Circuit works with BC557 PNP transistor, but not TIP32C?
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.







simulate this circuit &ndash; 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.