Halogen light dimmer


Posted on Mar 24, 2013

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.



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 disturban




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