Working Principle of Incandescent Light Bulb

This article presents the origin and the principle of the bulb incandescent. We can understand why end-of-life bulbs often slam when you turn them on.

Origin of the incandescent light bulb

Before Thomas Alva Edison who invented the first incandescent light lighting bulb based on a thread of carbon placed in a vacuum, Heinrich Goebel made a filament of carbon in 1854 from carbonized bamboo filament. He mentioned a period of 400 hours. Goebel is sometimes considered to be the inventor of the incandescent lamp.

Joseph Wilson Swan did tests on lamps incandescent based on carbon in 1850 but their life expectancy was short because of not enough vacuum.

Edison incandescent bulb

Since then, the incandescent light bulb is enhanced by the use of filaments of tantalum and tungsten (evaporate more slowly than that of carbon). Today, incandescent and halogen bulbs use tungsten.

Principle of the incandescent light bulb

The filament of an incandescent bulb behaves like an electrical resistance which converts electrical power into heat (Joule effect). The temperature of the filament and settled in a value such that the heat from the filament is equal to the heat generated by the Joule effect. Ideally, all the heat is radiated. In practice, a small part of the heat is also evacuated by thermal conduction in the branches that hold the filament. That is why it is not interesting to the thick filament that would lead too much heat to the lugs out of the bulb. That’s why we don’t make less than 6V halogen.

The temperature of the filament is very high. In a conventional incandescent bulb, it is 2500 ° C about (2800K). In halogen lamps (12V or 24V), high-performance exceeds 3000 ° C (up to 3400K). Tungsten, meanwhile, melts at 3410 ° C. This is the highest possible theoretical temperature to shine an incandescent bulb.

More a filament is hot and best is luminous efficacy (lumen light emission / electric power consumed in Watts). A traditional (and forbidden for sale today!) incandescent bulb of 100W reached 13.4 lumens per Watt (1340 lumens).

Bulb incandescent 100W and 1340lm (prohibited for sale since 2009)

Bulbs to vacuum or inert gas?

Initially, a vacuum must reign in incandescent bulbs to avoid oxidation or even burning the metal filament.

Burning of the filament of an incandescent bulb

To achieve these photos, voluntarily break a new incandescent bulb or at least that works. The filament is air lilbre. You turn the broken bulb and we take a picture very shortly after. It is a work of art in photo… The white smoke is made up of particles of tungsten oxide.

An inert gas in the bulb slows down the evaporation of the filament. The carbonated tungsten atoms can be redeposited on the filament itself, which restores. This gas can be argon or argon-nitrogen mixture. The filament can be brought to temperature higher than for an incandescent bulb vacuum. The luminous efficiency of the filament is best.

However, the inert gas led a part of the heat emitted by the filament. Evacuated heat thus cannot be radiated, where a low luminous efficiency. If this loss of heat in the gas is not compensated by the increase in efficiency due to the higher temperature of the filament, it is not interesting to fill incandescent of an inert gas.

The thermal conductivity of the filament to gas is roughly proportional to the length of the filament, but equally amazing as this sounds, varies little with its diameter. This means that the filaments purposes (low current) and long are more effective in a vacuum while the thick filaments have advantage to be used in an inert gas. The threshold is around 6 to 10 Watts per cm² of filament. This threshold depends somewhat on the temperature of the filament and the size of the bulb. More the bulb, the more the convection in the bulb increases the heat (which will not be radiated) junk removal.

Incandescent to krypton: + 10% light

A light bulb 100W Krypton offered 1450 lm instead of 1340 lm.

As inert gas, krypton or xenon are used. Their atoms are bigger and better to redeposit the evaporated on the filament tungsten atoms. These gases conduct less heat than argon. Xenon is more efficient, but more expensive. With the xenon, the luminous efficiency of incandescent bulbs is increased and the life also.

Why incandescent bulbs burn out

Because of the evaporation of the tungsten filament. In practice, the filament does not uniformly evaporate. Some places evaporate a little faster than others, it’s inevitable. The filament becomes thicker and thinner in some places to others.

These fine places have higher resistance locally. This results in a warming up more important where a greater evaporation at this place. It’s a vicious circle until the filament breaks!

When incandescent bulbs burn out?

Often when we turn on them. The thin parts of the filament of a worn lamp are lighter and will warm up more quickly in the ignition.

Tungsten has a 10 to 15 times less cold resistance (at startup). A current appeal crosses the filament when you light the bulb. When the thick places of the filament have not yet reached their temperature, thin places reached it already. At this stage, the filament is not hot and the current flowing through is still a little higher. The thin places in the filament will overheat as long as the rest of the filament has not reached its temperature. That’s why the older bulbs jump when you turn them on.

Why the breakdown is sometimes spectacular?

When the filament of an incandescent bulb breaks, sometimes an electric arc forms. Also, the current flowing through the arc through the filament of the bulb. The ends of the filament based and an arc appears with a bright blue flash. Most of the bulbs have a built-in fuse that jumps when the bulb blows, because of the current peak caused. Circuit breakers are not obvious but sometimes Light dimmers to jump because the electronic components are much more sensitive. So the bulb, in his death, carries with it its drive.

Often a light bulb: over-consumption?

  1. People think that a light bulb made over-consume the flow. The filament heats up very quickly:

Current to the ignition of a bulb

In 0.1 second typical, the filament of the bulb is hot. Overconsumption because you light a light bulb is less than the savings if it is turned off 1 second!

However, if the goal is to save money by turning off a few seconds, think of usury (10 000-100 000 cycles) switch.

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