I'm coming to electronic from sw side so I have holes in my e-knowledge… please help solve this situation.
I have an MCU pin connected to a BC327 pnp base through an 1K resistor. I wish to switch the power of something else with that. The emitter of transistor connected to +5V. The collector is not connected to anywhere yet.
When I pull the MCU pin to 0V base gets low and the transistor gives +5V on its collector – it is what I expected.
When I pull up to +5V the MCU pin (and therefore the base) I can measure about -10V at the collector of the transistor relative to the 0V. Ups, it's not what I expected. I expected 0V.
I'm afraid of damaging the whole stuff and I don't dare to make more experimentations. So my questions are:
– is it normal when collector is not connected to anywhere?
– is it refers to a damaged transistor potentially?
– if the answers to the above questions were "no" what else can cause this symptom?
Thanks in advance,
F
Best Answer
It's not normal. When the transistor is 'off' it is effectively an open circuit save for a bit of leakage to the +5V rail. A normal meter provides a an resistance of maybe 10M or 10G+ in some cases to the 0V rail. In any case, the voltage should be between 0V and 5V (and most likely close to 0V).
If you have a negative supply somewhere in your circuit, maybe there is a resistance between that and the PNP collector, if not then maybe something is up with your meter- it should read 0V when the test leads are open.