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Dimmer circuit using MOSFETs

We have seen here several circuits of dimmers, CC, CA, engines, for LEDs, ultimately Dimmers for almost all purposes. But this time we will present a electronic circuit dimmer which has very different characteristics of the other circuits. But this is special not because of using a Mosfet, as already published one in Light Dimmer using DC Power MOSFET, this is special by reason of its control be done low power and a PWM signal.

Usually the dimmer that controls the household AC mains uses Triac, but this circuit a transistor Mosfet, BUZ41A is used for load control, it is able to withstand voltages up to 500 Volts with a load of up to 4.5A. In the above article MOSFET dimmer it was used only for controlling a current of low voltage DC, it controls the AC current home network.

Dimmer com Transistor Mosfet

In this circuit the MOSFET pulse width modulation (PWM) uses a diode bridge for controlling the voltage that its output goes to, if an incandescent lamp. Pulse width modulation, PWM (English Pulse-Width Modulation) is a technique used to encode data in a pulsating signal, its main use is to allow the power control fixtures.

Dimmer usando Transistor Mosfet
Dimmer usando Transistor Mosfet

The operation of the PWM Dimmer

Ton Giesberts was the developer of this project and it was published in revista Elektor Electronics. The operation is simple, D6, R5 and C2 to form a rectifier optocoupler and the gate of the drive system of the Mosfet. R5 limits the current pulses through diode D6 to about 1,5 A. The voltage across the capacitor C2 is set to a maximum value of 10 Volts by R3, R4, E C1 D1. An optical coupler and resistor R2 are used to drive the gate of MOSFET.

Resistor R1 protects the Optical coupler LED and also functions as a normal current limiter, so that a voltage can be applied safely. The transistor in the optocoupler is connected to the positive power supply so that T1 can conduct as soon as possible. In order to reduce peak switching as a consequence of parasitic inductance, the value of resistor R2 is selected to be not too low, near 22 kohm.

A additional advantage this circuit is that the MOSFET will reach a power slightly higher than can be expected using a PWM signal . When the voltage across T1 reduces, the voltage across D1 remains equal to 10 Volts to a work cycle 88%. A higher duty cycle results in a lower voltage, example, a 94% strain is 4,8 Volts that proved to be just enough to drive T1. This value can be taken as the maximum duty cycle, wherein the transistor has 100% driving.

Uso do Dimmer PWM

A 230 Volt supply voltage, the voltage across the lamp output of the circuit is only 2,5 Volts inferior, measured with a lamp 100 Watts. Just to be clear, note that this circuit can not be used for controlling inductive loads, also electronic lamps fluorecentes can not be regulated with this circuit. O Mosfet T1 is connected asynchronously with the grid frequency and this can cause DC currents.

The resistors R3 and R4 should be 1Watt, and the other of 1/4 the Watt. The optical coupler used is CNY65, having an insulation class II , this ensures the safety circuit. The Mosfet must be mounted on a good heatsink to avoid problems. The circuit can be controlled with a Arduino, a microcontroller PIC or even a chip 555 as PWM controller without any problems. As this circuit is connected directly to mains Great care is and should only be installed by experienced people.

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