Power supply for low power fluorescent lamps. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power supply for low power fluorescent lamps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Lighting

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The article shows a diagram and a printed circuit board of a power source for connecting low-power (up to 15 W) fluorescent lamps to a 220 V network. The circuit is made on a transistor half-bridge inverter.

Low-power (up to 15 W) fluorescent lamps or fluorescent lamps (LDS) are widely used in desktop and portable lamps, scanners, copiers. A feature of the LDS power supply is the need to supply a voltage of 500-800 V to the lamp for ignition, and after ignition, it is necessary to ensure the rated current through the lamp, since the lamp burning voltage is 60 V.

These requirements are met by the proposed power supply, which allows you to connect low-power LDS to a 220 V network. It is based on a half-bridge voltage converter [1]. The primary winding of the converter transformer is included in the diagonal of the bridge formed by two transistors connected in series and two capacitors (Fig. 1).

Power supply for low power fluorescent lamps

In the working circuit of the power supply (Fig. 2), the L1C5 circuit is connected in series with the primary winding of the transformer. In addition, a rectifier with a filter on capacitor C1 and a special start circuit on elements R2, C4, VT3 and R7 are built in. Transistor VT3 operates in avalanche mode. After applying voltage to the converter, the charge of the capacitor C4 begins through the resistor R2. When the voltage at the collector of the transistor VT3 reaches 40 ... 60 V, it breaks like an avalanche. The discharge current of the capacitor C4 turns on the transistor VT2, starting the converter. Negative pulses from the winding III of the transformer periodically open the transistor VT3 and keep the capacitor C4 practically discharged.

(click to enlarge)

The start circuit can be slightly simplified if a dinistor is installed instead of a transistor (Fig. 3).

Power supply for low power fluorescent lamps

At the moment the voltage is applied to the converter in the L1C5 circuit, a shock excitation voltage appears, which ignites the lamp. After ignition, the current that passes through the lamp sharply reduces the quality factor of the circuit, shunting C5. The converter operates at a high frequency, and the inductive reactance of the inductor L1 limits the lamp current.

The printed circuit board (Fig. 4, a) has dimensions of 120x65 mm, the layout of the elements is shown in Fig. 4, b. Transistors VT1 and VT2 are mounted on metal risers 9 mm high and threaded fastening is used.

The following capacitors are used in the power supply: C1 - type K50-27 for a voltage of 350 V; C2, C3 - type K73-17 for 400 V; C4 - type KM4; C5 - type K3111. Transformer T1 is wound on a K10x6x5 ring made of M2000NM39 ferrite and contains 7 turns of a single-core wire with a diameter of 0,23 mm in PVC insulation in the I winding, and 4 turns of the same wire in II and III. The inductor L1 is wound on an armored core of the B22 type made of M2000NM1 ferrite and contains 130 turns of wire with a diameter of 0,33 mm in enamel insulation. Choke inductance 5 mH. The central core of one of the cups of the core was sawn off by 0,2 mm. This gap, which is formed after the assembly of the core, makes it possible to obtain a stable magnetic permeability of the ferrite and, accordingly, a stable inductance of the inductor. When installing the transformer T1 in the printed circuit board, you should pay attention to the correct connection of the winding leads. The beginning of all windings is indicated in the diagram by black dots.

Setting the power supply is reduced to setting the current through the lamp by changing the value of the inductance of the inductor L1. But it is easier to measure not the current through the lamp, but the current consumption of the power source. To do this, you must connect an AC ammeter in series with resistor R1. If we assume that the efficiency of the converter is 0,9, then for the LB81 lamp, the power of which is 8 W, the required current consumption of the source is 8 / 220x0,9 = 40 mA. The inductance of the inductor can be changed by the number of turns of the coil, by changing the gap, and also by introducing a core.

References:

Biryukov S.A. Digital devices on integrated circuits. - 2nd ed., revised. and additional - M .: Radio and communication, 1987, pp. 134, 135.

Author: V.Samelyuk

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