# Full-bridge or half-bridge for a DC motor

So I have a DC motor which draws around 16A at 12V. I want to have some room to play with in the top values. I need to control the spinnig direction as well as speed.

I understand how half-bridges and full-bridges work. From that it seems to me that half-bridge is not an option for a DC motor as it can be used only for creating AC and not DC. On the other hand full-bridge can be used for both AC and DC. Am I right?

I am asking because many sites mention the differences between the two, but I have not found a site that would mention the difference when using it to control a DC motor.

• You can use a half bridge for directional control, as well as speed control, if you have a bipolar supply. This is generally practical only with low power motors. – EinarA Jul 31 at 6:46

For bidirectional control of a DC motor you typically need a full bridge made up of four FETs or transistors. Each lead of the motor is connected to a half bridge where the upper transistor can take it to the positive supply or the lower transistor can take it to the negative. By setting either side high and the opposite low, you can make the motor spin in either direction.

For unidirectional control, you really only need a single FET or transistor. You can leave one side of the motor permanently connected to the supply, and use the transistor to connect or disconnect the other.

Half bridges that are not packaged in pairs as full bridges are somewhat associated with AC motors particularly because of the situation of 3-phase motors where you end up needing 3 half bridges. In that case you have three wires coming out of the motor, each of which gets driven by its own half bridge so it can be connected to either side of a DC supply. But rapidly pulse-width-modulating the half bridges, something approximating an AC sine wave can be produced, and if the three bridges do this in an appropriate phase relationship (0, 120, 240 degrees) then you can make the motor spin just as if it were connected to a 3-phase AC dynamo - but with the added benefit that you can vary the frequency of the synthesized AC to control speed.

Finally most bipolar stepper motors have two independent coils and require two full bridges.

• For unidirectional control, you really only need a single FET or transistor. ... and a freewheel diode. – Huisman Jul 30 at 18:18
• Thanks, I king of know all of this. I just wanted to make sure I have everything needed for a bidirectional control as it requires a full-bridge while half-bridge is not enough. – MStarha Aug 1 at 10:18

With half bridge you can't change the polarity of the motor and thus you can't change the direction. If you don't need to change the direction a half bridge is fine, otherwise you need an H bridge.

• True, but misses the point. Even a half bridge is more than is needed for unidirectional control, as it still has two FETs. Unidirectional control only requires 1; while bidirectional control required a full bridge made of 4. – Chris Stratton Jul 30 at 18:05
• @ChrisStratton It depends how you define control. 1 transistor is needed for bang-bang-control (I like this term in this situation as you get the full inrush current each time), 2 are needed for unidirectional PWM voltage control. – Huisman Jul 30 at 18:13
• No, two are not needed for PWM, that's not how it works at all. You may be overlooking the role of the winding inductance. – Chris Stratton Jul 30 at 18:15
• Two transistors are needed if you want some sort of active braking, not for PWM. – Vladimir Cravero Jul 30 at 18:18
• Yes, both of you are right. PWM can be done with 1 transistor. My previous comment was wrong in case of PWM, (but ill leave it there for the context of the comments) – Huisman Jul 30 at 18:19

I understand how half-bridges and full-bridges work. From that it seems to me that half-bridge is not an option for a DC motor as it can be used only for creating AC and not DC. [...]
Am I right?

The bold marked statement is incorrect.
Considering a half bridge being supplied by a grounded DC voltage:

• If the upper transistor in the half bridge is continueously conducting and the lower is never conducting, the half bridge voltage will be equal to the upper rail voltage, so, equal to the DC supply voltage.
• If the lower transistor in the half bridge is continueously conducting and the upper is never conducting, the half bridge voltage will be equal to the lower rail voltage, which is ground, which is DC as well.
• If the upper and lower transistor are conducting in turns, the half bridge voltage will be equal to a PWM signal and can be considered as an AC wave with a DC offset.

So, if the supply voltage is DC, the half bridge voltage always has a DC component.

• Yeah it is not a geniune AC (not positive and negative voltage, just positive and ground), but my point was that it cannot be used for bidirectional control of a DC motor. – MStarha Aug 1 at 10:15