Electrical – Mixed NPN and PNP H-Bridge DC Motor Driver

h-bridgemotornpnpnptransistors

I've briefly looked at including an H-Bridge circuit in one of my designs and I see that most (if not all) have N-Type transistors (either Darlington
or MosFets). This however needs two separate control signals to turn one half of the circuit on while the other is off.

Would the following circuit work? The theory (in my head at least) is that when the "Motor-Direction" signal is high N-Type Transistors T2 and T3 would be enabled while P-Type transistors T4 and T1 are disabled. Alternatively When the same signal is Low the opposite would happen. T5 would simply cut the power to the entire circuit and allow the motor to slow down and stop via the "Motor-Enable" line. I get that you would not have the Motor-Stop or fast reverse capabilities, but for a simple circuit that doesn't need that, would this not work just fine?
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Best Answer

No, your circuit will not work at all, so don't even try it.

Let's just for a second leave Motor-Direction unconnected (open circuit) and inspect what will happen if T5 (Motor-enable) is turned on, the voltage at the top of the H-bridge would want to be at about 11V (T5 saturated).

However, if you examine T3 and T4 they represent a current path of 4 Base-Emitter junctions with a forward voltage of about 2.4V. Therefore T3, T4 would draw current based solely on the limitation of T5 and your power supply and hold the top of the H-Bridge at about 2.4V. If your power supply can deliver enough current then T5 would be held out of saturation.

T5 will get very hot and eventually be thermally destroyed .....and if T5 short circuits then you would likely destroy either T3 and T4 next.

The Base-Emitter junctions are simply acting as diodes and it makes almost no difference to the problem if you provide a current drive or not.

If you want a simple circuit like to work, you'd have to use FETs which are voltage driven.