Curs_4+5_Lamps en_UEE_G3+

8/10/2019 Curs_4+5_Lamps en_UEE_G3+

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Fluorescent lamps use phenomenon of electro-and photo-luminescence as result of electrical discharge in inert gases and lowpressure vapor mercury.

The lamp is constructed from a cylindrical glass tube . At the ends thereare two electrodes and the tube is filled with low pressure inert gas and afew milligrams of mercury . The electrodes are from Wolfram and they are inspiral, having a high thermoelectrical emission at high temperature.(900 C).

The inert gas can be Argon, at low pressure. It is easy to ionize andthus it results an wavelength emission on the mercury spectrum, that isultraviolet, 1=184nm and 2=253nm). These radiations are converted invisible radiation by a luminofor on the inside wall of the lamp.

LIGHTING FLUORESCENT LAMPS


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Mercury is a metal that is liquid at normal room temperature, butinside an operating lamp (which is hot), the mercury is in a vapourform, but the pressure is extremely low only about 0.0007% of

atmospheric pressure.Fluorescent tubes (and CFLs) are technically referred to as 'low

pressure gas discharge lamps'.


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Operation

1. When the lamp is started, the cathodes are heated for a shorttime (1 s) in order to heat the cathodes so they release electrons.

2. A high voltage is then applied across the two cathodes and adischarge is created as the gas and mercury vapour conducts theelectrical current.

3. The flow of electrons energizes the vaporized mercury atoms tomake them give off ultraviolet (UV) radiation .

4. The inside of the glass tube is coated with a fluorescent powder.The UV radiation makes it give off visible light but only whilst exposedto the UV (i.e. the process of fluorescence).

The discharge in argon/krypton gas causes the gas to give off abluish glow, which can only be observed in a tube without thephosphor coating. This light constitutes only about 3% of the total lightoutput from the lamp, the remaining 97% is generated by thephosphor.


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Fluorescent lamps are not designed to be operated directly from themains supply.

All fluorescent lamps require a device to generate a high voltage(more than 230V) to initiate the discharge and an additional device tocontrol the discharge current.

Control gear/ballast is essentially a device connected in series with

a lamp to limit the current it draws down. All discharge lamps have a negative current-voltage characteristic,which means that voltage decreases with increasing current and, unlikeincandescent lamps, their electrical resistance decreases with increasingtemperature. A discharge lamp without control gear would draw an ever

increasing current as it runs up, and in the process destroy itself.

The key components in the circuit are :- Ballast (or choke) - the device thatcontrols the current through the lamp;

- Starter - the switch that starts the lamp;- Capacitor - the component that correctsthe power factor.


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Ballast can also be referred to as a choke. It is a device forrestricting (or 'choking') the current through the lamp. It is alwaysconnected in series with the lamp.

In electrical terms, the ballast is a self -inductance and consists of acoil of copper wire wound around a heavy iron core. As the alternatingmains current passes through the coil, it generates an alternating magneticfield in the iron core. This alternating magnetic field induces a current in thecoil opposing the mains current. The net effect is a limited current throughthe ballast and the lamp. The current limiting effect is very dependent uponthe frequency of the supply current the higher the frequency, the greateris the 'choking' effect.

The inherent resistance of the copper wire coil absorbs some of thepower (given out as heat). Typically, the power absorbed by the ballast(known as 'ballast losses '), is about 25% of the rated power of the lampbeing operated.


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The starter

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plac borne, 7 -condensator

This is the switching device used to startfluorescent lamps. Essentially, it is a switch that firstcompletes a circuit to heat up the lamp cathodes andthen instantly breaks the circuit, which induces a veryhigh voltage across the ballast and lamp. This high

voltage starts the discharge in the lamp which thenruns up to its normal operation.

If the lamp fails to light first time, the starterautomatically repeats the process until the lampstrikes. This is what causes a fluorescent tube to flashduring start-up. Once the lamp is running, the starterno longer attempts to start the lamp.

The modern fluorescent starter is known as a'glow' starter because it glows in operation and is

designed to be an easily replaceable item.


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Capaci tor The current restricting effect of the ballast prevents the alternating

mains current from being synchronous with the alternating mains voltage.The current is said to 'lag' behind the voltage and the magnitude of this'lagging' is referred to as the 'power factor' of the circuit.

A capacitor has the opposite effect in that it makes the alternatingcurrent 'lead' the alternating supply voltage. By choice of suitablecapacitor, the 'lagging' effect of the ballast can be offset by the 'leading'effect of the capacitor.

Such capacitors are referred to as 'power factor correction'capacitors (or PFC capacitors).

The ideal power factor is 1. Without a PFC capacitor, the powerfactor is usually less than 0.5 in a fluorescent circuit.

The fluorescent lamp circuit will operate normally without the PFC

capacitor, but the power meter would register less than half of theapparent power being transmitted - a situation not encouraged by theelectricity generation companies.


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Compact Fluorescent Lamps

Compact Fluorescent Lamps offer many advantages: Energy cost savings up to 70% vs. incandescent lamps of

comparable light output; Long life 8 to 10 times longer, than standard incandescent lamps;

Good color rendering rare earth tri -phosphor provides such highquality color you wont believe its fluorescent. Most types offer a choice of

color options, from warm to cool, to select the tone and atmosphere youneed;

A choice of wattages, shapes and sizes to meet lighting needs.Designed to fit everything from table lamps to wall sconces and ceiling

fixtures.


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CFLs contain a small amount of mercury sealed within the glass tubing 4milligrams (mg). By comparison, older thermometers contain about 500milligrams of mercury.

No mercury is released when the bulbs are intact (not broken) or in use

CLEANUP AND DISPOSAL OVERVIEW 1. Before cleanupa. Have people and pets leave the room.b. Air out the room for 5-10 minutes by opening a window or door.c. Shut off the central forced air heating/air conditioning (H&AC) system.d. Collect materials needed to clean up broken bulb.2. During cleanupa. Be thorough in collecting broken glass and visible powder.b. Place cleanup materials in a sealable container.3. After cleanupa. Promptly place all bulb debris and cleanup materials outdoors in a trashcontainer or protected area until materials can be disposed of properly.

Avoid leaving any bulb fragments or cleanup materials indoors.b. For several hours, continue to air out the room where the bulb wasbroken and leave the H&AC system shut off.


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Basic Physics of the Incandescent Lamp (Lightbulb)

Incandescence occurs when electrical

resistive heating creates thermally excited atoms.Some of the thermal kinetic energy is

transferred to electronic excitations within thesolid. The excited states are relieved by photonic

emission. When enough of the radiation emitted isin the visible spectrum so that we can see anobject by its own visible light, we say it isincandesc ing .

In a solid, there is a near-continuum ofelectron energy levels, resulting in a continuousnon-discrete spectrum of radiation.


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Bulb envelope is made of soft glass; topoperating temperature ~ 400 C. Fill gas usually argon to retard filamentevaporation with some nitrogen to eliminate

arcing. Exhaust tube extends through bulb base andis used to evacuate, and fill bulb before beingsealed off. Base , made of aluminum, is cemented to the

bulb. Eyele t is contact point to which electrical hotwire is soldered. Fuse protects circuit.

Glass rod that supports wires placed in it. Lead -in wires made of three welded metalsections carry current to and from filament,

passing through glass seals called the stem press.


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Coiled-coil tungsten filament is designed to maintain temperatureand glows yellow/orange hot at 3000 K.

The intricate coiled coil mechanical design of the filament isdesigned to retain as much thermal energy as possible while increasingsurface area.

The preferred krypton gas is too expensive, except for specialty

lamps where long life span is a priority (traffic-signal lights).

Lightbulbs smaller than 25 W require no fill gas, just apartial vacuum free of oxygen and water vapor.


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Filaments gradually degrade, and lightbulbs darken as tungstenevaporates from the filament surface to be deposited on the innersurface of the glass envelope. Convection currents in the fill gases willcarry the tungsten atoms to the top of the envelope to blacken thebulb.

Tungsten evaporates from the filament at higher temperaturelocations, creating a thermal runaway cycle: since filament hot spotsevaporate faster, locally thinned filament locations will develop higher

electrical resistances that rise in temperature, thereby reinforcinglocalized evaporation.

At startup, tungsten filaments are so cool that the initial inrushcurrent is 10 times greater than operating current, leading to strong

magnetic forces between adjacent coils of the filament. This thermaland mechanical stress ensures that most degraded household bulbswill fail or burn out in their first second of cold startup.

Expensive quartz infrared lamps counter this effect by preheatingwith a low voltage during startup.


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The Tungsten Halide Cycle

The blackening of a bulb due to evaporated tungsten depositscan be reduced by introducing traces of a halogen gas such asiodine. These halogens engage in a temperature-dependent cyclewith tungsten vapor in which tungsten halide forms at lowertemperatures.

Tungsten halide dissociates at the higher temperatures on thefilament. This halide cycle will return tungsten atoms from the fillgas and the silica envelope to the filament. While bulb blackening isreduced, tungsten is unfortunately not returned to the thinnest

parts.


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Halogen lamp filaments not only can be run hotter and moreefficiently, but must be run at higher temperatures to initiate andsustain the halogen cycle. Therefore, for halogen bulbs, a verysmall tubular envelope made of fused silica (a noncrystalline

quartz) is operated at temperatures up to 1200 C, together with ahigh-pressure fill gas (about five atmospheres).

To ensure full lamp life, halogen filaments should be run atleast 20 minutes to initiate the halogen gas cycle and fill-gas

convection.Halogen lamps are often not tolerant of changing orientations

because of their strong dependence on convection gas currents.Halogen lamps are whiter and hotter (usually 3000 to 3500 K) thanordinary bulbs, and the system is more efficient (10 to 12% of thespectral energy is in visible wavelengths). Halogen lamps havebecome standard in automobile headlamps and projector bulbs,and for photographic use.

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