It’s no secret that halogen lamps used in automobiles often fail. This happens as a result of a current surge that inevitably occurs every time they are switched on, when the filament coil is still in an unheated (cold) state.
For example, a “halogen” fog lamp is designed to operate in normal mode with a power consumption of 55 watts (at 12-volt supply). According to the law for an electrical circuit known to every high school student, the filament resistance should be about 2.6 ohms. And indeed this is so, but only with a heated filament. In a cold lamp, the resistance measured with an ohmmeter slightly exceeds 0.2 ohms. This means that at the moment this “halogen” is switched on, there will inevitably be a 60-ampere current surge!
The device I propose is intended to extend the service life of incandescent lamps in a low-voltage network. The soft warm-up time — the lamp reaching operating mode — depends on the resistance of resistor R1 and the capacitance of capacitor C1. With the values specified in the circuit, it does not exceed 2.5 s.
The saturation voltage of the composite transistor VT1 — VT2 can be set by adjusting the rotor of resistor R2. This allows selecting the necessary time to reach operating mode (depending on the load power) in the range from zero to maximum delay.
The choice of КТ872А as a power semiconductor triode is not accidental. This transistor manufactured by NPO “Transistor” (Minsk) is capable of withstanding significant current surges for a long time at Icp up to 10 A. For optimal operation in our circuit, it should be installed together with КТ972Б on a common heat sink with an area of about 100 cm2, at a current consumed by the lamp up to 6 A.
If switch SA1 is replaced with a jumper, and a micro-toggle switch or micro-button is connected in series with resistor R1, then an additional convenience appears — the absence of a powerful power switch. Its role will now be performed by the power transistor.
Due to the small number of parts used in this device, there is no printed circuit board. All mounting is done by point-to-point wiring (reliable and well-known to everyone from school physics laboratory work).
A. FILIPOVICH, Dzerzhinsk, Belarus
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