INDEX

L298
Stepper motor driving with ULN2003
Stepper motor diriving with L293D
Another  motor driving circuit
 

MOTOR DRIVING CIRCUITS(H-BRIDGE)

In roboticsindia and many other forums i have seen questions about connecting motors directly to parallel port ,output of IC555,opamp etc.. The answer is that if you connect motor directly to parallel port,555,324 etc. then the IC may be damaged, same with parallel port(sometimes burning your motherboard). So if you want to connect motor to the output parallel port etc.. you should have a motor driving circuitary. The reason is that a normal motor current rating is greater than 250mA, for steppers it is 600mA. The output current capacity of these IC's are very low. For IC555 the maximum output current is 200mA. So if you connect a 250mA motor to the output, it drives more current from the IC and the IC is not able to provide enough current motor requires and finally this may result in the burning of IC. So if you start with an IC, see its maximum voltage, current ratings etc.. Same problem occur with 741, its maximum power dissipation is 500mW and the  voltage of 741 is about 12V . For DC I=P/V=500mW/12v which is less than 100mA, so it is not able to drive a motor. A parallel port basically consist of digital IC's of 74LS series having maximum source current of 2mA and sink current 24mA(see IBM definitions for parallel port). In digital IC's there are two current source current, i.e, when output is LOGIC HIGH and sink current when output is LOGIC LOW. So if you want to drive a motor the output should be LOGIC HIGH, if you use parallel port voltage for the motor. You can do other way connecting external voltage at one terminal and other terminal of motor to parallel port. In this case output should be LOGIC LOW(0) to the motor to
run. But in both cases the required current is greater than the source and sink current, so it is not possible to drive. But in case of a power transistor you can see that we connect motor at the collector side and maximum collector current in the range of Amperes. So a power transistor is able to drive the motor. See the other parameter VCEO it is greater than 50V for a normal power transistor. Therefore a power transistor ratings is decided by comparison between the maximum voltage and current ratings of the motor and VCEO(max) and maximum collector current of transistor. Here we are talking about driving a DC motor. Stepper motor driving will be discussed later. In DC motor driving H-bridges are used. While choosing an H-bridge compare the maximum current and voltage ratings of your motor and that of H-bridge.