Build This Low Cost H-Bridge

Keeping The Design Simple By Using A Relay

The relay in the circuit switches the negative with the positive voltage across the motor saving the use of 3 MOSFETs, 3 diodes and 3 or more heat sinks. Since the H-bridge only uses 1 pin for direction and 1 pin for P.W.M. It also saves pins on your microcontroller.

When you are wiring the relay to your motor make sure that the normally close contacts of the relay make the motor drive your robot forward. Since most robots will spend most of their time going forward. In this way the relay will NOT take any power while the robot is going forward.

There is one down side to this H-bridge, that is if the H-bridge is being run from a circuit you have no control of. Most circuits will use 2 direction signals to control a H-bridge. So that type of circuit would cause the motor to stop very fast using this H-bridge which your gearbox may not like. (see note at bottom)

There is no problem if you are running it from a PIC (microcontroller) you programmed. When you want the robot to stop; just put a low on the P.W.M. pin to coast to a stop.

The PIC and voltage regulator in the photo are only there to show how to interface a PIC with the H-bridge. If you wanted to run the H-bridge at 6 volts then the microcontroller would have to be run from another battery since the voltage regulator would not be able to keep the noise pulses down. Also you would need a 6 volt relay instead of the 12 volt relay.

Major Parts Used:

1 12 volt relay DPDT contact rating 10 amps.
1 power rectifier
1 signal NPN transistor
2 heat sinks

Note: This H-bridge circuit can be made into a 2 direction signals input H-bridge by using two SPDT relays with two signal transistors driving them. See 2nd photo. This may be cheaper since SPDT relays made for cars can handle 40 amps and only cost about $3 each.

Home Page