Electronic Design

Wide-Range Pulse-Width Modulator Uses 555 Timer

Simply adding a diode and potentiometer to a 555 timer operating in the asynchronous mode yields a pulsewidth modulator (PWM) with a duty factor adjustable from 1% to 99% (Fig. 1). Applications would include speed control of electric motors where the switching drive can be power efficient.

The output of this circuit could drive a MOSFET to control the current through the motor, resulting in smooth control of the motor speed at efficiencies on the order of 90%. This also applies to lighting control, where the light intensity can be controlled efficiently.

Another application is in switching power supplies, where the PWM adjustment permits a variable output voltage. Feedback to the VC terminal of the 555 timer (pin 5) can be used to regulate the voltage. An output voltage in excess of the regulation threshold would force an early termination of the PWM signal on a cycle-by-cycle basis by pulling down on the VC terminal to maintain the output voltage.

A microprocessor could adjust the PWM directly through a digital potentiometer to control motor speed, light intensity, or power-supply output voltage.

For the duty factor (DF):

see equation

where:

see equation

and a is the ratio of the resistance between terminals 2 and 1 to the resistance between terminals 3 and 1 of R2. Choose R3 = R1, and R2 = 100 X R1. This will allow a duty factor of 1% to 99%.

As mentioned, a digital potentiometer can be substituted for R2. Limited current flow is the main constraint in using a digital potentiometer in this application. For a 100-kO digital potentiometer, R1 and R3 would be 1 kO, resulting in a peak current flow of 5 mA.

A standard diode may be used for D at reduced linearity. For an ideal diode, k = 0.693 and:

see equation

resulting a linear relationship between DF and a. Figure 2 shows the VOUT waveform as a is varied.

See figure 2

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