Electronic transformer dims halogen lamp

Posted on Aug 24, 2012

Figure 1 shows a simplified schematic of a typical low-voltage halogen-lamp transformer without the protection circuits and EMI filter. Q1 and Q2 with C2 and C3 comprise a classic, half-bridge topology that works in self-oscillating mode. The circuit provides positive feedback by placing the primary windings of transformer T1 in series with the bridge output. To achieve a high power-factor value, a rectified but unfiltered mains voltage supplies power to the circuit. The working frequency is approximately 30 to 40 kHz.

This simple circuit does not allow for correctly and conveniently dimming the lamp because any attempts to manipulate the time constant of the R1/C1 start-up circuit worsen the power factor. A possible solution that involves making taps in the primary win

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