Sodium lamps: design, principle of operation, types, application

The designs of the first lights were rather primitive. They consisted of two electrodes, between which an arc discharge burned. There were two significant drawbacks in these designs: due to burnout, the electrodes needed constant adjustment, and the radiation spectrum captured a significant portion of ultraviolet radiation. Therefore, incandescent lamps, and later sodium lamps very quickly occupied their niches in the lighting of rooms and streets.

In fairness, I must say that even these lighting devices still compete with brands of more economical LED lamps.

But there are areas where the use of sodium light bulbs will be a priority for a long time to come. Optimism adds a high flux to discharge lamps, duration of operation and high efficiency indicators of these devices.

Design and principle of operation

The action of the sodium discharge lamp is based on the property of sodium vapor, capable of emitting monochromatic bright light in the yellow-orange spectrum. This gaseous substance is enclosed in a special flask (tube) called a burner. Since sodium vapor heated to a high temperature acts aggressively on glass surfaces, the tube made of more stable substances - borosilicate glass or polycrystalline alumina (depending on type of lamp).

On each side of the burner are electrodes designed to create arc discharges that heat sodium vapor. This design is housed in a vacuum glass flask ending with a threaded base.

It is appropriate to note here that there are two types of such lighting devices: NLND (low pressure) and NLVD (high pressure). The design described above gives a general idea of ​​the construction of sodium discharge lamps of both types. These lamps differ in the design of the burners and the working vapor pressure inside the tubes.

In low-pressure sodium lamps, its value does not exceed 0.2 Pa, and in NLVD - about 10 kPa. Correspondingly, the working temperatures of sodium vapors also differ: 270–300 ° С for NLND and 650–750 ° С in high-pressure burners. From this it is clear that the NLVD burners have rather high levels of light flux, that is, they shine quite brightly.

There is nothing surprising in the fact that high-pressure sodium lamps gradually displaced NLND-type lighting fixtures from the market. Although the light spectrum corresponding to low pressure is more pleasing to the eye, NLND burners gave way to more powerful models with fairly high light emission.

Given this circumstance, we will focus on the NLVD type lamps. The design of such a light source is shown in Figure 1. Here is a diagram of a tubular lamp DNaT.

The numbers indicate:

• 1 - external flask;
• 2 - nickel plated base;
• 3 - contact plates;
• 4 - gas discharge tube (burner);
• 5 - molybdenum electrodes;
• 6 - sodium vapor mixed with inert gases (argon or xenon);
• 7 - sodium amalgam;
• 8 - sealed niobium input;
• 9 - metal conductors;
• 10 - molybdenum plates;
• 11 - getters (getters).

In fig. 2 shows a photo of a sodium lamp of this type.

Flasks of sodium lamps are cylindrical (as in Figure 2), elliptical, coated inside with a thin layer of light-scattering substance (DNaS). They can be frosted (DNaMT) or contain a mirror reflector next to the burner (DNaZ).

Operating principle.

The ignition of the sodium lamp burner comes from an electric arc arising between the electrodes. In the channel of the electric discharge, a stream of charged particles from sodium vapor is formed. Strictly speaking, inside the gas discharge tube is not pure sodium, but a mixture of gases. For better arc ignition add argon or xenon or mercury vapor.

Mercury-free luminaires already exist today. They have a more complex design so far, but development is ongoing and they will probably someday replace conventional mercury lamps.

After a high pulse voltage is applied to the cathodes, ignition of the NLVD occurs. For a while, the lamp shines dimly. After about 7 to 10 minutes, after the sodium vapor has warmed up to operating temperature, the lamp goes into maximum light output mode.

The principle of operation is similar to the operation of mercury lamps, but to turn on a luminaire filled with sodium vapor, a higher voltage pulse is required than to turn on DRL. After heating the burner, pulse currents must be limited. Therefore, for this type of lighting fixtures, NLVD manufacturers developed special ballasts with built-in pulse ignition devices. Without the use of an IZU, it is impossible to ignite a sodium lamp by connecting it directly to the electrical network.

Classification of sodium lamps

As noted above, sodium lamps are of two types: NLND and NLVD. They can also be classified by the type of flask, by the composition of impurities, and radiation power. Since the vapor pressure of sodium directly affects the light output of the lamp, we will briefly review the luminaires precisely in this parameter.

Low Pressure (NLND)

The first appeared NLND (low pressure in the burner). They provide low color rendering, but have a pleasant spectrum of radiation for humans. They were massively used in the 30s of the past century. Low-pressure lamps can be found today, but they are replaced by more advanced sodium lamps, which we will dwell on in more detail.

High Pressure (NLVD)

The high efficiency of NLVD has made them a leader among other gas-discharge light sources. The luminous efficiency of such lamps reaches 150 lumens / watt. They can work up to 28500 hours. True, at the end of their service life, their light output decreases, and the color shifts to the red side of the spectrum.

For a number of parameters, NLVD surpass the quality of fluorescent lamps that emit cold glow and metal halide lamps that consume a lot of electricity. Among modern electric light sources there are few luminaires that can make a sodium lamp worthy of competition.

Advantages and disadvantages

The advantages of sodium lamps are as follows:

• profitability of tubular lamps;
• long term of operation;
• stability of electrical parameters over almost the entire service life;
• warm shades of sodium radiation (see fig. 3);
• a fairly wide range of temperatures at which sodium lamps operate stably - from –60 to +40 degrees Celsius.

Unfortunately, there are disadvantages that limit the scope of NLVD:

• the annoying frequency of flickering light;
• inertia when turned on;
• explosiveness of NLVD;
• the presence of mercury content in most models;
• resonant radiation weakens during operation;
• increase in power consumption nearing the end of its service life;
• the need to use ballasts for connecting lamps.

Ballasts are sometimes a source of noise and consume up to 60% of power consumption. They also require additional maintenance.

Despite the presence of the above disadvantages, in some areas where the color rendering of the light source is not significant, the use of NLVD is very beneficial, and in some cases simply irreplaceable.

Application area

The yellow-orange light of the lighting devices is pleasing to the eye, but its monochromaticity muffles the colors of the interior colors. Therefore, sodium lamps are not used in residential premises as the main lighting device. They can serve only as elements of decorative lighting.

Figure 3 shows a photo of such a backlight .:

Studies have shown that yellow luminescence tends to beneficially influence the development of plants. At the same time, their growth intensifies, and productivity increases. In summer, vegetation receives such lighting from sunlight. But in the greenhouses where vegetables are grown in winter, sunlight is clearly not enough. NLVD is ideally suited for these purposes (see figure 4).

The use of sodium lamps for lighting greenhouses not only increases productivity, but also saves energy.

Pay attention to the monochromatic light of sodium lamps. The muffled color of the plants indicates that almost all the light from the lamps is spent on the production of chlorophyll.

Monochromaticity is very useful in street lighting. Such light is not scattered in the fog. The use of street lights for highway lighting can improve traffic safety. Park zones and paths with street lighting based on NLVD, which have a yellow spectrum of light, increase the comfort of vacationers at night.

Less commonly, such luminaires are used in industrial premises (usually in warehouses), as well as in the design of advertising signs and decorations.

Connection

Since a high pulse voltage (sometimes up to 1000 V) is required to set the burner on fire, this complicates the wiring of sodium lamps. We have to use additional equipment. Ballasts for NLVD are of two types: EMR (electromagnetic) and ballasts (electronic).

IZU are connected in parallel to the lamp circuit, and chokes are connected in series, sometimes through a pulsed ignition device.

Figure 6 shows the connection of the NLVD.

Pay attention to how the throttle (ballast) and IZU are connected.

Please note that for self-connection, you must comply with the requirement: the wire length from the inductor to the lamp base must not exceed 100 cm.

Some foreign manufacturers supply sodium lighting devices with integrated starting devices in the lamp bulb to the market.

Safety and Disposal Considerations

Risks in the operation of sodium lamps are associated with high pressure and temperature inside the burner. Even the surface of the flask heats up to 100 ° C and can cause burns if carelessly handled. There is a possibility that the flask will burst under the influence of hot gases escaping from the burner.

In order to protect against the consequences of destruction, lamps are made in which the lamps are behind thick glass. Pay attention to the design street lighting fixtures (fig. 5).

Due to the presence of mercury in sodium lamps, special requirements apply for their disposal. Used appliances must not be disposed of in trash bins. They must be sent to special enterprises for disposal and processing.

Video in addition to the article