Scheme of a lamp with an electromagnetic ballast. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electromagnetic ballasts. Scheme of a lamp with an electromagnetic ballast. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Ballasts for fluorescent lamps

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For an AC network, an ordinary choke with a special core is suitable as a current limiter. The type of inductor must match the type of lamp being switched on, otherwise the lamp may be overloaded and burn out much ahead of its time.

The most common and simple switching circuit LL is a starter (with a simple inductive resistance - a choke, shown in Fig. 3.1). Elements LL1, E1, C1, C2 shown in this diagram form ballast (Abbreviated PRA).

ballast - an electrical device that provides ignition modes and normal operation of a fluorescent lamp.

Scheme of a lamp with an electromagnetic ballast
Rice. 3.1. Starter circuit of electromagnetic ballast: E1, C1 - starter; C2 - serves to increase the power factor, it is also an interference suppressor) capacitor; LL1 - current limiting choke; SA1 - switch/switch

Consider principle of operation of the circuit. An EL cold fluorescent lamp has a high resistance between its electrodes. Therefore, when turned on, the mains voltage, passing through the incandescent electrodes of the lamp, falls entirely on the key element of the starter. Key element - This is a small neon light bulb with two electrodes. One of the electrodes is rigid and fixed, and the other is bimetallic (both can be bimetallic, depending on the type of starter), bending when heated and closing the circuit when heated. When cold, it is open.

Since a difference in electrical potential appears on the electrodes of this key, the gas in the starter bulb is ionized and heats up the bimetallic plate. At some point, the starter key closes, and the electric current that appears in the circuit begins to "pump" energy into the throttle L. Energy is stored in the inductive element in the form of a magnetic field.

In addition, the current heats up the electrodes of the fluorescent lamp. Heated electrodes are inherent effect of thermionic emission, widely used in electronic tubes, kinescopes, vacuum indicators. So, free charges appear in the gas filling the bulb of the lamp.

At the same time, after the key element of the starter is closed, the discharge in it goes out, the bimetallic plate cools down, and the key opens.

After the starter contacts open and the inductor current breaks, the magnetic field, decreasing, crosses the turns of the inductor and induces EMF self-induction, the value of which is proportional to the quality factor of the inductor.

Therefore, at the moment of opening the starter contacts, a high-voltage pulse appears on the throttle, the value of which is already sufficient to ignite the lamp. The voltage of this surge is added to the instantaneous mains voltage at the moment the starter opens. Therefore, a voltage pulse appears on the lamp, which is the sum of the inductive surge of the inductor and the instantaneous mains voltage.

The surge amplitude depends on the accumulated energy, and the magnitude of this energy is proportional to the inductor current at the moment the starter contacts break. Therefore, the high voltage surge may not be sufficient to ignite.

For example, if the inductor current drops and reaches zero, just at the moment the starter opens, then there will be no surge at all. Taking into account the phase shift between current and voltage, the lamp will only have an instantaneous mains voltage - no more than 300 V. The lamp does not light up, and the starter closes again and additional heating occurs. That is, there was a false start (visually - a short-term flash).

Hack and predictor Aviator. If, when the starter was broken, the current was not zero, but the accumulated power was enough to get a high-voltage surge, then the lamp ignites.

Ignition is characterized by a sharp drop in the resistance of the gas gap of the fluorescent lamp. After ignition, the starter turns off, since its resistance is much greater than the resistance of a burning lamp. The inductor, being an inductive reactance, maintains the operating voltage at the lamp electrodes (limits the current passing through the lamp).

This circuit, like other classical electromagnetic ballasts, has a number of significant shortcomings:

harmful and unpleasant flickering of 100 Hz, and in the near-electrode areas - 50 Hz, the lamp is powered by a low-frequency alternating voltage, and in pauses, when the mains voltage passes through zero, the gas has time to deionize, which can be described as a characteristic flickering;
the presence of a bulky and noisy choke and an unreliable starter (a failed starter causes a false start of the lamp - several flashes before stable ignition, which sharply reduces the life of the fluorescent lamp);
increased noise and heat generation that occurs during the operation of the throttle;
low power factor;
high power loss;
instability of the luminous flux during fluctuations in the mains voltage.

Council. The first step to modernize the electromagnetic ballast and eliminate some of its shortcomings is to replace the conventional starter with an electronic one.

Author: Koryakin-Chernyak S.L.

See other articles Section Ballasts for fluorescent lamps.

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