A Constant Torque DC Motor Speed Controller Circuit Using IC556 Explained

Controlling or varying a DC motor speed may appear to be not so difficult and you may find plenty of circuits for it. However these circuits do not guarantee consistent torque levels at lower motor speeds, making the functioning quite inefficient. Moreover at very low speeds due to insufficient torque, the motor tends to stall. Another serious drawback is that, there’s no motor reversal feature included with these circuits. The proposed circuit is completely free from the above shortcomings and is able to generate and sustain high torque levels even at lowest possible speeds.

Circuit Description

Before we discuss the proposed circuit, we would want also to learn the simpler alternative which is not so efficient, yet may be considered reasonably good as long as the load over the motor is not high and as long as the speed is not reduced to minimum levels.

The figure shows how a single 555 IC can be employed for controlling DC motor speed, we won’t go into the details, the only notable drawback of this configuration is that the torque is directly proportional to the speed of the motor.

Coming back to our main design, here we have used two 555 ICs instead of one or rather a single IC 556 that contains two 555 ICs in one package.

Briefly the proposed DC motor controller includes the following interesting features:

Speed can be varied continuously right from zero to maximum, without stalling.

The torque is never affected by the speed levels and remains constant even at minimum speed levels.

The motor rotation can be flipped or reversed within a fraction of second.

The speed is variable in both the directions of the motor rotation.

The two 555 ICs are assigned with two separate functions. One sections is configures as an astable multivibrator generating 100 Hz square wave clocks which is fed to the preceding 555 section inside the package.

The above frequency is responsible for determining the frequency of the PWM.

The transistor BC 557 is used as a constant current source which keeps the adjoining capacitor at its collector arm charged.

This develops a saw-tooth voltage across the above capacitor, which is compared inside the 556 IC with the sample voltage applied externally over over the shown pin-out. 

The sample voltage applies externally can be derived from a simple 0-12V variable voltage power supply circuit. 

This varying voltage applied to the 556 IC is used to vary the PWM of the pulses at the output and which eventually is used for the speed regulation of the connected motor.

The switch S1 is used to instantly reverse the motor direction whenever required.

Parts List

R1, R2, R6 = 1K,

R3 = 150K,

R4, R5 = 150 Ohms,

R7, R8, R9, R10 = 470 Ohms,

C1 = 0.1uF,

C2, C3 = 0.01uF,

C4 = 1uF/25V

T1, T2 = TIP122,

T3, T4 = TIP127

T5 = BC557,

T6, T7 = BC547,

D1---D4 = 1N5408,

Z1 = 4V7 400mW

IC1 = 556,

S1 = SPDT toggle switch


The above circuit was inspired from the following circuit which was published long back in elecktor electronic India magazine.