Making a pin sink?

[Dave]

kevj, Mister E drew a nice picture of what I was talking about. The diode I mentioned was because I had no idea of what potential you were trying to switch. I didn't think i had to draw you a picture.....

Dave Purola,
N8NTA

[ErnieM]

The “bridge to Vcc” resistor is needed cause transistors ain’t perfect. There is always a leakage current from B to C that needs come from somewhere. If you don’t provide a source for it then it comes from E to B, which is real base current, which makes the device begin to turn on, which increases current out the collector, which burns power, which heats the device, which increases the B to C leakage that needs come from somewhere …

The bridge resistor gives the current a source path to keep it from going E to B. I personally go much higher, like 100x.

In this specific case, if Vcc also supplies the PIC you don’t need the bridge resistor, because it is only needed when you turn the device off, and at that time the base resistor is connected to Vcc (thru the output)… so you actually get the same benefit of the bridge from the base resistor.

The same is true driving grounded emitter NPN’s thru an R from an output, you don’t really need the bridge.

But if in any doubt, resistors are cheap; put it in.

[kevj]

The “bridge to Vcc” resistor is needed cause transistors ain’t perfect. There is always a leakage current from B to C that needs come from somewhere. If you don’t provide a source for it then it comes from E to B, which is real base current, which makes the device begin to turn on, which increases current out the collector, which burns power, which heats the device, which increases the B to C leakage that needs come from somewhere …

Outstanding! Thank you Ernie - that actually helps a lot - I'm much more clear now. Total lightbulb moment.

[mister_e]

while i can't 100% agree with explanation for a simple G.P. NPN/PNP, i will suggest you a simple test... configure an I/O as input,and measure the voltage between it and GND.. what do you get?

What's the trip point of a transistor?

[kevj]

a) a simple G.P. NPN/PNP

b) configure an I/O as input,and measure the voltage between it and GND.. what do you get?

c) What's the trip point of a transistor?

a) What is a G.P. ???

b) With the power on or off?

c) Don't know.

d) Can I buy a vowel?

[ErnieM]

The last one is easy!

c) What's the trip point of a transistor?

It's a trick question. Transistors are analog devices; they don't have anything like a well-defined trip point.


And I'll go out on a limb here and guess "GP" is General Purpose.