Halogen light dimmer provides infinite control

-March 16, 2000

Modern lighting systems use halogen lamps, most of which run on 12V ac from a transformer. The dimmer circuit in Figure 1 can change the intensity of the light from zero to maximum. The dimmer operates at approximately 12V, unlike the usual ones that function by adjusting the firing angle of the 110 or 220V mains supply.

The dimmer works to inject a constant current into the halogen lamp and to regulate that current using pulse-width modulation (PWM) according to a potentiometer-controlled input, or a 0 to 5V signal, or even an analog output from a µC. 12V ac from the transformer, converted to 16.8V dc, powers the SG3524 PWM circuit (IC1). An RC circuit sets the approximately 10-kHz operating frequency. The output of the PWM IC drives the power transistor (Q1), a pnp Darlington. The collector of Q1 connects to a 10-turn ferrite-core inductor; a 1000-µF capacitor affords filtering to provide the bulb with dc current. Op-amp IC2 amplifies the drop across the shunt resistor and feeds the amplified signal back to IC1. IC1 compares the feedback signal with the desired input level from the potentiometer, 0 to 5V, then controls and regulates the current in the bulb.

Current regulation is important, not only because it makes dimming possible, but also because it protects the bulb at start-up (when the bulb is cold). The constant current gives the filament longer life and makes the bulb immune to line-voltage disturbances. Some designs use "electronic transformers," which are basically switching power supplies that drop the mains voltage from 110/220V ac to 12V dc that's pulsed at high frequency. These systems generate higher RFI than the design in Figure 1. In this design, because the controlled variable is current, not voltage, you could use supplies higher than 12V ac to compensate for the drop in the connecting wires in case you wish to place the halogen bulb and dimmer at some distance from the transformer. (DI #2497).

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