How to make an H-bridge

Step 2 The truth about H-bridges.

An H-bridge is a series of four controllable switches in which there are two sets of two switches.

One set of switches when closed allows electricity to flow one way. The other set of switches allows electricity to flow in the opposite direction.

Another important characteristic of an H-bridge is that it typically can use a smaller voltage (5VDC from a micro controller, for instance) to control a much larger voltage (12VDC used to power a motor). These two separate voltage sources are kept isolated from one another.

H-bridges can be made with either 4 relays or 4 transistors.

The best type of H-bridge is made with transistors since these can easily be used to control a motor's speed (by using the smaller voltage to modulate the much larger motor voltage). An H-bridge made using relays (like the one we are making) cannot easily be used to change a motor's speed (unless, of course, the motor voltage is being supplied from a power source capable of being modulated... i.e. the H-bridge itself cannot modulate the power source to down the motor. The power must be decreased to slow down the motor before it goes through the H-bridge).