What is an H-Bridge?

      An H-Bridge is a circuit that is commonly used to control DC motors and stepper motors. We used an L293D H-bridge chip made by ST Microelectronics for our project. The basic idea is that five electrically controlled switches (transistors) are arranged to control an output, usually a motor, using a separate input voltage. Two of the inputs, 'channel 1' (CH1) and 'channel 2' (CH2), control two switches each. If one channel is a logic high and the other is a logic low, then there is a voltage across the output terminals equal to the input voltage. If the channels are switched then the output voltage changes polarity (reverses the motor direction). If both channels are at the same (high or low) then the output terminals are short circuited. If the circuit is being used to drive a DC motor, then this setting will make the motor stop quickly. The fifth switch is connected in series with one of the output terminals and is controlled by the 'enable' (EN) input. If enable is a logic high (on) then the switch closes and the output behaves according to the guidelines above. If the enable is logic low (off) then there is an open circuit (very high resistance) between the output terminals. This state supercedes anything done with the input channels. When using this circuit to control a DC motor, the enable = low setting allows the motor to float to a stop.

Truth Table for a basic H-bridge