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Halogen or (Quartz) Tungsten / Halogen lighting evolved from incandescent filament lighting .
Essentially, the Tungsten filament used in an incandescent filament light is mounted in a
light bulb similar to an incandescent filament light, but with a small quantity of a "halogen" element
added (compounds of either Iodine or Bromine) to the bulb once the air has been pumped out.
The halogen gas and the tungsten filament produces a halogen cycle chemical reaction
inside the glass bulb and redeposits evaporated tungsten back on the filament, increasing the filament's life.
A halogen lamp can (and does) operated at a higher temperature than a standard incandescent filament lamp
and because it operates at a higher temperature, the glass in the bulb needs to be quartz glass.

When handling Halogen lamps, you must be careful not to contaminate the glass with the natural oils on your fingers
as this greatly reduces the life of the lamp.

Always wear gloves, or use a paper tissue or cotton wool when replacing Halogen lamps to avoid touching the glass.

photograph showing how to correctly replace a halogen capsule lamp

How to correctly (and incorrectly) handle a halogen capsule lamp.

photograph of a halogen G4 capsule compared to a 5p coin

The most useful and common type of tungsten halogen light for domestic purposes comes in "capsule" form.
The capsules are available in various sizes and shapes (the one shown above is a 12V, 20W, G4 capsule).
Capsules are also incorporated into other lamps (e.g. eco halogen lights and GU10 spotlamps).

photograph of a halogen G4 capsule powered up

Tungsten halogen capsule (left) with a low current flowing and (right) at full power.

photograph of an eco halogen lamp

The nearest equivalent to an incandescent lamp is the "eco halogen" lamp as shown here, where a
tungsten halogen capsule is used inside an incandescent style bulb.

photograph of an eco halogen lamp at full power

When powered up, the eco halogen lamp behaves similarly to an incandescent lamp .
Has slightly better light output for the same power as a similar incandescent lamp .

photograph of GU10 mains powered halogen spotlamp

Another popular version of the halogen lamp is the GU10 mains powered spotlamp as shown here.

photograph of GU10 mains powered halogen spotlamp showing the capsule

On closer examination (and the removal of part of the mirrored reflector),
the halogen capsule can be seen at the centre of the GU10 spotlamp.

Diagram of an Aluminium and Dichoric reflector construction

Not all GU10 spotlamps are the same - there are two different methods of construction:

Aluminium Reflector (on the left in the diagram):
This lamp has an Aluminium Reflector built into the lamp with the purpose of directing Heat & Light in the same direction.

Dichoric (on the right in the diagram):
This lamp does not have an Aluminium Reflector and the Heat from the lamp radiates in the opposite direction to that of the Light.

It is important to use the correct type of lamp (Aluminium Reflector or Dichoric) in a light fitting to avoid
overheating and the risk of fire.

Photograph showing the difference between Aluminium and Dichoric reflector GU10

The difference is apparent when the lamps are illuminated.
The lamp on the left (furthest away in the photo) is an Aluminium Reflector lamp and
the lamp on the right (nearest the camera in the photo) is a Dichoric lamp.

A method of checking MR16 lamps for Dichoric or Aluminium Reflector

MR16 Halogen lamps are widely available in the Dichoric variety, but Aluminium Reflector types
are only available from specialist lighting outlets or good Electrical Wholesalers.
If you get your MR16 Halogen lamps from one of the DIY sheds - they are Dichoric.
If you get your GU10 Halogen lamps from a DIY shed - they could be either type.
The photo above shows another method of checking the lamp type:


One of the biggest problems with light fittings at the moment is trying to identify what type of lamp is needed for replacement.
This quick guide shows the most common types on the market at the moment.

photograph of a halogen G4 capsule compared to a 5p coin

G4 Capsule, 12V used in under cupboard lights and various other decorative lighting.

photograph of a halogen G9 capsule compared to a 5p coin

G9 Capsule, 230V (mains voltage) used in various decorative lights.

photograph of a halogen GU10 capsule compared to coins

GU10 Spotlight, 230V (mains voltage) used in downlighters and various other light fittings.

photograph of a halogen MR16 capsule compared to coins

MR16 Spotlight, 12V used in downlighters and various other light fittings.
Note: The MR16 is also known as a GU5.3

photograph of a halogen MR11 capsule compared to coins

MR11 used mainly for display lighting (e.g. in cabinet lighting).
Note: The MR11 is also known as a GU4

photograph of a halogen floodlight lamp compared to a 5p coin

A typical tungsten halogen lamp as used in security floodlights.


You need the correct lamp (light bulb) base to replace a blown one.
Here is the most common type of bases in use.

photograph of a BC base compared to a coin

Bayonet Cap Base - the most common type of light fitting found in domestic lighting.
Also known as BC or BC22d

photograph of an ES base compared to a coin

Edison Screw Base - one of the common types of light fitting.
Also known as ES or E27

photograph of an SES base compared to a coin

Small Edison Screw Base - one of the common types of light fitting.
Also known as SES or E14

photograph of an SBC base compared to a coin

Small Bayonet Cap Base - one of the common types of light fitting.
Also known as SBC or B15d


From Issue 11 of the Electricity Safety Council's "Switched On" magazine (available here) and our own experiments, the cause is:

Fuses and Circuit-breakers provide protection in the event of an overloaded or faulty circuit.
Whilst the majority of circuit-breakers in a consumer unit will rarely trip, circuit-breakers
controlling lighting circuits, often trip when an incandescent or halogen light bulb "blows".
This unwanted tripping is due to the operating characteristics of the circuit-breaker.

The operating mechanism of circuit-breakers has a magnetic part and a thermal part.
The magnetic part causes very rapid operation of the circuit-breaker in the event of a fault,
the current causing such operation being called the "instantaneous tripping current".
Instantaneous tripping of a circuit-breaker when a fault occurs is an essential characteristic,
so that the supply to a faulty circuit is quickly cut off to remove the electric shock hazard and to prevent the risk of fire.

One way of preventing the unwanted tripping of circuit-breakers is to replace incandescent & halogen light bulbs
with one of the equivalent "low energy" types, which are unlikely to cause a short-circuit when they fail.

However, unwanted instantaneous tripping may occur when there is a sudden increase in current flow for other reasons,
as can happen when an incandescent or halogen light bulb blows.

The circuit-breaker trips because the filament in the bulb becomes ionised, causing the filament support wires
to be temporarily short-circuited.
Nevertheless, if a circuit-breaker operates, it always makes sense to check the light fittings just in case there is a fault.
Fuses take longer to react to an overload condition and rarely blow as the fault caused by a lamp blowing clears fairly quickly.

Our tests show that a cold 50W GU10 spotlamp has a resistance of 91 Ohms.
From our GCSE science, we would expect a resulting current of 230 Volts ÷ 91 Ohms = 2.5 Amps.
But 230 Volts x 2.5 Amps = 575 Watts, more than 10 times what we expect (we expect 50 Watts).

As the filament inside the lamp warms up, the resistance increases to 1060 Ohms,
giving us a more reasonable current of 230 Volts ÷ 1060 Ohms = 0.22 Amps and
using 230 Volts x 0.22 Amps = 50 Watts of power as expected.

From switching on the lamp to the filament warming up, there must be a surge of current due to the initial low resistance.
This surge of current causes rapid heating of the filament and mechanical stress because some parts of the filament are still cold.
Sometimes the mechanical stress causes the filament to break, leaving a small gap between the two ends.
As the two ends of the filament are connected to a supply of current, an electrical arc forms,
resulting in a cloud of conductive gas (plasma) inside the glass envelope of the lamp.
As this plasma is electrically conductive, a high current flows between the filament ends acting as electrodes.
This high current is "seen" by the circuit breaker as a fault, resulting in disconnection of the supply.
The fault condition does not continue for long (less than a second) and fuses usually take at least a second to react to an overload.
Sometimes you can see the light glowing very brightly when first switched on for a fraction of a second before it blows.

Diagram of the test setup to measure the GU10 switch on surge current

Our experiment to measure the surge current when powering up a 50W GU10 spotlamp.
WARNING:     Do not attempt to reproduce this experiment.
It was done by someone who has spent a lifetime safely investigating faults in electrical & electronic systems.
Electricity has the ability to kill you, and it will unless you know exactly what you are doing.

Oscilloscope trace of a GU10 switch on surge current

This Oscilloscope trace shows the surge current when switching on a GU10 lamp from cold.


We all know from Albert Einstein that light can be made to change direction by interaction with gravity,
however that's not really the affect we have in mind here.

When a light bulb from a table lamp is used to replace a blown ceiling light, it doesn't last very long.
This because the filament in the bulb was subjected to the force caused by gravity stressing the filament
in the direction of the base.
When the same bulb is now turned upside down and fitted in a ceiling light, the stress on the filament
is now in the direction away from the base.

From our experiments on surge currents on startup above, and the fact that the filaments get more brittle with age
there is a greater risk that the bulb will "blow"
sooner rather than later after relocation.

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