800W-soft-start-light-dimmer

The zener diode maintains a constant voltage of 20 V for the unijunction transistor Q1, except at the end of each half-cycle of the input when the line voltage drops to zero. Initially, the voltage across capacitor C1 is zero, and capacitor C2 cannot charge sufficiently to trigger Q1. C1 will start to charge, but due to the low voltage, C2 will only charge to a level adequate to trigger Q1 near the end of the half-cycle. Although the lamp resistance is low at this point, the voltage applied to the lamp is also low, resulting in a small inrush current. Subsequently, as the voltage on C1 increases, C2 is able to trigger Q1 earlier in the cycle. During this period, the lamp is gradually heated by the increasing applied voltage. By the time the peak voltage applied to the lamp reaches its maximum value, the bulb has been heated sufficiently to maintain the peak inrush current at a reasonable level. Resistor R4 regulates the charging rate of C2 and provides a means to dim the lamp. Diode D6 and resistor R7 enhance performance at low conduction angles.

The circuit described involves a zener diode functioning as a voltage regulator, ensuring a stable 20 V supply to the unijunction transistor Q1. This configuration is particularly useful in applications where consistent triggering of the transistor is critical. The operation begins with C1, which remains uncharged initially, preventing C2 from reaching the threshold voltage required to activate Q1. As the AC input signal progresses, C1 gradually accumulates charge, albeit slowly due to the low initial voltage.

The charging dynamics of C1 are crucial since they determine when C2 can trigger Q1. Near the end of each half-cycle, C2 charges up sufficiently to activate Q1, allowing the circuit to conduct. During this brief moment, the low voltage across the lamp results in minimal inrush current, preventing damage to the filament from sudden surges. However, as the cycle continues, the voltage across C1 rises, enabling C2 to trigger Q1 earlier in the cycle. This early triggering allows for a more gradual increase in voltage across the lamp, which results in a smoother heating process and mitigates the risk of excessive inrush current.

Resistor R4 plays a pivotal role in controlling the time constant of C2's charging process. By adjusting the resistance value of R4, the rate at which C2 charges can be modified, thus allowing for dimming capabilities of the lamp. This feature is particularly beneficial in lighting applications where varying brightness levels are desired.

Additionally, diode D6 and resistor R7 are implemented to enhance the circuit's performance during low conduction angles. This arrangement ensures that the circuit remains responsive even when the input voltage is low, improving the overall efficiency and reliability of the system. The careful selection of these components and their values is essential for achieving optimal performance in the intended application.The zener provides a constant voltage of 20 V to unijunction transistor Ql, except at the end of each half-cycle of the input when the line voltage drops to zero. Initially, the voltage across capacitor Cl is zero and capacitor C2 cannot charge to trigger Ql. Cl will begin to charge, but because the voltage is low, C2 will be charged to a voltage adequate to trigger Cl only near the end of the half cycle.

Although the lamp resistance is low at this time, the voltage applied to the lamp is low and the inrush current is small. Then the voltage on Cl rises, allowing C2 to trigger Ql earlier in the cycle. At the same time, the lamp is being heated hy the slowly increasing applied voltage. By the time the peak voltage applied to the lamp has reached its maximum value, the bulb has been heated sufficiently to keep the peak inrush current at a reasonable value. Resistor R4 controls the charging rate of C2 and provides the means to dim the lamp. Diode D6 and resistor R7 improve operation at low-conduction angles. 🔗 External reference