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Efficient dc lamp dimmer - 74C14

This circuit uses a 74C14 to form an efficient dc lamp dimer. This circuit uses a pulse-width-modulated method to yield an efficiency of 96% or better. Inverter IC1A forms a narrow pulse oscillator with a frequency of about 200 Hz, and a pulse width of approximately 0.1ms. Potentiometer R1 sets the oscillator's fequency as well as the end limit of R2, which is the brightness control. The charge time on C1, set by adjusting R2, varies the pulse width from narrow to wide. R2 provides a good linear brightness adjustment that includes the extremes of fully off to 100% on. With lamp loads of up to 24W, there's no need for a heat sink on the power MOSFET. The circuit's inefficiency is due mainly to the voltage drop across the MOSFET (using a power MOSFET with a lower RDS-on produces a more efficient circuit). The circuit is suitable for controlling heating elements as well as some dc motors.

Source: "Electronics Design" Magazine, Mar, 7, 94

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