Hello,
please take a look at the following circuit:
http://oi57.tinypic.com/256h0fa.jpg

I use a high voltage (500V) PNP transistor as a high side switch.
With a PIC pin I turn it ON or OFF, through the NPN high voltage (500V) tra=
nsistor.
The 200K resistor is to partially turn ON the transistor when the PIC is in=
 tri-state (e.g. during brief reset time). Then the PIC will be either ON o=
r OFF, so it will be able to withstand also the leakage from that 200K resi=
stor (2 mA worst case, the protection diodes can withstand it). 200K and 2K=
 resistors are of course rated for 400V (actually I use a couple of 200V eq=
ual resistors (half resistance) in serie).

Basicly the PIC knows the voltage of both sides, and turns ON the switch wh=
en the left side voltage is higher than the right side voltage, with some e=
xceptions.
It's like a diode, but being in software it allows me to do things that a d=
iode cannot do.
Switching speed can be in the KHz region. The impedance is not really low (=
currents are 100 mA peak max, and limited by the source anyway).

Now the question: do you think that the Rb resistor is really necessary?
If so, why?

It's not for the cost :D it's to learn something and improve my knowledge.

Thank you very much.

Cheers,
Mario

--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.