Friday, February 6, 2009

Learned all about transistors and "NPN open-collector ouputs"!

Transistors consist of a base, emitter, and collector. When no current is applied through the base to the emitter, no current flows from the collector to the emitter. However, when current does flow through the base to the emitter, a proportional amount of current flows from the collector to the emitter. The relationship is: Ib = B*Ic.
So... a little base current allows a much larger collector current to flow. (B approx = 100). Oftentimes, Ic can only go so high (depending on its source power). Thus, even if Ib is very high and via the relationship Ic should be huge, Ic will only go as high as it can based on the circuit its hooked up to.

NPN open-collector output:
Now, take the idea of a transistor and put it at the output of some circuit. More specifically, connect the circuit output up the the transistor "base", connect ground to the "emitter", and leave the collector open (hence open-collector output). Now, you have a connection (the collector) that is either a short circuit or an open circuit, depending on whether the base is receiving current from the circuit.
Use this feature to hook up any other circuit you want to the collector, without worrying about affecting the internal circuit!

NPN vs PNP: these are two types of transistors. They differ in that a NPN is typically closed unless a current is applied to "base", where as PNP is open unless a current is applied to "base"