A Highly Efficient DC Lamp Dimmer

The simplest lamp dimmer cir- cuit consists of a rheostat, in se- ries with the lamp, which one may adjust to obtain the required brightness. Such linear regulators are quite inefficient since a lot of power is wasted in them. Moreover, in the rheostat the moving contacts are likely to get damaged in the long run, as its value is frequently adjusted by moving the slider. Such linear control circuits provide an overall efficiency of no more than 50 per cent. This wastage of power can be avoided if one uses pulse width modulation (PWM) which can be made to control an electronic rheostat. The circuit shown here is based on PWM principle. Gate N1 and its associated components constitute an oscillator producing oscillations of approximately 200 Hz with a pulse width of 0.1 ms. This output is fed to transistor T1 for level shifting. At the output of this transistor is a potentiometer VR2, using which a DC component can be added to the pulses emerging from transistor T1. By adjusting this potentiometer/trimmer, one can have a good linear control of the lamp brightness from completely off state to 100 per cent on state. The signal is inverted by gate N2 and fed to MOSFET 12N10. IC CD40106 provides six inverting buffers with Schmitt trigger action. The buffers are capable of transforming slowly changing input signals into sharply defined jitter-free output signals. They are usually used as wave and pulse shapers. IC CD40106 possesses high immunity and low power consumption of standard CMOS ICs along with the ability to drive 10 LS-TTL loads. In this circuit loads up to 24W can be connected between MOSFET drain and 12V supply without using a heatsink. The loads can even be DC motors, miniature heating elements, etc. If one uses a low RDS (on) MOSFET, a higher efficiency can be achieved. By using the components as shown in the circuit, an efficiency of approximately 95 per cent can be achieved. The flexibility of the design makes it possible to change the MOSFET with a similar one, in case of non-availability of 12N10. The circuit by itself does not draw much current when the load is disconnected. Ensure proper ESD protection while handling the MOSFET to prevent damage. Lab note: The circuit was tested using MOSFET IRF640 with RDS (on)=0.18 ohm.

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