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05/13/15 |
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Controlling of the light intensity of a incandescent lamp is an old idea. In
the past, this goal was achieved using a potentiometer (rheostat). This
system is not efficient due to its power losses. Furthermore, such device is
not easy to be automatically controlled. Because of these reasons,
solid-state devices were used to control the input power to the lamp. Triacs
and Thyristors are the two common types of such devices. There are many
commercial integrated circuits to control this switches, e.g., TCA785. A
photocell is used as a light sensor for automatic dimming. A photocell is a
light-sensitive sensor and when the light intensity increases, its
resistance reduces. The idea behind controlling the input power of a lamp is to apply the AC voltage in a fraction of the cycle and remove it in the other fraction, referred to as the duty cycle. The total harmonic distortion of load current for this method is high and a filter is usually used to reduce the harmonics. Note that this kind of controllers can not be used for fluorescent lamps. In the past, electromagnetic ballasts were used to drive fluorescent lamps. These ballasts have a poor power factor. Also, the lamp life time is reduced due to the lack of a preheat stage. Electronic ballasts, on the other hand, have many advantages. A sensor can be used to control the intensity of the light automatically, providing energy savings. The built-in preheating stage in an electronic ballast increases the life time of a lamp to three or four times, compared to the electromagnetic ballasts. We were approach by a local company (Afrough & Arvin Illumination Industrial Group), to develop dimmable electronic ballasts. A circuit was constructed using the half-bridge technology and the two MOSFET switches are controlled by an IC to produce a 100 kHz input to drive the lamp. The grid AC voltage should be converted to a DC voltage using a diode rectifier. The photocell is adopted to sense the light intensity of the room and the power input to the lamp is controlled accordingly. |
This site was last updated 05/13/15