Filament protection device for headlight bulbs. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Electronic devices
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It is known that most often incandescent lamps burn out at the moment of switching on. This is due to the fact that a cold filament has a low resistance, about 10 times less than that of a heated lamp filament. Consequently, at the moment of switching on, the current through the filament, respectively, is 10 times greater than in the stationary mode. The headlight lamp filament with a rated current of 5 A heats up in 0,03 s. Since the diameter of the filament is not the same in length, thinner places heat up faster. And since the temperature of the conductor is inversely proportional to the fourth power of the diameter, it is clear how much overheating occurs in thin places. This is what causes the thread to burn out.
What to do? First, it is necessary to carry out the filament with high accuracy in diameter. This is the business of lamp manufacturers, but they are not at all interested in an “eternal lamp.” Then you need to turn on the lamp, limiting the starting current.
The most simple, and therefore reliable and cheap circuit is proposed (Fig. 1), on which FU is a regular fuse; SA - switch; K1 - relay with normally open contact K 1.1; R1 - resistor, it is also an inertial fuse; HL1 - incandescent lamp.
The scheme works as follows. After SA is closed, the voltage at point "a" increases as the HL1 lamp warms up. Since the resistance R1 is approximately 2...3 times greater than the resistance of the cold thread, the starting current is also approximately 2...3 times less. When the voltage at point "a" reaches the trigger voltage of relay K1, contact K1.1 closes and the lamp is connected to the current source completely. Resistor R1 is selected so that in the event of a relay failure or a short circuit in the lamp circuit, it will burn out. The heating time of the conductor of the resistor R1 before burnout is chosen 10 times longer than the heating time of the lamp, i.e. 0,3 s
To turn on the high and low beam headlight lamps, it is recommended to make two lamp switching blocks, introducing two standard relays and one resistor into them, as shown in Fig. 2, where K1 is a shunt relay; K2 - headlight relay.
The device for switching on the lamps is made in the form of a block and is placed either directly in the headlight block or near the headlight blocks one at a time.
The circuit solution is protected by RF patent 2156044 dated 04.06.98/XNUMX/XNUMX.
Author: A. Belyavsky, Cherkasy
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