![]() While the concept of a high current H-bridge is fairly simple, many practical considerations complicate implementation. For more information on PWM, see Driving a DC Motor using PWM and Pulse Width Modulation. Generally 20kHz is a good choice for PWM frequency because it is well beyond of the dynamic range of motors and just beyond the range of human hearing. If the voltage reversals are at a high enough frequency, the cycling is unnoticeable. To keep the motor stationary, forward voltage is applied half of the time and reverse voltage is applied half of the time ( PWM=50% duty). To make the motor spin in reverse, 1=4=off and 2=3=on (PWM=0% duty cycle). To apply a forward voltage across the motor, mosfets 1=4=on and 2=3=off, causing the motor to spin in the forward direction ( PWM=100% duty cycle). The mosfets are used as switches and are activated in diagonal pairs. In the circuit diagram we see that the 4 mosfets surrounding the motor form an “H” shape. The H-bridge described in this write-up is capable of currents up to about 40A at 24V, but requires the assembly of a PCB. Full H-bridge circuits capable of a few Amps can be purchased in convenient IC packages (see PWM and H-bridge chips). The convenience of an H-bridge is that a low current digital signal can be used to control a high current motor (or other device). An H-bridge is a circuit configuration commonly used to control the speed and direction of a brushed DC motor. ![]()
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