Using a MOSFET for Mains Voltage Dimming
In this circuit, it’s demonstrated that dimmers designed for mains voltage don’t always require a triac. Instead, a MOSFET (BUZ41A, 500 V/4.5A) in a diode bridge is employed to achieve voltage control over an incandescent bulb through pulse-width modulation (PWM). A comprehensive PWM controller can be located in another section of this publication. The gate’s power supply voltage is derived from the voltage across the MOSFET. D6, R5, and C2 collaborate as a rectifier, with R5 serving to limit current spikes through D6 to approximately 1.5 A, making it no longer a pure peak rectifier. The voltage across C2 is kept regulated to a maximum of 10 V by the combined action of R3, R4, C1, and D1.
Optocoupler-Enhanced Gate Drive
An optocoupler and resistor (R2) are harnessed for controlling the gate. R1’s primary function is to safeguard the LED in the optocoupler, but it also operates as a standard current-limiting device, ensuring that a ‘hard’ voltage can be safely applied. The optocoupler in use here is a well-known model, the CNY65, known for providing class-II isolation, which is essential for the regulator’s safety.
Optimizing Transistor Conduction in Optocoupler Circuit
To ensure swift conduction of T1, the optocoupler’s transistor is linked to the positive power supply. This configuration facilitates rapid activation of T1. A delicate balance is struck in selecting the value of R2; 22 kΩ is chosen to mitigate switching spikes caused by parasitic inductance. This value serves as a compromise, minimizing inductive voltages while managing switching loss during conduction transitions. An interesting outcome of this choice is the prolonged conduction of T1 beyond what the PWM signal alone would dictate.
As the voltage across T1 diminishes, D1 maintains a constant 10 V until an 88% duty cycle. Beyond this point, higher duty cycles result in reduced voltage. At a 94% duty cycle, a 4.8 V voltage level proves sufficient to sustain T1’s conduction. This value represents the maximum duty cycle, where the transistor operates at nearly 100% conduction. Despite a 2.5 V drop in lamp voltage at 230 V mains, this circuit cannot control inductive loads due to asynchronous switching with mains frequency, potentially causing DC current flow. Similarly, electronic lamps like PL types, operating internally on DC, cannot be dimmed using this setup.
Balancing Resistance Values for Efficient Operation
A crucial compromise is established concerning the lowest current consumption (when the lamp is off) and the permissible highest duty cycle. When the duty cycle hits zero, the voltage across the resistors peaks, reaching approximately 128 V with a 230 V mains supply. To prevent potential issues due to voltage ratings (which might be below 300 V, depending on the specific resistor), two resistors are connected in series. Each resistor dissipates a maximum of 0.5 W. To enhance longevity, it’s prudent to employ two 1-W rated resistors in this configuration.