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Varying Brightness AC Lamp Circuit Design
Description
Within this circuit, a Silicon Controlled Rectifier (SCR) is implemented to gradually modify the power delivered to a 120 volt light bulb through controlling the duration the alternating current (AC) line voltage is applied during each cycle.
Caution:
The circuit is connected directly to the AC power line and ought to be situated within a protective enclosure preventing any contact with the individual components. To avert electric shock, it is imperative not to touch any part of the circuit while it is connected to the AC power line. A 2K, 10 watt power resistor is utilized to diminish the line voltage to 9 volts DC. This resistor will release approximately 7 watts and necessitates sufficient ventilation.
Operation:
Two NPN transistors are deployed for detecting the commencement of each half cycle, subsequently activating a delay timer which then activates the SCR at the termination of the predetermined delay period. The delay duration is defined by a current source, regulated by a 4017 decade counter. The initial count (pin 3) sets the current at a minimum, correlating to around 7 milliseconds of delay, corresponding to almost the entire half cycle so that the bulb is nearly extinguished. Maximum brightness is achieved on the sixth count (pin 1), which is unconnected, allowing the current to be maximal and ensuring minimal delay, consequently activating the SCR near the beginning of the cycle. The remaining 8 counts progressively increase the brightness in 4 steps up and 4 steps down, transitioning between maximum and minimum levels. Each step up or step down provides roughly twice or half the power, creating an illusion of linear brightness variation. The brightness level of each step can be tuned using the 4 resistors (4.3K, 4.7K, 5.6K, 7.5K) connected to the counter outputs.
The circuit has been constructed by Don Warkentien (WODEW) who suggested adding a small 47uF capacitor from ground to the junction of the current source transistor (PNP) to reduce the digital stepping effect, thus resulting in a smoother illumination. The value of this capacitor will depend on the 4017 counting rate; a faster rate would necessitate a smaller capacitor.
Circuit diagram
