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
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Purchase