how fluorescent lamps work by nafees

7/25/2019 How Fluorescent Lamps Work by nafees

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How Light Bulbs Work

Before the invention of the light bulb, illuminating the world after the sunwent

down was a messy, arduous, hazardous task. It took a bunch

of candlesor torchesto fully light up a good-sized room, and oil lamps, while fairly

eective, tended to leave a residue of soot on anything in their general vicinity.

When the science of electricity really got going in the mid !""s, inventors

everywhere were clamoring to devise a practical, aordable electrical home lightingdevice. #nglishman $ir %oseph $wan and &merican 'homas #dison both got it right

around the same time (in !)! and !)*, respectively+, and within years,

millions of people around the world had installed electricallighting in their homes.

'he easy-to-use technology was such an improvement over the old ways that the

world never looked back.

'he amazing thing about this historical turn of events is that the light bulb itself

could hardly be simpler. 'he modern light bulb, which hasnt changed drastically

since #disons model, is made up of only a handful of parts. In this article, well see

how these parts come together to produce bright light for hours on end.

/ight Basics

/ightis a form of energy that can be released by an atom. It is made up of many

small particle-like packets that have energy and momentum but no mass. 'hese

particles, called light photons, are the most basic units of light. (0or more

information, see 1ow /ight Works.+
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&toms release light photons when their electrons become e2cited. If youve

read 1ow &toms Work, then you know that electrons are the negatively charged

particles that move around an atoms nucleus (which has a net positive charge+. &n

atoms electrons have dierent levels of energy, depending on several factors,

including their speed and distance from the nucleus. #lectrons of dierent energy

levels occupy dierent orbital3s. 4enerally speaking, electrons with greater energymove in orbitals farther away from the nucleus. When an atom gains or loses

energy, the change is e2pressed by the movement of electrons. When something

passes energy on to an atom, an electron may be temporarily boosted to a higher

orbital (farther away from the nucleus+. 'he electron only holds this position for a

tiny fraction of a second5 almost immediately, it is drawn back toward the nucleus,

to its original orbital. &s it returns to its original orbital, the electron releases the

e2tra energy in the form of a photon, in some cases a light photon.

'he wavelength of the emitted light (which determines its color+ depends on how

much energy is released, which depends on the particular position of the electron.

6onse7uently, dierent sorts of atoms will release dierent sorts of light photons. In

other words, the color of the light is determined by what kind of atom is e2cited.

'his is the basic mechanism at work in nearly all light sources. 'he main dierence

between these sources is the process of e2citing the atoms.

In the ne2t section well look at the dierent parts of a light bulb.

/ight Bulb $tructure
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/ight bulbs have a very simple structure. &t the base, they have two metal contacts,

which connect to the ends of an electricalcircuit. 'he metal contacts are attached

to two sti wires, which are attached to a thin metal 8lament. 'he 8lament sits in

the middle of the bulb, held up by a glass mount. 'he wires and the 8lament are

housed in a glass bulb, which is 8lled with an inert gas, such as argon.

When the bulb is hooked up to a power supply, an electric current 9ows from one

contact to the other, through the wires and the 8lament. #lectric current in a solid

conductor is the mass movement of free electrons (electrons that are not tightly

bound to an atom+ from a negatively charged area to a positively charged area.

&s the electrons zip along through the 8lament, they are constantly bumping into

the atoms that make up the 8lament. 'he energy of each impact vibrates an atom --

in other words, the current heats the atoms up. & thinner conductor heats up more

easily than a thicker conductor because it is more resistant to the movement of

electrons.

Bound electrons in the vibrating atoms may be boosted temporarily to a higher

energy level. When they fall back to their normal levels, the electrons release the

e2tra energy in the form of photons. :etal atoms release mostly infrared light

photons, which are invisible to the human eye. But if they are heated to a high

enough level -- around ;,""" degrees 0ahrenheit (,"" degrees 6+ in the case of a

light bulb -- they will emit a good deal of visible light.

'he 8lament in a light bulb is made of a long, incredibly thin length

of tungsten metal. In a typical


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when one of the chemicals has reached its ignition temperature. =n #arth,

combustion is usually a reaction between o2ygen in the atmosphere and some

heated material, but other combinations of chemicals will combust as well.

'he 8lament in a light bulb is housed in a sealed, o2ygen-free chamber to prevent

combustion. In the 8rst light bulbs, all the air was sucked out of the bulb to create anear vacuum -- an area with no matter in it. $ince there wasnt any gaseous matter

present (or hardly any+, the material could not combust.

'he problem with this approach was the evaporation of the tungsten atoms. &t such

e2treme temperatures, the occasional tungsten atom vibrates enough to detach

from the atoms around it and 9ies into the air. In a vacuum bulb, free tungsten

atoms shoot out in a straight line and collect on the inside of the glass. &s more and

more atoms evaporate, the 8lament starts to disintegrate, and the glass starts to

get darker. 'his reduces the life of the bulb considerably.

In a modern light bulb, inert gases, typically argon, greatly reduce this loss of

tungsten. When a tungsten atom evaporates, chances are it will collide with an

argon atom and bounce right back toward the 8lament, where it will re>oin the solid

structure. $ince inert gases normally dont react with other elements, there is no

chance of the elements combining in a combustion reaction.

6heap, eective and easy-to-use, the light bulb has proved a monstrous success. It

is still the most popular method of bringing light indoors and e2tending the day

after sundown. But by all indications, it will eventually give way to more advanced

technologies, because it isnt very e?cient.

Incandescent light bulbs give o most of their energy in the form of heat-carrying

infrared light photons -- only about " percent of the light produced is in the visible

spectrum. 'his wastes a lot of electricity. 6ool light sources, such as 9uorescent

lampsand /#@s, dont waste a lot of energy generating heat -- they give o mostly

visible light. 0or this reason, they are slowly edging out the old reliable light bulb.

How Fluorescent Lamps Work

Aou see 9uorescent lighting everywhere these days -- in o?ces, stores, warehouses,

street corners... Aoull even 8nd 9uorescent lamps in peoples homes. But even

though theyre all around us, these devices are a total mystery to most people. %ust

what is going on inside those white tubes

In this article, well 8nd out how 9uorescent lamps emit such a bright glow without

getting scalding hot like an ordinary light bulb. Well also 8nd out why 9uorescent

lamps are more e?cient than incandescent lighting, and see how
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Let There Be Light

'o understand 9uorescent lamps, it helps to know a little about light itself. /ight is a

form of energy that can be released by an atom. It is made up of many small

particle-like packets that have energy and momentum but no mass. 'hese particles,called light photons, are the most basic units of light. (0or more information,

see 1ow /ight Works.+

&toms release light photons when their electronsbecome e2cited. If youve

read 1ow &toms Work, then you know electrons are the negatively charged particles

that move around an atoms nucleus (which has a net positive charge+. &n atoms

electrons have dierent levels of energy, depending on several factors, including

their speed and distance from the nucleus. #lectrons of dierent energy levels

occupy dierent orbitals. 4enerally speaking, electrons with greater energy move

in orbital farther away from the nucleus.

When atom gains or losses energy, the change is e2pressed by the movement of

electrons. When something passes energy on to an atom -- heat, for e2ample -- an

electron may be temporarily boostedto a higher orbital (farther away from the

nucleus+. 'he electron only holds this position for a tiny fraction of a second5 almost

immediately, it is drawn back toward the nucleus, to its original orbital. &s it returns

to its original orbital, the electron releases the e2tra energy in the form of a photon,

in some cases a light photon.
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'he wavelengthof the emitted light depends on how much energy is released,

which depends on the particular position of the electron. 6onse7uently, dierent

sorts of atoms will release dierent sorts of light photons. In other words,

the colorof the light is determined by what kind of atom is e2cited.

'his is the basic mechanism at work in nearly all light sources. 'he main dierencebetween these sources is the process of e2citing the atoms. In an incandescent

light source, such as an ordinary light bulbor gas lamp, atoms are e2cited by

heat5 in a light stick, atoms are e2cited by a chemical reaction. 0luorescent lamps

have one of the most elaborate systems for e2citing atoms, as well see in the ne2t

section.

Down the Tubes

'he central element in a 9uorescent lamp is a sealed glass tube. 'he tube

contains a small bit of mercuryand an inert gas, typically argon, kept under very

low pressure. 'he tube also contains a phosphor powder, coated along the inside

of the glass. 'he tube has twoelectrodes, one at each end, which are wired to an

electrical circuit. 'he electrical circuit, which well e2amine later, is hooked up to an

alternating current (&6+ supply.

When you turn the lamp on, the current 9ows through the electrical circuit to the

electrodes. 'here is a considerable voltage across the electrodes, so electrons will

migrate through the gas from one end of the tube to the other. 'his energy changes

some of the mercuryin the tube from a li7uid to a gas. &s electrons and charged

atoms move through the tube, some of them will collidewith the gaseous mercuryatoms. 'hese collisions e2cite the atoms, bumping electrons up to higher energy

levels. When the electrons return to their original energy level, they release light

photons.

&s we saw in the last section, the wavelength of a photon is determined by the

particular electron arrangement in the atom. 'he electrons in mercury atoms are

arranged in such a way that they mostly release light photons in
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the ultravioletwavelength range. =ur eyes dont register ultraviolet photons, so

this sort of light needs to be converted into visible light to illuminate the lamp.

'his is where the tubes phosphor powder coating comes in. Phosphorsare

substances that give o light when they are e2posed to light. When a photon hits a

phosphor atom, one of the phosphors electrons >umps to a higher energy level andthe atom heats up. When the electron falls back to its normal level, it releases

energy in the form of another photon. 'his photon has less energy than the original

photon, because some energy was lost as heat. In a 9uorescent lamp, the emitted

light is in the visible spectrum -- the phosphor gives o white lightwe can see.

:anufacturers can vary the color of the light by using dierent combinations of

phosphors.

6onventional incandescent light bulbs also emit a good bit of ultraviolet light, but

they do not convert any of it to visible light. 6onse7uently, a lot of the energy used

to power an incandescent lamp is wasted. & 9uorescent lamp puts this invisible light

to work, and so is more efcient. Incandescent lamps also lose more energythrough heat emission than do 9uorescent lamps. =verall, a typical 9uorescent lamp

is four to si2 times more e?cient than an incandescent lamp. Ceople generally use

incandescent lights in the home, however, since they emit a DwarmerD light -- a light

with more red and less blue.

&s weve seen, the entire 9uorescent lamp system depends on an electrical current

9owing through the gas in the glass tube. In the ne2t section, well see what a

9uorescent lamp needs to do to establish this current.

ooking with !as

In the last section, we saw that mercury atoms in a 9uorescent lamps glass tube are

e2cited by electrons 9owing in an electrical current. 'his electrical current is

something like the current in an ordinary wire, but it passes through gas instead of

through a solid. !as conductorsdier from solid conductors in a number of ways.

In a solid conductor, electrical charge is carried by free electrons >umping from atom

to atom, from a negatively-charged area to a positively-charged area. &s weve

seen, electrons always have a negative charge, which means they are always drawn

toward positive charges. In a gas, electrical charge is carried by "ree

electronsmoving independently of atoms. 6urrent is also carried by ions, atoms

that have an electrical charge because they have lost or gained an electron. /ikeelectrons, ions are drawn to oppositely charged areas.

'o send a current through gas in a tube, then, a 9uorescent light needs to have two

thingsE

. Free electrons and ions

. # di$erence in charge between the two ends o" the tube (a voltage+
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4enerally, there are few ions and free electrons in a gas, because all of the atoms

naturally maintain a neutral charge. 6onse7uently, it is di?cult to conduct an

electrical current through most gases. When you turn on a 9uorescent lamp, the

8rst thing it needs to do is introduce many new "ree electronsfrom both

electrodes.

%tart it &p

'he classic 9uorescent lamp design, which has fallen mostly by the wayside, used a

special starter switch mechanism to light up the tube. Aou can see how this system

works in the diagram below.

When the lamp 8rst turns on, the path of least resistance is through the bypass

circuit, and across thestarter switch. In this circuit, the current passes through

the electrodes on both ends of the tube. 'hese electrodes are simple 'laments,

like you would 8nd in an incandescent light bulb. When the current runs through the

bypass circuit, electricity heats up the 8laments. 'his boils o electrons from the

metal surface, sending them into the gas tube, ionizing the gas.

&t the same time, the electrical current sets o an interesting se7uence of events in

the starter switch. 'he conventional starter switch is a small discharge bulb,

containing neon or some other gas. 'he bulb has two electrodes positioned right

ne2t to each other. When electricity is initially passed through the bypass circuit,

an electrical arc(essentially, a 9ow of charged particles+ >umps between these

electrodes to make a connection. 'his arc lights the bulb in the same way a larger

arc lights a 9uorescent bulb.
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=ne of the electrodes is abimetallic stripthat bends when it is heated. 'he small

amount of heat from the lit bulb bends the bimetallic strip so it makes contact with

the other electrode. With the two electrodes touching each other, the current

doesnt need to >ump as an arc anymore. 6onse7uently, there are no chargedparticles 9owing through the gas, and the light goes out. Without the heat from the

light, the bimetallic strip cools, bending away from the other electrode. 'his opens

the circuit.

(nside the casing o" a conventional )uorescent starter there is a small gas

discharge lamp*

By the time this happens, the 8laments have already ionized the gas in the

9uorescent tube, creating an electrically conductive medium. 'he tube >ust needs a

voltage kick across the electrodes to establish an electrical arc. 'his kick is provided

by the lamp3sballast, a special sort of transformer wired into the circuit.

When the current 9ows through the bypass circuit, it establishes a magnetic

'eldin part of the ballast. 'his magnetic 8eld is maintained by the 9owing current.

When the starter switch is opened, the current is brie9y cut o from the ballast. 'he

magnetic 8eld collapses, which creates a sudden >ump in current -- the ballast

releases its stored energy.


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+apid start and starter switch )uorescent bulbs have two pins that slide

against two contact points in an electrical circuit*

Light +ight #way

'oday, the most popular 9uorescent lamp design is therapid startlamp. 'his

design works on the same basic principle as the traditional starter lamp, but it

doesnt have a starter switch. Instead, the lamps ballastconstantly channelscurrent through both electrodes. 'his current 9ow is con8gured so that there is a

charge dierence between the two electrodes, establishing a voltage across the

tube.

When the 9uorescent light is turned on, both electrode 8laments heat up very

7uickly, boiling o electrons, which ionize the gas in the tube. =nce the gas is

ionized, the voltage dierence between the electrodes establishes an electrical arc.

'he 9owing charged particles (red+ e2cite the mercury atoms (silver+, triggering the

illumination process.

&n alternative method, used in instant,start9uorescent lamps, is to apply a veryhigh initial voltage to the electrodes. 'his high voltage creates a corona discharge.

#ssentially, an e2cess of electrons on the electrode surface forces some electrons

into the gas. 'hese free electrons ionize the gas, and almost instantly the voltage

dierence between the electrodes establishes an electrical arc.

Fo matter how the starting mechanism is con8gured, the end result is the sameE a

9ow of electrical current through an ionized gas. 'his sort of gas dischargehas a

peculiar and problematic 7ualityE If the current isnt carefully controlled, it will

continually increase, and possibly e2plode the light 82ture. In the ne2t section, well

8nd out why this is and see how a 9uorescent lamp keeps things running smoothly.
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The ballast- starter switch and )uorescent bulb are all wired together in a

simple circuit*

'his surgein current helps build the initial voltage needed to establish the

electrical arc through the gas. Instead of 9owing through the bypass circuit and

>umping across the gap in the starter switch, the electrical current 9ows through the

tube. 'he free electrons collide with the atoms, knocking loose other electrons,

which create ions. 'he result is plasma, a gas composed largely of ions and free

electrons, all moving freely. 'his creates a path for an electrical current.

'he impact of 9ying electrons keeps the two 8laments warm, so they continue to

emit new electrons into the plasma. &s long as there is &6 current, and the

8laments arent worn out, current will continue to 9ow through the tube.

'he problem with this sort of lamp is it takes a few seconds for it to light up. 'hese

days, most 9uorescent lamps are designed to light up almost instantly. In the ne2t

section, well see how these modern designs work.

Ballast Balance

We saw in the last section that gases dont conduct electricity in the same way as

solids. =ne ma>or dierence between solids and gases is their electrical

resistance(the opposition to 9owing electricity+. In a solid metal conductor such as

a wire, resistance is a constant at any given temperature, controlled by the size of

the conductor and the nature of the material.

In a gas discharge, such as a 9uorescent lamp, current causes resistance to

decrease. 'his is because as more electrons and ions 9ow through a particular area,

they bump into more atoms, which frees up electrons, creating more charged

particles. In this way, current will climb on its own in a gas discharge, as long as

there is ade7uate voltage (and household &6 current has a lot of voltage+. If thecurrent in a 9uorescent light isnt controlled, it can blow outthe various electrical

components.

0luorescent lamps ballastworks to control this. 'he simplest sort of ballast,

generally referred to as magneticballast, works something like an inductor. & basic

inductor consists of a coil of wire in a circuit, which may be wound around a piece of

metal. If youve read 1ow #lectromagnets Work, you know that when you send
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electrical current through a wire, it generates a magnetic 8eld. Cositioning the wire

in concentric loops ampli8es this 8eld.

'his sort of 8eld aects not only ob>ects around the loop, but also the loop itself.

Increasing the current in the loop increases the magnetic 8eld, which applies a

voltage opposite the 9ow of current in the wire. In short, a coiled length of wire in acircuit (an inductor+ opposes change in the current 9owing through it.

'he trans"ormer elementsin a magnetic ballast use this principle to regulate the

current in a 9uorescent lamp.

Ballast can only slow down changes in current -- it cant stop them. But the

alternating current powering a 9uorescent light is constantly reversingitself, so

the ballast only has to inhibit increasing current in a particular direction for a short

amount of time. 6heck out this sitefor more information on this process.

:agnetic ballasts modulate electrical current at a relatively low cycle rate, which

can cause a noticeable 9icker. :agnetic ballasts may also vibrate at a low

fre7uency. 'his is the source of the audible humming sound people associate with

9uorescent lamps.

:odern ballast designs use advanced electronics to more precisely regulate the

current 9owing through the electrical circuit. $ince they use a higher cycle rate, you

dont generally notice a 9icker or humming noise coming from electronic ballast.

@ierent lamps re7uire specialized ballasts designed to maintain the speci8c

voltage and current levels needed for varying tube designs.

0luorescent lamps come in all shapes and sizes, but they all work on the same basic

principleE &n electric current stimulates mercury atoms, which causes them to

release ultraviolet photons. 'hese photons in turn stimulate a phosphor, which

emits visible light photons. &t the most basic level, thats all there is to itG

How +elays Work

& relay is a simple electromechanical switchmade up of an electromagnet and a

set of contacts. Helays are found hidden in all sorts of devices. In fact, some of the

8rst computers ever built used relays to implement Boolean gates.

In this article, we will look at how relays work and a few of their applications

+elay onstruction

Helays are amazingly simple devices. 'here are four parts in every relayE

.lectromagnet

#rmaturethat can be attracted by the electromagnet

%pring
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%et o" electrical contacts

'he following 8gure shows these four parts in actionE

In this 8gure, you can see that a relay consists of two separate and completely

independent circuits. 'he 8rst is at the bottom and drives the electromagnet. In

this circuit, a switch is controlling power to the electromagnet. When the switch is

on, the electromagnet is on, and it attracts the armature (blue+. 'he armature is

acting as a switch in the second circuit. When the electromagnet is energized, the

armature completes the second circuit and the light is on. When the electromagnet

is not energized, the spring pulls the armature away and the circuit is not complete.

In that case, the light is dark.

When you purchase relays, you generally have control over several variablesE

'he voltage and current that is needed to activate the armature

'he ma2imum voltage and current that can run through the armature and thearmature contacts

'he number of armatures (generally one or two+

'he number of contacts for the armature (generally one or two -- the relay

shown here has two, one of which is unused+

Whether the contact (if only one contact is provided+ is normally open (/0+

or normally closed (/+

+elay #pplications

In general, the point of a relay is to use a small amount of power in

theelectromagnet-- coming, say, from a small dashboard switch or a low-power

electronic circuit -- to move an armature that is able to switch a much larger amount

of power. 0or e2ample, you might want the electromagnet to energize using volts

and " milliamps (" mill watts+, while the armature can support " &6 at

amps (;" watts+.

Helays are 7uite common in home appliances where there is an electronic control

turning on something like a motor or a light. 'hey are also common in cars, where

the supply voltage means that >ust about everything needs a large amount of

current. In later model cars, manufacturers have started combining relay panels into

the fuse bo2 to make maintenance easier. 0or e2ample, the si2 gray bo2es in this

photo of a 0ord Windstar fuse bo2 are all relaysE

In places where a large amount of power needs to be switched, relays are

often cascaded. In this case, a small relay switches the power needed to drive a

much larger relay, and that second relay switches the power to drive the load.
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how fluorescent lamps work by nafees

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