Function | Symbols |
Selecting | Types of lamp |
Connecting
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) |
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.
P = I × V or I = P / V |
where: | P = power in watts (W)
I = current in amps (A) V = voltage in volts (V) |
Type of Lamp | Example |
MES Miniature Edison Screw These are the standard small lamps. The bulb diameter is usually about 10mm, but tubular bulbs are also available. MES lamps have one contact on the base and the body forms the other contact. They are available with a good range of voltage and power (or current) ratings. Lens ended versions are available to produce a focused beam of light.
LES Lilliput Edison Screw Photograph © Rapid Electronics |
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MCC Miniature Centre Contact These have a bayonet style fitting, like a standard mains lamp in the UK. They have one contact on the base and the body forms the other contact. The bulb diameter is about 10mm. Photograph © Rapid Electronics |
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SBC Small Bayonet Cap These have a bayonet style fitting, like a standard mains lamp in the UK. They have two contacts on the base so the metal body is not connected in the circuit. SBC lamps have high power ratings (24W for example) and their bulbs are large with a diameter of up to about 40mm. Note the two filament arrangements in the lamps shown, horizontal on the left, vertical on the right. Photograph © Rapid Electronics |
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Pre-focus This type of lamp is used in torches and lanterns. The flange at the top of the metal body is used to hold the lamp in place. Lampholders are not readily available so this type is unsuitable for most projects. Photograph © Rapid Electronics |
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Wire ended These are very small lamps with a bulb about 3mm diameter and 6mm long. Take care to avoid snapping the wires where they enter the glass bulb. Photograph © Rapid Electronics |
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Grain of Wheat These are similar to the wire ended lamps above but they have stranded wire leads usually about 150mm long. The bulb is about 3mm diameter and 6mm long - the size of a grain of wheat! Photograph © Rapid Electronics |
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Rapid Electronics have kindly allowed me to use their photographs on this page. We buy most of our components and tools for the Electronics Club from Rapid Electronics. The photographs are from their Image Gallery CD-ROM. |
screw terminals | solder tags |
Lampholders Photographs © Rapid Electronics |
Most lamps are designed to be used in a lampholder but the small 'wire ended' and 'grain of wheat' lamps have wires which may be soldered directly onto a circuit board.
Lampholders usually have screw terminals or solder tags to attach wires.
Some small holders have contacts which may be soldered directly to a circuit board.
A disadvantage of connecting lamps in series is that if one lamp blows all of them will go out because the circuit is broken. Christmas tree lamps have a special feature to overcome this problem; they are designed to short circuit (conduct like a wire link) when they blow, so the circuit is not broken and the other lamps remain lit, making it easier to locate the faulty lamp. Sets also include one 'fuse' lamp which blows normally.
WARNING! The Christmas tree lamps may seem safe because they use only 12V but they are connected to the mains supply which can be lethal. Always unplug from the mains before changing lamps. The voltage across the holder of a missing lamp is the full 240V of the mains supply! (Yes, it really is!)