The lamp in slide projectors is normally controlled with the aid of a triac in series with it. This method reduces the brightness of the lamp appreciably since the triac drops about 1.5 V, which is more than 6% of the nominal lamp voltage. To compensate this loss, the mains voltage to the projector should be increased by that percentage. The transformer and ventilator can cope easily with that kind of increase, and so can any electronic circuits, since these are invariably powered via a regulator.
The a.c. voltage to the projector may be increased by connecting the secondary winding of a mains transformer in the live wire of the mains. The voltage developed across this winding is then added to the mains voltage (provided that the transformer is connected correctly as regards phase).
The voltage the secondary winding must deliver is calculated as follows. The current through the winding depends on the number of projectors and their rating. Assuming there are four, each rated at 250 W (power consumption: 300 W), the total power consumption during normal operation is 1200 W. With a mains voltage of 240 V, the current will be 5 A. Since the projectors do not all use full power simultaneously, the secondary winding may be rated at 6 A. The secondary voltage, Us, should be the ratio of the loss across the triac and the nominal lamp voltage times the mains voltage, that is,
Us= 1.5/24-240 = 15 V.
Fuse Fl should be rated at 1.25xthe maximum current of all projectors. Assuming the earlier stated values, that is,
Ifuse1 = 1.25 1200/240 = 6.25 A.
or, rounded oil to the next standard value, 6.3 A (delayed action). The rating of fuse F2 is calculated from
Ifuse2= 1.2514.1s/240 = 487 mA,
or, rounded off upwards, 500 mA. The circuit should preferably be fitted with a man-made fiber enclosure. If the output voltage is lower, rather than higher, than the mains voltage, the connections to either the primary or the secondary of the additional transformer must e Interchanged.