Halogen Lamp Dimmer With Soft Start

Posted on Feb 7, 2014

Most dimmers use pulse width modulation (PWM) to control the amount of power that is delivered to the lamp. Those that come bundled with a switch faceplate control the firing angle of a Triac on the 240V mains side. These work fine with resistive loads but may not be suitable for inductive loads such as low-voltage halogen lamp transformers. This

Halogen Lamp Dimmer With Soft Start
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circuit also employs PWM but it switches at a high frequency (22kHz) on the low-voltage side of the lamp transformer. This high frequency also simplifies EMI filtering. Furthermore, because this circuit is isolated from the mains by the transformer, it is relatively safe to build and install. IC1 is a standard 555 astable oscillator with a high duty cycle. It produces a narrow negative-going pulse at its pin 3 output approximately every 45 µs (ie, the frequency of oscillation is about 22kHz). These pulses trigger IC2, another 555 timer, this time wired as a variable monostable. IC2`s pin 3 output is normally low which means that its internal discharge transistor is on and the 1nF capacitor on pins 6 & 7 is discharged. However, when the monostable is triggered (by IC1), its output goes high, the internal discharge transistor turns off and the 1nF capacitor charges via VR1 & VR2 until it reaches 2/3Vcc. At this point, the output at pin 3 switches low again. Each time pin 3 of IC2 goes high, it turns on power Mosfet transistor Q1 which in turn switches on the lamp. Potentiometer VR2 is used to control the time it takes the 1nF capacitor to charge to the threshold voltage and thus sets the width of the output pulses. At maximum resistance, the pulse width is 55ms. This is longer that the 45ms period of oscillator IC1, and so IC2`s pin 3 output is high for 100% of the time and the lamp operates with maximum brightness. Now...

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