Lamp Control in Slide Projectors
Traditionally, slide projectors regulate lamp brightness using a triac placed in series with the lamp. However, this method significantly diminishes brightness as the triac incurs a voltage drop of approximately 1.5 V, equivalent to over 6% of the lamp’s nominal voltage. To counter this loss, the mains voltage supplied to the projector should be raised by the same percentage. This adjustment is well tolerated by the projector’s transformer, ventilator, and any electronic circuits, all of which are invariably powered through a regulator.
Increasing a.c. Voltage for the Projector
To elevate the a.c. voltage supplied to the projector, one can connect the secondary winding of a mains transformer to the live wire of the mains circuit. The voltage generated across this winding is then added to the mains voltage, assuming the transformer is correctly connected concerning phase orientation.
Calculating Secondary Winding Voltage
Determining the voltage the secondary winding must provide involves several considerations. The current flowing through the winding is contingent on the number of projectors and their individual ratings. Assuming there are four projectors, each rated at 250 W (with a power consumption of 300 W), the total power consumption during normal operation equals 1200 W. With a mains voltage of 240 V, the current amounts to 5 A. As the projectors do not always operate at full power simultaneously, the secondary winding can be rated at 6 A. The secondary voltage, Us, is calculated as the ratio of the triac’s voltage drop to the nominal lamp voltage, multiplied by the mains voltage:
Us = 1.5/24-240 = 15 V.
Fuse Ratings Calculation
Fuse Fl needs to be rated at 1.25 times the maximum current of all projectors. Considering the previously mentioned values, the calculation becomes:
Ifuse1 = 1.25 1200/240 = 6.25 A.
Rounding up to the nearest standard value, the fuse rating becomes 6.3 A (delayed action). The calculation for fuse F2 is as follows:
Ifuse2= 1.2514.1s/240 = 487 mA,
Rounded up, this value becomes 500 mA. It’s advisable to enclose the circuit within a man-made fiber enclosure. If the output voltage is lower than the mains voltage, the connections to either the primary or secondary of the additional transformer must be interchanged.