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crutschow

Low Power CMOS PWM Circuit With Buffer Output Requires Only 5 Components

CD4093 PWM circuit uses only 2 diodes, 1 capacitor, and 1 potentiometer

  1. crutschow
    PWM (Pulse Width Modulation) is frequently used for efficient control of DC voltage or current.
    Here's a simple, low-power circuit to generate a PWM signal that requires only 1 CD4093 NAND CMOS chip, 1 potentiometer, 2 diodes, and 1 capacitor.
    The pot provides for adjustment of the duty-cycle.

    The microwatt power consumption of this circuit is desirable for any small battery powered circuits that use PWM.
    It's similar in operation to some designs using a 555 timer but consumes much lower power than a standard 555, although probably comparable to the circuit with a CMOS 555 chip.
    Which circuit is preferable is likely mostly determined by the parts' availability and cost.

    The LTspice simulation of the CD4093 circuit is below.

    upload_2017-4-25_22-29-40.png

    U1 is a CD4093 CMOS Schmitt NAND gate configured as an astable multivibrator.
    The hysteresis on the Schmitt input establishes the high and low switch level for the oscillation trigger points.
    The feedback for the oscillation is fed back through two diodes, D1 and D2, and pot, U5.

    The setting on the pot affects the charge time versus the discharge time for C1 which determines the PWM duty-cycle.
    The pot can vary the duty-cycle to within a percent or so of both 0 and 100 percent, depending upon the operating frequency
    The simulated PWM duty-cycle for pot settings of 0% and 100% was about 0.8% and 99.4% respectively for a PWM frequency of ≈930Hz.
    The plot shows the duty-cycle for pot settings of 1% (Red), 50% (Yellow), and 99%(Green).

    The other gates in the package, U2, U3, and U4, are connected in parallel to provide a buffer output for the PWM signal.
    This isolates the output load from the multivibrator circuit, and provides a higher current drive for faster switching of high capacitance loads such a MOSFET gate.

    This circuit can also be built with the hex inverting Schmitt gates 4584 or 40106.
    That would give an additional two gates in parallel to increase the buffer drive capability.

    Required power and ground connections for the CD4093 are not shown.

    Attached Files:

    spec likes this.