Electrical – Circuit works with BC557 PNP transistor, but not TIP32C

pnptransistors

I am high side switching a 5v collector with a 12v base. On the emitter side is just an led and 10k ohm resistor to ground. The base resistor was 1k ohm.

This circuit works just as I expected it with a BC557 PNP transistor, (when 12v base is removed led goes off, with 12v base applied led is on @ 5v) but as my intended load will have a current of 2A or so I need to use the tip32c instead. Problem is, I have no voltage at all on the emitter pin when I try it. I've checked the pinout multiple times (EBC on bc557 and BCE on TIP32C) but it seems that not the issue. I've also experiment with every combination of pinout. I guess what's left is the base resistor, but does that need to be varied with the transistor?

Thanks

Best Answer

You are not using a PNP transistor the correct way. With the base raised to 12V and the emitter as the output, the PNP B-E junction is being reverse bias. The BC557 B-E junction breaks down and current is supplied to the load through the break down. The TIP32C displays a different behavior probably because it breaks down at a higher B-E voltage.

With the description of your circuit, you want to use a NPN transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

Better yet, use a N-MOSFET. Since the gate of a MOSFET needs essentially zero DC current, the 12V signal does not have to supply a big BJT base current (order of 100mA) for the 2A output case.