It is a completer solid state time dependent light dimmer that can be used with a 230V lamp load up to 150 Watts. This dimmer can be used in bedroom, theater hall, or any auditorium to avoid the blinking of the eyes which occurs when bright lights are suddenly switched on.
The circuit operation of this time dependent dimmer are consisting of three parts. The first circuit is the full wave bridge rectifier. The second is the timing circuit and the third part is the triggering stage using unijunction transistors. This triggering stage control is depending upon the timing circuit and the triggering of the silicon controlled rectifier (SCR) which in turn controls the intensity of the lamp.
The bridge rectifier provides full wave rectification of the mains supply. The operative 18V DC supply could be obtained from the Zener Diode. The pulsating nature of the supply keeps the triggering circuit in synchronization with the mains supply frequency. Depending upon the setting of the intensity control potentiometer and the timing potentiometer, the capacitor C2 starts charging. The intensity control potentiometer determines the final voltage level of the capacitor C2 will reach. The maximum and minimum setting of the potentiometer will provide full and zero charging respectively of C2.The timing potentiometer have been provided for the adjustment of the rate of charging of C2. The voltage developed on C2 when it gradually charges by the two stage directly coupled transistorized amplifier.
The second stage of this amplifier acts as a current source to charge to charge the capacitor C3. The magnitude of the current supplied by this stage depending upon the voltage level of the capacitor C2. When the voltage of capacitor C2 rises the charging rate of capacitor C3 rises and C3 reaches to the peak point voltage of the UJT earlier during each successive positive half cycle.
The peak voltage of the UJT stage is about 13V. When this is reached the UJT fires and C3 discharges rapidly through the 47 Ohms resistor. As C3 discharges the point is reached where the emitter current falls below the UJT’s minimum holding current and the UJT turns off. Then after C3 starts to charge again and the process repeats. The circuit itself acts as a free running oscillator that generates a saw-tooth wave from across C3 and a pulse wave form across the R12 resistor. The pulse output of the R12 is used to trigger the SCR. The full wave pulsating supply to this stage keeps the circuit in synchronization of the mains supply.
If the capacitor C2 is initially in the fully discharged condition the intensity control at its maximum position and the timing control in the maximum resistance position then the capacitor C2 will take approximately ten minutes to charge fully. This means the lamp connected to the device will take this much time to reach its full brightness. The reverse operation can be obtained by simply bringing the intensity control potentiometer to the minimum position. This will gradually discharge the capacitor C2 and it reduce the intensity of the lamp to zero, after about same time. The in between setting of these potentiometer can give intermediate brightness as well as duration.
Caution: As the full circuit operates directly from the 230V mains supply full precautions must have to be taken into consideration and suggest not to touch any part of the circuit when it is in operative conditions.