Olin, Yes, I agree with lowering the base resistor to allow more current in the base circuit to drive the transistor on harder, which is what I suggested in the beginning, (as did several others). About the heating effects, that is a very likely possibility. And I agree with your conclusion there. Maybe I am doing things wrong when I do this, but I have always put a pullup resistor on an NPN transistor to pull the base high when I want to make sure the transistor is on when I want it to be on, or a pull down resistor when I want to make sure it is off until I drive it high, regardless of what I have driving it. And so far, when I have done this, I have yet to have any problems with heating or false turn on's or off's. Maybe it's not good design practice, but it work well, so I probably won't change. Anyway, when I suggested he add a resistor from base to emitter, it was my experience talking. As I said before, I have always done this and haven't had any problems. And I also suggested he change the base resistor to 1K ohms to get the base drive needed. And someone suggested using a heavier transistor. I believe this would be a good idea. Someone also suggested that an Hfe of ~20 would be good for a power transistor. The transistor he is using is not a power transistor, but a small signal transistor. So, again the heftier transistor would be a good idea. So, bottom line from my point of view is I am going on my experience and how I would do it. You on the other hand have a much higher education than me, so who am I to argue. Regards, Jim On Wed, 13 June 2001, Olin Lathrop wrote: > > > Since he is using a PNP transistor, he has it configured in the > > upside down form which is correct for a PNP transistor. This > > is so that you can relate it to an NPN transistor so that the bias > > polarities are correct. So, in connecting a resistor from Base to > > Emitter, this is similar to connecting a resistor from Base to > > Collector on an NPN transistor. Except that on a PNP, this resistor > > will force the transistor to turn off until driven low, which turns > > it on. In an NPN transistor, this resistor would keep it turned on > > until drive off by a low. > > Yes, I understand all that (and have for about 25 years, some of which were > spent earning a masters degree in electrical engineering from RPI). > > > And as far as the PIC pin is concerned, it's general knowledge that > > a TTL/CMOS output is better at sinking current than at sourcing it. > > So, therefore, pull the pin up, and drive it low. > > I agree with your facts but not your conclusion. PICs use totem pole CMOS > outputs. These actively pull both down and up. It may be true that the > pullups are a bit weaker than the pulldowns, but shouldn't matter much > because there should be virtually no current when the base is driven to very > near the emitter. In other words, we are looking at the PIC's open circuit > high output voltage, which is very close to the power rail. The base won't > start drawing meaningful current until it is forward biased at least a few > 100mV. > > I think what was happening is that the transistor wasn't driven hard enough. > This kept it from going into saturation, which caused the voltage drop > accross it to increase, which caused it to heat up. Its temperature > probably exceeded specs, which caused it to leak accross the base-emitter > junction, which caused it to stay on. The correct solution is therefore to > provide more base current in the on state by lowering the base resistor to > 1K ohms. > > > ******************************************************************** > Olin Lathrop, embedded systems consultant in Littleton Massachusetts > (978) 742-9014, olin@embedinc.com, http://www.embedinc.com > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. jim@jpes.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.