It is known that most incandescent bulbs burn out at the moment of inclusion. It due to the fact that the cold filament has a low resistance, about 10 times less than that of the heated filament of the lamp. Therefore, at the time the inclusion of current on the filament, respectively, 10 times more than in a stationary mode. Heating of the filament lamp lamps with a rated current of 5 A occurs for 0,03 p. since the diameter of the filaments unequal in length, more thin places heats up quickly. And since the temperature of the conductor back proportional to the fourth power of the diameter, it is clear how strong overheating occurs in thin places. This is the cause burnout of the filament.
What to do? First, it is necessary to perform the filament with high accuracy by the diameter. This is the business of manufacturers of lamps, but they are not at all interested in “eternal the lamp". Then you need to turn on the light, limiting inrush current.
Offered in the most simple, and therefore reliable and cheap scheme (Fig.1), where FU - regular fuse; SA - switch; K1 - relay with normally open contact To 1.1; R1 - resistor, aka inertial fuse; HL1 - filament lamp.
The scheme works as follows. After the circuit SA, the voltage at point "a" increases as heating lamp HL1. Because the resistance of R1 is approximately 2...3 times greater than the resistance of the cold filament, starting current also about 2...3 times less. When the voltage at point "a" voltage reaches operation of relay K1, contact K1.1 is closed, and the lamp is connected to a source current completely. The resistor R1 is selected so that in case of failure of a relay or short circuit in the circuit of the lamp he burned out. The heating conductor resistor R1 to burnout selected 10 times more than the heating time of the lamp, i.e. 0.3 sec.
To turn on lamps high beam and low beam, it is recommended to perform two block lamp by entering into the composition of two regular relay and one resistor, as it is shown in Fig.2, where K1 relay bypass grafting; K2 - relay switching.
The device of the bulb is in the form of a block and place or directly in the headlights, or near headlight units one by one.
Schematic protected by RF patent 2156044 from 04.06.98 G.
Author: A. Belyavsky, Cherkassy