Soft Start For Torch - Increases The Life Of Torch Bulbs


Posted on Feb 5, 2014

The halogen or krypton bulbs in modern torches (USA and Canada: flashlights) have a limited life and are not particularly cheap. A simple modification in the torch lengthens the life appreciably. It is a fact of nature that any incandescent bulb has anite life. However, the bulbs in modern torches (US and Canada:‚ashlight) have a less-than-


Soft Start For Torch - Increases The Life Of Torch Bulbs
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average life. The reason for this is that the halogen or krypton bulbs used are operated at over-voltage to give as bright a light as feasible. The life of these bulbs may be extended simply by connecting a resistor in series with the bulb. For instance, when the battery voltage is 6V and the bulb is a 500mA type, a series resistor of 1 will reduce the voltage across the bulb by about 0. 5V. This will certainly lengthen the life of the bulb, but it will also cause a reduction in the available brightness. Also, energy is wasted in the resistor (evinced by heat production). Clearly, this is not a very good solution to the problem. A better one is shunting the bulb with a transistor in series with a resistor. Another well-known fact is that incandescent bulbs normally burn out when they are being switched on. This is because the resistance of the cold filament is significantly lower than that during normal operation. This results in a switch-on current that is much higher than the normal operating current. Clearly, much is to be gained by damping the switch-on current. The switch-on current may be limited by a simple circuit that is small enough to allow it to be built into most types of torch. As the diagram shows, such a circuit consists of nothing more than a metal-on-silicon-field-effect-transistor, or MOSFET, and a resistor. The transistor may be almost any current n-channel type that can handle the requisite power. The...




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