--- In email@example.com
, "B. Degnan" <billdeg@...> wrote:
> Pin 1-2 should be -12V (base to collector)
> Pin 1-3 should be 5V (base to emitter)
> my base to emitter = 0 = bad?
The base to emitter is the input to the transistor.
It can be anywhere from 0V up to .7V (or 1.4V for darlington type transistors). Typical is .6V. This is really a current input, the B-E junction acts like a diode. So this means, between 0V and ~.4V, the input current is minimal and the transistor is off. Between ~.4 and ~.7 the input current increases exponentially and the transistor starts to conduct until it's fully on.
The collector to emitter is the output from the transistor. When the transistor is OFF, this voltage will be the highest it can be (like voltage across an open switch). When the transistor is ON, the voltage drops to Vce saturated (1V for this transistor as per datasheet).
Negate/Invert all voltages for PNP type transistors (ie, 0V to -.7V)