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.
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.
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.