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Lamps

Function and Construction
Lamps emit light when an electric current passes through them. All of the lamps shown on this page have a thin wire filament which becomes very hot and glows brightly when a current passes through it. The filament is made from a metal with a high melting point such as tungsten and it is usually wound into a small coil. Filament lamps have a shorter lifetime than most electronic components because eventually the filament 'blows' (melts) at a weak point.

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Circuit symbols
There are two circuit symbols for a lamp, one for a lamp used to provide illumination and another for a lamp used as an indicator. Small lamps such as torch bulbs can be used for both purposes so either circuit symbol may used in simple educational circuits.

Lamp used for lighting
(for example a car headlamp or torch bulb)

Lamp used as an indicator
(for example a warning light on a car dashboard)

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Selecting a Lamp
There are three important features to consider when selecting a lamp:
Voltage rating - the supply voltage for normal brightness.
Power or current rating - small lamps are usually rated by current.
Lamp type - please see the table below.
The voltage and power (or current) ratings are usually printed or embossed on the body of a lamp.

Voltage rating
This is the supply voltage required for normal brightness. If a slightly higher voltage is used the lamp will be brighter but its lifetime will be shorter. With a lower supply voltage the lamp will be dimmer and its lifetime will be longer. The light from dim lamps has a yellow-orange colour.
Torch lamps pass a relatively large current and this significantly reduces the output voltage of the battery. Some voltage is used up inside the battery driving the large current through the small resistance of the battery itself (its 'internal resistance'). As a result the correct voltage rating for a torch lamp is lower than the normal voltage of the battery which lights it!

For example: a lamp rated 3.5V 0.3A is correct for a 4.5V battery (three 1.5V cells) because when the lamp is connected the voltage across the battery falls to about 3.5V.

Power or current rating
This is the power or current for the lamp when connected to its rated voltage. Low power lamps are usually rated by their current and high power lamps by their power. It is easy to convert between the two ratings:
P = I × V
or
I = P / V where: P = power in watts (W)
I = current in amps (A)
V = voltage in volts (V)

Examples:

A lamp rated 3.5V 0.3A has a power rating P = I × V = 0.3 × 3.5 = 1.05W
A lamp rated 6V 0.06A has a power rating P = I × V = 0.06 × 6 = 0.36W
A lamp rated 12W 2.4W has a current rating I = P / V = 2.4 / 12 = 0.2A

by http://www.kpsec.freeuk.com/

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