Neon indicators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The neon lamp belongs to the class of glow discharge devices. It is a glass container (Fig. 1), inside which two metal electrodes are placed. The electrodes can be flat, cylindrical, and in the form of straight or curved rods. The cylinder is filled with an inert gas (neon, argon, or a mixture of them with helium), which is under low pressure (several mm of mercury).

Fig. 1

One of the electrodes of the lamp is the cathode, the other is the anode. For lamps designed to operate on alternating current, each electrode is alternately anode and cathode.

We assemble a simple installation in accordance with the diagram shown in Fig. 2, from a power source, a potentiometer R1 and a voltmeter with a measurement limit of 150 V, connected in parallel with a neon lamp L1.

Fig. 2

As a power source, you can use a battery or a low-power rectifier that provides a constant voltage of at least 80 V.

As long as the voltage across the electrodes of the lamp is low, the gas gap between the electrodes is an insulator. As the potentiometer slider moves to the left (according to the circuit), the voltage at the lamp electrodes gradually increases. At a certain voltage for a given lamp, a glow discharge occurs in it, while the internal resistance of the lamp decreases sharply, and the current through it increases. The voltage at which a glow discharge occurs in the lamp is called the ignition voltage. Its value depends on the composition and pressure of the gas in the lamp, the material and shape of the electrodes, and the distance between the electrodes.

The appearance of a glow discharge can be explained as follows. In a gas, even at ordinary temperature, some of the molecules will be ionized, that is, in a gas, among neutral molecules, there will be electrons and positive ions - gas molecules that have lost some of their electrons.

When a constant voltage is applied to the electrodes of the lamp, an electric field is created between them. Electrons move in this field to the positive electrode - the anode, and positive ions to the negative electrode - the cathode. If the strength of the electric field between the electrodes of the lamp is large enough, the electrons acquire such a speed that when they collide with a gas molecule, they ionize it; in turn, the nones, bombarding the cathode, knock out new electrons from it. As a result of ionization, the gas becomes electrically conductive, but unlike metals, where the current is created by electrons, here both electrons and ions participate in the creation of the current.

Due to the fact that gas molecules, both during ionization and during recombination (reduction of an ion into a neutral molecule as a result of electron capture), can emit light, the gas near the cathode begins to glow. The color of the glow can be red or red-orange, depending on the composition of the gas.

When passing through an alternating current neon lamp, the glow is observed at both electrodes.

The area of ​​the glow depends on the strength of the current through the lamp. With an increase in current, all new sections of the cathode are included in the work and the glow area expands. The voltage at the electrodes of the lamp remains almost constant until the entire cathode is covered by the glow.

Neon lamp - an indicator of the electrification of the body. It is possible to determine whether the body is charged not only with an electrometer, but also with a neon lamp. When the output of the electrode of a neon lamp approaches an electrified body, for example, a glass or ebonite rod electrified by friction, a glow discharge occurs in the lamp. Hold the lamp follows the output of the second electrode.

With the help of a neon lamp, you can make sure that during the operation of the school electrophore machine only sectors of aluminum powder deposited on the disks are electrified - for this, you should bring the lamp to the sector of the disk. If the lamp is brought close to the disk between the sectors, the lamp will not light up.

Neon lamp - polarity indicator. Taking advantage of the fact that the glow occurs at the cathode, that is, the electrode under a negative potential, it is possible to determine the polarity of the direct current source using a neon lamp. To do this, the lamp is connected to the terminals of the current source and it is determined which electrode of the lamp glows.

Previously, when connecting a neon lamp to a direct current source, the polarity of which is known, it is necessary to establish exactly how the lamp electrodes are connected to the base.

Neon lamp - phase wire indicator. The apartment has two electrical wires. One of them is connected to the ground, it is called the neutral wire. Touching it is safe. The other wire, called the live wire, is fully energized with respect to ground and can be life-threatening if touched. You can distinguish these wires from one another using a probe with a neon lamp (Fig. 3).

Fig. 3

The probe can be mounted in a screwdriver handle made of transparent plastic, while one lamp electrode is connected through a resistor R1 to the screwdriver blade, the other electrode is connected to a metal ring worn on the screwdriver handle.

Touching the neutral wire with a screwdriver blade does not cause the lamp to ignite; if the phase wire is touched, the lamp will light up. The screwdriver should be held so that contact between the hand and the metal ring is ensured.

Neon lamp - fuse blown signaling device. When a fuse blows - "plugs" you have to turn all the fuses out of the sockets one by one in search of a blown one. If, in parallel to each fuse, a neon lamp and resistor R1 are connected (Fig. 4), then if the fuse blows, the mains voltage through the switched on electrical appliances and resistor R1 will be applied to the neon lamp, causing it to ignite.

Fig. 4

Neon lamp - mains voltage indicator. During the day, the voltage of the electrical network usually changes within certain limits. In the evening, when the total number of electrical appliances connected to the network increases, the voltage drops somewhat. During the day, when the network load is low, the voltage becomes normal or slightly higher than normal.

For some devices, such as a TV or radio, the mains voltage change should not exceed certain values ​​in order to avoid their failure. You can control the mains voltage with a voltmeter, but it is better to do this with a voltage indicator made on neon lamps.

The indicator scheme is shown in fig. 5.

Fig. 5

Two voltage dividers from resistors R220, R1 and R2, R3 are included in an alternating current network with a voltage of 4 V. Neon lamps L1 and L2 type MN-3 are connected in parallel with resistors R1 and R3. The resistances of the resistors R1 and R2 are chosen so that the voltage drop across the resistor R1 is sufficient to ignite the L1 lamp when the mains voltage is equal to the minimum allowable (200 V). The voltage drop across the resistor R3 should be equal to the ignition voltage of the L2 lamp when the mains voltage increases to the maximum allowable (230 V).

Therefore, if the mains voltage is within acceptable limits, one L1 lamp is lit. If none of the lamps is lit, it means that the mains voltage is not enough for the normal operation of the TV, while the burning of both lamps indicates an increase in voltage above the set limits, in both cases the TV must be disconnected from the network.

Author: V.Shilov

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