Varying Brightness AC LampIn this circuit, the SCR is required to be able to slowly range the intensity of an 120 volt lamp through managing plenty of time which the AC line voltage will be applied to the light while in every half cycle.
Caution:
This circuit can be straight connected to the AC power line and also needs to be inserted inside of a enclosure which will reduce direct contact with every of the parts. To prevent electric shock, never touching every component to the circuit while it is connected to the AC power line. A 2K, 10 watt electric power resistor is required to fall the line voltage as a result of 9 volts DC. This resistor could desolve related to 7 watts and also requirements a few ventilation.
Operation:
A couple of NPN transistors are employed to identify the start of each one half cycle and trigger the delay timer which often triggers the SCR by the end of the delay time. The delay time is set up by the current supply which can be controlled by a 4017 decade counter. The 1st be counted (pin 3) packages the current into a minimal that refers to about 7 milliseconds of delay, as well as almost all half cycle time so that the lamp is nearly off. Complete brightness is purchased to the sixth count (pin 1) which can be not connected so the current shall be highest and offer a minimum delay plus trigger the SCR close to the beginning of the cycle. The remaining 8 counts increment the brightness 4 steps up and 4 steps down in between highest and also minimal. Every step up or down supplies about twice or half the energy, so the intensity shows up to modify linearly. The brightness of every stage could be modified with all the 4 resistors (4. 3K, 4. 7K, 5. 6K, 7. 5K) connected on the counter outputs.
This circuit has become designed through Don Warkentien (WODEW) which suggsted introducing a tiny 47uF capacitor through ground to the junction on the current supply transistor (PNP) to relieve the a digital treading impact and so the lamp can lighten up and fade in a smoother style. The value of the capacitor depends on the 4017 counting rate, a quicker charge could need a small capacitor.
