Online repair manual

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 finite life. However, the bulbs in modern torches (US and Canada: flashlight) have a less-than-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 1Ω 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.

MOSFET:

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 popular BUZ11 or BUZ10 is eminently suitable for the present application. The requisite limiting of the start-up current is provided by the internal gate capacitance of the transistor in conjunction with the large gate resistor. If needed, a small capacitor may be added between gate and drain. Once the transistor is conducting hard, the remaining losses are negligible. This is true also when the torch is switched off: the quiescent current flowing through the transistor is much smaller than that caused by the self-discharge of the batteries.

Finally:

Since it is much simpler to break into the positive supply line of a torch than into the negative line, the addition of the limiting circuit makes it necessary for the batteries to be inserted into the torch the other way around from normal (as indicated by the manufacturer). Also, the on/off switch of a modified torch works the other way around from normal. Fitting the modification in some of the popular Mag-Lite torches is fairly straightforward.

After the rubber cover of the on/off switch has been removed, the entire push-button switch mechanism may be removed by releasing a central hexagonal bolt. The switch terminals may serve as soldering supports for the transistor-resistor series network. If it proves impossible to obtain a 47 MΩ resistor, four or five surfacemount-technology (SMT) resistors of 10 MΩ may be linked in series. Such a link works just as well and is almost as small as a normal 47MΩ resistor.