The H-bridge

Basic Theory-

An H-bridge is an electronic circuit which enables a voltage to be applied across a load in either direction. These circuits are often used in robotics snd other applications to allow DC motors to run forward and backwards. H-bridges are available as intergrated circuits, or can be built from discrete components.
H-bridge, sometimes called a full "full bridge" the H-bridge is so named because it has four switching elements at the "corners" of the H and the motor forms the cross bar. Here is a figure showing a basic gridge.

Structure of an H-bridge (highlighted in red)

An H-bridge is built with four switches (solid-state or mechanical). When the switches S1 and S4 are closed (and S2 and S3 are open) a positive voltage will be applied across the motor. By opening S1 and S4 switches and closing S2 and S3 switches, this voltage is reversed, allowing reverse operation of the motor.

The switches S1 and S2 should never be closed at the same time becayse this will cause a short circuit on the input voltage supply course. the same applies to the S3 and S4. This condition is known as shoot through.

Operation-
The H-Brifge arrangement is generally used to reverse the polarity of the motor, but can also be used to brake the motor, where the motor comes to a sudden stop, as the motors terminals are shortened, or to let the motor 'free run' to a stop, as the motor is effectively disconnected from the circuit.  The following table summarises operation.

S1     S2     S3     S4          Result
1        0        0      1     Motor moves right
0        1        1      0     Motor moves left
0        0        0      0     Motor free runs
0        1        0      1     Motor brakes
1        0        1      0     Motor brakes

Thats all about H-bridges.  Now hopefully I will be able to soon hack the RC car!!!