Electronic – Why are bipolar transistors often used with the base at vcc

Consider the operation of the circuit.

When the transistor is on current is flowing in the coil from top to bottom as the circuit is drawn we now switch the transistor off. The current in the coil still wants to flow.

For the circuit on the left this current can now flow back to Vcc via the diode the voltage across the coil has reversed direction and is limited by the diode the current can decay to zero safely.

For the circuit on the right the diode does not help. The current flowing in the coil will force the voltage on the collector to rise to the point where the transistor (or possibly the diode) breaks down and starts to conduct. At this point the current can start to decay in the coil but the energy in the broke down transistor (or less likely diode) will be excessive and may well result in the transistors death. Note a zener diode here will work because you allow the voltage on the coil to reverse so the current can decay to zero while limiting the voltage across the transistor to a safe value.

It should be noted the allowing the voltage across the coil to reverse to an higher voltage means the current can decay more quickly which is why you sometimes see a zener in the right hand circuit or more than one diode in series in the left hand one.

The problem seems to be as obscure as expected. Not related to having the supply loaded or anything. I am simply using a capacitor to smooth out the signal to the base and the circuit works. Discovering further, this external power supply is intended to be earthed and wasn't. Even still, the capacitor seems to be necessary for startup to be successful.

I edited the above schematic, adding the 187uOhm resistor to pull the base down and open the main circuit, also to discharge the 680nF capacitor. With those the circuit works well, controlling the ATX PSU with on and off signals.