e005072.pdf

GENERAL INTEREST

Soft start for torch

Increases the life of torch bulbs

Design by K. Viernickel

47M

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.

BUZ11

000002 - 12

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-aver-

age life. The reason for this is that the halogen or

krypton bulbs used are operated at overvoltage 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 6 V and the

bulb is a 500 mA type, a series resistor of 1 Ω will

reduce the voltage across the bulb by about 0.5 V.

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.

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.

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 surface-

mount-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 47 MΩ resistor.

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 lim-

iting circuit makes it necessary for the

batteries to be inserted into the torch the

other way around from normal (as indi-

cated by the manufacturer). Also, the

[000002-1]

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 fila-

ment 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 sim-

ple 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 MOS-

FET, 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

Text (Dutch original): S. van Rooij

original configuration

4 - 6 cells

after modification

4 - 6 cells

BUZ11

000002 - 11

Elektor Electronics

5/2000

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