and
learn something in the process.

As some pointed out correctly, it's for a RS232->CMOS simple interface.

Thanks,
Andrea


>
>
>
>        Russell McMahon
>
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