Motor Driver H-BRIDGE Concept Pt-1

H-BRIDGE:

The H-Bridge is designed to drive a motor clockwise and anticlockwise. To reverse a motor, the supply must be reversed and this is what the H-Bridge does. An H-Bridge can be made with SWITCHES, RELAYS, TRANSISTORS or MOSFETS.

H-Bridge with switches:

Do not make circuit "A." It can easily create a SHORT-CIRCUIT. It is only a demonstration circuit.

Switch A and D will make the motor rotate clockwise. 
Switch B and C will make the motor rotate anti-clockwise. 
Switch A and B will create a BRAKE. 
Do not close switch A and C at the same time. 
Do not close switch B and D at the same time. 

An improved design is shown in Circuit C. It does not create any short-circuit:

H-bridge Motor Control

So how does it work?. The four transistors are connected together in a “H-bridge” configuration with the motor connected in the middle. To make the motor rotate in the forward direction, a high (logic “1″ or +5V) signal is applied to the forward input, while no signal is applied to the reverse input (applying a voltage to both inputs at the same time is not allowed). The speed of the motor is controlled by using the pulse width modulating signal as before. Transistors TR1 and TR4 conduct. Current flows from terminal A through to terminal B (left-to-right direction) of the motor. To reverse the motor’s direction a high signal is applied to the reverse input and transistors TR3 and TR2 conduct, allowing current to pass through the motor in the opposite direction from terminal B through to terminal A (right-to-left direction). The flywheel diodes, D1 to D4 across the transistors of the H-bridge motor control circuit help to protect the transistors from any induced back emf generated by the motor during braking. Any suitable PNP or NPN transistor can be used other than the ones above, but all the transistors should have high power ratings, if not use heatsinks.While it is possible to construct the H-bridge motor control circuit from scratch using individual components, there are lots of IC’s and “black boxes” available off-the-shelf to make motor speed control and design far easier and usually cheaper. Two H-bridge motor control IC’s that are popular and easy to connect are the National Semiconductors LMD18200 and the Texas Instruments L293D motor-driver chips. Both are easy to use with in-built diodes, shorted circuit protection and are TTL and CMOS compatible.