The BE resistor in high voltage circuits it's always necessary just to improve the transistor behavior. To understand this please read in your transistor datasheet the signification and value of Vce0, the collector emitter voltage with the base in the wind (the base opened or not connected). Vasile On Sat, Feb 28, 2015 at 12:10 PM, Electron wrote: > > 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) > transistor. > 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 > or OFF, so it will be able to withstand also the leakage from that 200K > resistor (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 equal resistors (half resistance) in serie). > > Basicly the PIC knows the voltage of both sides, and turns ON the switch > when the left side voltage is higher than the right side voltage, with so= me > exceptions. > It's like a diode, but being in software it allows me to do things that a > diode 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 > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=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 .