> what happens to small signal NPN bipolar transistors (BC548C) if I apply
to the base -12V and +12V, when the collector is always at +5V respect to
the (grounded)emitter? Currents are small, few tens mA max.
>
> Should I fear damage or bad behaviour? I need to use the transistor as a
RTL (Resistor Transistor Logic) simple switch.


What? - still alive ?
Wonders will never cease
:-)

DON'T do the mains trick again!!!

Re the transistor.
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.
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.
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?



        Russell McMahon

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