In this article we will discuss about:- 1. Classification of Lamps 2. Insulation Resistance of a Lamp 3. Power Ratings of Different Types of Lamps 4. Relation between Lamp Voltage and Intensity of Illumination 5. Neon Night Lamp or Indicator Lamp 6. How a Gas-Discharge Lamp Glows 7. Cold Cathode and Hot Cathode Lamps and Other Details.
Contents:
- Classification of Lamps
- Insulation Resistance of a Lamp
- Power Ratings of Different Types of Lamps
- Relation between Lamp Voltage and Intensity of Illumination
- Neon Night Lamp or Indicator Lamp
- How a Gas-Discharge Lamp Glows
- Cold Cathode and Hot Cathode Lamps
- Difference in the Functioning of D.C. and A.C. Lamps
- Characteristics of Gas-Discharge Lamps other than Neon Lamp
1. Classification of Lamps:
Electric lamps may be classified into three different groups:
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(1) Arc lamps;
(2) Filament lamps;
(3) Gas discharge lamps in which discharge may take place in sodium vapour or mercury vapour or neon gas.
Of these three classes, arc lamps are practically no more in use nowadays. The luminosity of these lamps are so bright that they are totally unsuitable for indoor illumination. Formerly these lamps were often used in railway station, jetty, railway yard etc. which were either open spaces or partially open spaces. In these days, however, very powerful filament lamps or gas discharge lamps have replaced the arc lamps. Arc lamps are usually used in Search Lights of army and ships as well as in many cinema projectors and magic lanterns.
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Filament Lamps:
Lamps usually found in domestic houses are mostly filament lamps. In this lamp there is a glass bulb inside of which a filament made of special type of wire is used. Current flows through this filament which becomes white hot and emits rays of light. As light is emitted by the filament, this type of lamp is known as ‘filament lamp’.
Filament lamps available in the market are of three different types—
(a) Carbon lamp,
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(b) Metallic filament lamp, and
(c) Coiled-coil lamp.
Again, metallic filament lamps are of two types—
(i) Vacuum lamp, and
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(ii) Gas-filled lamp.
Coiled-coil lamp is also gas-filled type. Of these three types, carbon lamp appeared in the market first, then gradually the others followed.
Gas-Discharge Lamp:
Gas-discharge lamp (or fluorescent lamp) is different from filament lamp. An arc lamp is a very small instance of the lightning discharge that takes place in the sky. When two pieces of clouds in the sky are charged with electricity, the difference of potential (i.e. electrical tension) between them continuously goes on increasing until the smaller piece of cloud is no longer in a position to hold further charge.
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As a result the electric tension is brought to a steady balance by the flow of excess electricity from the smaller piece of cloud to the larger one piercing the air in the sky between the two clouds. When electricity discharges through air, flash of electricity takes place. This is called lightning stroke.
An arc lamp also works on the same principle. A bit away from each other there are two carbon rods in this lamp. After switching on these two rods are touched together for a moment and then they are separated by a very short distance which is instantly bridged by an electric arc through the air. This arc maintains continuous flow of current between two electrodes. Bigger the cross-sectional area of the rods, longer will be the length of the arc. This will carry more current and emit more amount of light.
The terminal from which current enters into air is called positive terminal or positive electrode of the lamp. The terminal through which current flows back to supply line from air is called negative terminal or negative electrode.
In an arc lamp current flows through air. If current is passed through any other gas, the lamp is known as gas-discharge lamp. It is not necessary to place the electrodes under a cover when current is passed through air. But when current flows through any other gas, the two electrodes are placed at the two ends of a closed glass tube from which air is taken out and the vacuum tube is then filled with a particular gas and finally the tube is so sealed as to be air-tight. Now, if suitable voltage is applied between two electrodes, the tube becomes a gas-discharge lamp.
Difference between Arc Lamp and Gas-Discharge Lamp:
(1) In an arc lamp current passes through air in open space. Hence, it has to overcome atmospheric pressure in its passage. But in a gas-discharge lamp gas remains in the sealed glass tube at a much reduced pressure.
(2) The amount of electrical energy spent in sending current by overcoming the ordinary atmospheric pressure is much more than that spent to obtain the same illumination by sending current through rarefied gas.
(3) The amount of heat evolved in a gas-discharge lamp is much less than that evolved in an arc lamp.
(4) The voltage required for an arc lamp is about 60 volts. But a gas-discharge lamp becomes luminous at a much higher voltage.
(5) Neon tube etc. takes only about 1% of current which flows through an arc lamp.
2. Insulation Resistance of a Lamp:
For a lamp one point is very essential, but nobody seems to be sufficiently watchful to it. The point is that there must be an insulation resistance of at least 50 megohms between the contacts attached to the cap and the external brass or aluminium covering of the cap. Otherwise a defective lamp, either in a stiff pendant holder or in a bracket holder of conduit wiring, adversely affects the insulation resistance of the entire wiring.
3. Power Ratings of Different Types of Lamps:
Three different types of glass bulbs are usually used for lamps,—clear bulbs, pearl bulbs and opal bulbs. A clear bulb should not be used unless it is covered by a reflector or a shade, lest it should adversely affect the eye. The inner surface of a pearl bulb is hazy and the lamp filament is not visible from outside.
The illumination emitted by this lamp is almost uniform in all directions. But the amount of illumination available is 2% less than that emitted by a similar lamp (same wattage and voltage) with a clear bulb. It is true that 10-12% less illumination is available from lamps having opal bulbs, but the most uniform distribution with same intensity in general illumination is also available from this lamp.
There is also another type of lamp with bulb made of blue glass. This lamp is known as day-light lamp. The light emitted by this lamp is almost like day-light (not direct sun light). To replace a 100-watt lamp with clear bulb, it is necessary to use a daylight lamp rated 140-150 watts.
Vacuum lamps are generally available in ratings of 15 and 25 watts. They are made to suit 100-130 volts and 200-260 volts supply pressure. Gas-filled lamps are made to suit all low pressure supply and are available in all ratings from 25 to 1500 watts. Coiled-coil lamps are generally suitable for 200-260 volts pressure and lamps rated from 40-watt up to 100-watt are available in the market.
4. Relation between Lamp Voltage and Intensity of Illumination:
It is very essential to observe whether the rated voltage is actually reaching the lamp terminals or not. If the terminal voltage of a lamp is more than the rated voltage, the lamp will indeed glow brighter, but the life of filament will be shorter. On the other hand if the terminal voltage is less, the lamp will surely last longer, but the intensity of illumination will be less. Hence, the correct practice should be at first to measure the voltage available to every lamp point by means of a voltmeter.
A lamp suitable for that voltage as far as practicable should then be fitted there. If this is not done, the calculations for determining correct sizes of lamps are useless. This, however, is not done in actual cases. In practice, lamps rated for the same voltage are connected everywhere without discrimination.
Now, from above discussion one may think that since a lamp lasts longer when its rated voltage is a bit more than that actually available at its terminals, we should bear with correspondingly a bit less illumination. This is alright from economical point of view. But from the point of view of illumination this argument is not at all correct.
The reduction in illumination is not directly proportional to the reduction in terminal voltage; it is much more than that. As for example, let us consider a 22C-volt, 60- watt, coiled-coil lamp. Let the voltage reaching the lamp terminals be 2% less.(i.e. 4.4 volts less).
In that case intensity of illumination will be 8% less than that obtained from a lamp having a terminal voltage of 220 volts. Thus, the illumination is much less than what is necessary. It may, be. tolerable in a common residence, but such illumination is considered to be quite insufficient for a workshop, drawing office, laboratory etc.
5. Neon Night Lamp or Indicator Lamp:
A tube filled with neon gas emits very bright red colour. The penetrating power of this light is very high. Usually red neon lamp is not used for domestic purposes. It is used to display big size letters or for decorative lighting.
Illumination is also available from a neon lamp.
Its estimate is as follows:
The amount of illumination available from a 6 to 7.5 meters (20 to 25 feet) long neon tube, 11 mm in diameter, is almost equal to that available from a 150-watt filament lamp.
This lamp is used where a little light is required. The bulb of this lamp contains a mixture of neon gas and a little helium gas. Two iron electrodes but no filament are used in it. Current from the supply line reaches one electrode, passes through the gas inside the bulb and finally goes out through the other electrode. As a result the gas becomes luminous and emits light. The light is somewhat reddish in colour.
The two electrodes form parallel coils maintaining a small distance between them, thus avoiding a direct contact. Alternatively, there may be a central iron disc surrounded by a thin sheet of iron. These are the two electrodes. It consumes little power,—only one half watt or one watt is consumed by this lamp.
A very high resistance is connected in series and placed inside the cap of the neon lamp. For this resistance the lamp draws always the same amount of current from the supply line. Otherwise the amount of current would have increased with the rise of temperature of the lamp. Those who must have to use a lamp at night in the bedroom, use this type of lamp.
6. How a Gas-Discharge Lamp Glows:
It has already been stated that lamp begins to work when vacuum is produced in a sealed glass tube by taking out air, a gas like neon etc. is filled inside the tube at a very low pressure and finally connecting the two electrodes at two ends to a suitable supply voltage.
Every substance in nature is made up of a large number of molecules—be it solid, liquid, gas or vapour. The finest particle of any substance is called a molecule. Usually no sign of presence of electricity appears in such molecules. But if current is passed through a gas, the molecules break up. When molecules break up, the substance no longer remains the same. Then two opposite kinds of electricity evolves out of the substance. One of these is called positive ion and the other is known as electron.
In other words, a molecule is produced as a result of combination of positive ion and electron together. Of these two, positive ion is positively charged and electron is negatively charged. A combination of these two together makes up a molecule of a substance, and no sign of electricity then appears in it.
When current is passed through a gas, it breaks up the molecules of the gas, and if, by any means, negatively charged particles, i.e. electrons can be separated, then what remain of the molecules are the ions with positive electric charge. All these positive ions, having the same kind of charge, repel each other. Whenever a gas molecule is broken up into positive ions and electrons in a sealed glass tube, the charged particles naturally come in close contact with each other and tend to recombine into a molecule again. As soon a§ they recombine, they emit a brilliant light. This light produced from the gas is the light from a gas-discharge lamp.
7. Cold Cathode and Hot Cathode Lamps:
Molecules can be broken up either by sending current through electrodes at high voltage or by heating electrodes in any way (e.g. with the help of a starter) till they become red hot. The lamp in which the former method is used is a ‘cold cathode discharge lamp’, and the latter method is used in a ‘hot cathode lamp’. Neon lamp is the example of cold cathode discharge lamp, while fluorescent lamp is a hot cathode discharge lamp.
Voltage:
As soon as the electrodes in a hot cathode lamp are charged with electricity, molecules of the gas are broken up and the negatively charged electrons rush towards the positive electrode, i.e. anode. (The process of breaking up of molecules of a gas into electrons and positive ions is known as ‘Ionisation’ of gas).
More the supply voltage, more is the speed at which electrons are attracted towards anode. If the supply voltage is not sufficiently high, the ionisation is very weak and the electrons are not attracted towards the anode with suitable speed. Hence, no current flows through the sealed tube. For this reason sufficiently high voltage is required, otherwise the lamp cannot emit light.
At the beginning of glowing of the lamp a pressure of the order of 15,000 volts is required, otherwise the gas molecules are not ionised. But another feature of this type of lamp is that, once it begins to glow, the voltage necessary to maintain the discharge is not as large as the voltage required to start the glow. If the voltage required to start the lamp continues to be applied even after start, an excess current will flow through the lamp resulting in excessive heat produced in the gas. The luminosity of the lamp will then be dull, span of life of the lamp will be shortened and the power consumed by it will increase.
The speed at which the electrons rush towards the anode is very high. Being attracted towards anode while on the point of running at very high speed, they collide with unbroken molecules which in turn break up due to this collision. The fresh electrons thus released also rush towards the anode with very high velocity and break up new molecules that come into collision with them.
The positively charged ion of these molecules recombine with a colliding electron and produce a molecule again. It is at the time of combination of a positive ion with an electron that a molecule is formed and the shine comes out. As soon as the switch is on, gas inside the tube is vigorously agitated on account of excessively high speed of electrons.
According to physical properties of each gas, one or more electrons are liberated from a molecule. These electrons rush towards the anode and, recombine with freshly released positive ions to form a molecule again. As a result the whole tube becomes luminous.
8. Difference in the Functioning of D.C. and A.C. Lamps:
Alternating current is more suitable for gas-discharge lamps than direct current. This type of lamp also works with d.c. supply, but advantages with a.c. supply are more. The main advantage of a.c. is that supply pressure can be easily increased by means of a static transformer. This is not normally possible with d.c. For d.c. supply a rotating machine is required. Besides, d.c. voltages of the order of 15,000 volts cannot be directly generated.
Another disadvantage with d.c. supply is that quality of illumination obtained from an a.c. lamp is much better than that obtained from a d.c. lamp. It has been observed that a cluster of disjointed shining spaces is found near and positive electrode. a dark zone near the middle and again a shining zone at some distance from the negative electrode of a d.c. lamp (see Fig. 208). Thus, the entire tube does not become luminous. But in case of a.c. supply, since the supply frequency is 50 hertz in our country, each electrode becomes 50 times positive and 50 times negative in a second.
With every change of polarity of an electrode the positions of shining zone and dark zone are interchanged. The speed of such changes is 100 times in a second. Therefore, the persistence of vision makes us unable to see these changes, and the entire tube appears to be brightly luminous all the time. A glimpse of the nature of shining zone as found in an a.c. lamp is shown in Fig. 209.
How an a.c. voltage can be very easily increased or decreased from 110 or 230 volts by means of a transformer is shown in Fig. 210, so that the supply pressure is suitable for a gas-discharge lamp. Here one terminal of an auto-transformer is connected to one line of a 110 or 230 volts supply system. Different tappings are available in this transformer to obtain different voltages.
The tapping which is just suitable for lamp voltage is connected in series with the lamp. Through a suitable fuse and a switch the lamp terminal is then connected to other terminal of the supply line. This arrangement ensures rated supply voltage for the lamp. But in this case voltage reaching the lamp terminals will be less than the supply voltage.
The auto-transformer can also be used in a different way so that a voltage greater than the supply voltage is available at the lamp terminals. The diagram for this connection is shown in Fig. 211. On account of inductive reactance of the transformer coil the power factor of the circuit as a whole is reduced to 0.58. To improve the power factor a capacitor is to be used across the supply lines. This capacitor helps to improve the power factor up to 0.95.
Current:
The amount of current that should be sent through a gas-discharge lamp in order to obtain sufficient illumination depends on the diameter of the tube, on the gas with which the tube is filled up, the voltage reaching the lamp terminals, the kind of electrode that has been used and on the pressure of the gas inside the tube. If each of these items is of just suitable size and quality, the lamp will draw necessary current to produce proper illumination. If the amount of current is less, the intensity of illumination will be less. With current greater than the rated value, the lamp will be excessively heated up and its span of life will be shortened.
Tube filled with neon gas takes 0.01 to 0.05 ampere (10 to 50 mA). In some special cases it may take 0.1 to 1.0 ampere (100 to 1000 mA).
The more is the pressure of the gas inside, the less is the amount of current through the tube. The luminosity of the lamp decreases with the decrease in amount of current. Again, the lamp takes less current for a very low pressure also. It has been found that the maximum current passes through the lamp at about 3 mm pressure.
9. Characteristics of Gas-Discharge Lamps other than Neon Lamp:
i. Yellow Colour:
The tube that emits yellow colour is filled with helium gas. It requires higher supply pressure than a neon lamp. Also, the power consumption of this lamp is nearly double, as the resistivity of helium gas is nearly twice that of neon gas. The pressure of the gas inside the tube is about 3 mm.
Yellow light is also available from Sodium Vapour Lamp. It has good penetration through fog. But any coloured light is not good enough for identifying colour of an object.
ii. Blue Colour:
Argon gas with a little mercury vapour inside the tube gives blue light. Blue colour is emitted when argon gas is ionised. At the time of ionisation, greenish blue colour is emitted by mercury vapour. This is why argon can be used in the same tube with mercury vapour. This removes the difficulties that arise when only mercury vapour is used.
iii. Mercury-Vapour Lamp:
The disadvantage with a mercury vapour lamp is that red colour cannot be properly identified in its light. This lamp emits bluish green light.
iv. Green Colour:
If a yellow coloured tube is filled with argon gas and mercury vapour, it will emit green light.
v. Neon Sign:
The connections of a neon sign is shown in Fig. 233. For the ionisation of the gas inside the neon tube, supply is given at a high pressure through a step-up transformer.
Single-phase a.c. supply lines at low pressure are connected to a S.P.N.I.C. (single-pole and neutral iron clad) switch used as a main switch. For the convenience of fireman, another S.P.N.I.C. Switch is installed just after the main switch.
The supply lines are drawn from the main switch to fireman’s switch, and from there the lines are drawn up to primary terminals of the transformer through a capacitor and a choke coil. The capacitor is connected across the lines for the improvement of power factor, while choke coil is connected in series with the live line for the surge voltage.
Since the transformer is a step-up one, its secondary coil has the larger number of turns than the primary coil. The mid-tapping of the secondary coil must be available for earth connection. In this circuit another suitable choke is used for suppression of interference. The connections of two choke coils, one for the surge voltage and the other for the suppression of interference, are shown in Fig. 233.
After ionisation of the gas when supply is required at a comparatively less pressure, necessary voltage drop takes place across the choke coil connected in series with the live line on the primary side. The choke coil for the suppression of interference is always connected on the secondary side of the transformer. The neon sign is connected across the secondary winding.
