pulse width modulation

Pulse Width Modulation, also called PWM, is an excellent method of controlling the amount of power delivered to a load without dissipating wasted power because the average power delivered to the load is proportional to the duty cycle of the modulation.

Many people try to control the speed of a DC motor by using a large variable resistor (called a rheostat) in series with the motor to control the amount of current and therefore the speed of the motor. While this may work, it creates a lot of heat and energy wasted on the resistor. A simple way to control the speed of a motor is to regulate the amount of voltage across its terminals. This is achieved using “Pulse Width Modulation” or PWM by driving the motor with a series of “ON-OFF” pulses instead of a continuously varying (analog) voltage. By increasing or decreasing the pulse widths, the PWM circuit regulates the power flow to the motor while keeping the frequency constant.

The power applied to the motor can be controlled by varying the width of these applied pulses and thus varying the average voltage applied to the motor terminals. By changing or modulating the timing of these pulses, the speed of the motor can be controlled, i.e. the longer the pulse is “ON”, the faster the motor will turn, and likewise the shorter the pulse is “ON”. ON”, the slower the motor will turn. to turn. The difference in the ratio between the “ON” pulse and the “OFF” pulse is called the duty cycle. Duty cycle is expressed as a percentage related to the fraction of time the output voltage is “ON” and can range from 0 to 100 percent. So for a 0 percent duty cycle, the motor is completely off, 50 percent of the motor is running at half speed, and 100 percent of the motor is fully on.

Using pulse width modulation to control a small motor or fan has the advantage that the power loss in the switching device is small because it is either fully “ON” or fully “OFF”, therefore the switching device switching has a much lower power dissipation. providing a type of linear control resulting in better speed stability. Also, the amplitude and frequency of the motor voltage remain constant, so the motor always runs at full power.

An added benefit of pulse width modulation is that the pulses reach full supply voltage and will produce more torque in a motor by being able to overcome internal motor resistances more easily. The result is that the motor can run slowly without stalling. Also, in a pulse-width modulation circuit, small commonly available potentiometers can be used to control a wide variety of motor loads, whereas large, expensive, high-power variable resistors called rheostats are needed for resistive controllers.

Pulse Width Modulation can also be used to control the brightness of light sources such as DC lamps or light emitting diodes. Light-emitting diodes (LEDs) turn on and off extremely quickly and appear to flicker if a low-frequency voltage is supplied to them. The effect of this flicker can be reduced by increasing the pulse width modulation frequency. If the “ON/OFF” switching frequency is fast enough, the human eye cannot see the action of this switch and only sees the brightness of the lamp without blinking.

Pulse width modulation is an excellent method of controlling the average amount of power delivered to a load without dissipating wasted power. PWM circuits can be used in many different applications to control the speed of motors and fans or to dim the brightness of DC or LED lamps. If you need to control it, use pulse width modulation to do so.