Electronic ballast for fluorescent lamps LBU 30 with a power of 30 W. Scheme, description

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Electronic ballast powered by low-voltage sources. Electronic ballast for fluorescent lamps LBU 30 with a power of 30 W. Encyclopedia of radio electronics and electrical engineering

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Designed to power LL when lighting a garage, garden house or other small spaces.

The ballast is made on accessible elements and can be easily repeated by medium-skilled radio amateurs.

К merits device, in particular, is its ability to operate at a supply voltage reduced to 5 V.

This electronic ballast is designed to power LL LBU 30 with a power of 30 W and has following specifications:

nominal supply voltage -13,2 V;
rated input current - 2,6 A;
conversion frequency - 20-25 kHz;
The efficiency of the device is 85%.

The block diagram of the converter is shown in fig. 3.52.

Electronic ballast for fluorescent lamps LBU 30 with a power of 30 W
Rice. 3.52. Structural diagram of the converter

The converter is made on the basis of a step-up voltage inverter loaded on a series oscillatory circuit formed by an inductor L1 and a capacitor C1, in parallel with which a fluorescent lamp EL1 is connected. The inverter converts the DC battery voltage of 13,2 V into AC in the form of rectangular pulses with an amplitude of 150 V, supplied to the series oscillatory circuit L1, C1.

The resonant frequency of the circuit is equal to the frequency of the supply voltage, and the current flowing through the load connected to the circuit capacitor does not depend on its resistance. At the same time, at the moment the supply voltage is applied, the resistance of the lamp EL1 is high, a high voltage is applied to the capacitor C1, and a current exceeding the nominal value flows through the inductor L1.

This current also flows through the filaments EL1, heating them up, which ensures reliable switching on of the lamp. When the lamp lights up, its resistance drops and shunts the capacitor C1. As a result, the voltage on it is reduced to a value that maintains the burning of the lamp, and the current through the inductor L1 is reduced to the nominal value.

The circuit diagram of the converter is shown in fig. 3.53.

Rice. 3.53. Schematic diagram of the converter (click to enlarge)

The oscillatory circuit is formed by elements L2, C7. The inverter is made according to the scheme of a push-pull oscillator with positive current feedback (POST) on the elements T1, T2, L1, VT1, VT2, VD1-VD6, C2-C5, R1-R4. This construction of the inverter allows you to minimize the energy spent on controlling the key transistors VT1, VT2, and reduce the effect of the power supply voltage on the stability of the converter.

In this case, optimal conversion frequencies are also easily provided. In addition to the above elements, the converter contains a fuse FU1, a capacitor C1 that protects the power supply from surge currents, and a circuit C6, R5 that suppresses high-frequency voltage fluctuations on the windings of the transformer T2.

The converter works as follows. At the moment the supply voltage is applied, the transistors VT1, VT2 are closed, and the voltage on their collectors is equal to the supply voltage. A current flows through the resistors Rl, R2, charging the capacitors C2, C3 in the opposite direction to their polarity indicated in the diagram.

After some time, the voltage at the base of one of the transistors (for example, VT1) will reach its opening threshold, and a current will flow through the collector circuit, which will also pass through the power source, winding I of transformer T2 and winding W of transformer T1. As a result, a current will also appear in the winding II of the transformer T1, which, in turn, will flow through the capacitor C2 and the base-emitter junction of the transistor VT1.

In this case, VT1 enters saturation mode, and the capacitor C2 is recharged in accordance with the polarity indicated in the diagram. Its recharge is limited by the diode VD1. Thus, the converter is started. Transistor VT1 will be in saturation until the base current stops, which can occur as a result of a decrease in current through the primary winding of transformer T2 or a short circuit of the windings of transformer T1.

The converter starts at the resonant frequency of the L2C7 circuit, and the transistors VT1, VT2 will switch at the moment the inductor current L2 passes through zero. After the lamp EL1 is ignited and the capacitor C7 is shunted by it, the energy transfer of the inductor L2 to the lamp and capacitor C7 is delayed, and the conversion frequency is reduced.

In this case, its stabilization occurs at a level determined by the magnetization reversal time of the inductor L1, which, saturating, short-circuits the winding of the transformer T1, which leads to the closing of one transistor and the opening of another. The tuning frequency of the oscillatory circuit is chosen to be 46 kHz, and the operating frequency of the converter is 20-25 kHz.

With this ratio of frequencies, maximum efficiency is ensured. Chains C4, VD5, R3 and C5, VD6, R4 serve to reduce the amplitude of the switching pulse on the collectors of transistors VT1, VT2 when they are closed.

The converter is mounted on a printed circuit board made of foil fiberglass with dimensions of 233x50 mm. A drawing of a possible version of the printed circuit board of the converter is shown in fig. 3.54.

Electronic ballast for fluorescent lamps LBU 30 with a power of 30 W
Rice. 3.54. Converter printed circuit board

The board is designed to install MLT resistors, capacitors K73-17 (C1, C4, C5), K50-35 (C2, C3) and K15-5 (others), diodes of the KD105 (VD1, VD2) and KD212 (VD3-VD6) series . Transistors VT1, VT2 are fixed using standard flanges and screws with M4 nuts on L-shaped heat sinks (shown in dash-dotted lines in Fig. 3.54). Each of them is bent from a plate of sheet aluminum alloy AMts-P 2 mm thick (workpiece dimensions - 85x50, shelves - 50x12 mm) and screwed to the board with screws and nuts MZ. The outputs of the transistors are connected to the printed conductors by pieces of the mounting wire. Resistors R3, R4 are installed perpendicular to the board.

The electronic ballast can be built into the luminaire or housed in a separate housing. During installation inductor L1 and transformer T1 should preferably be placed as far as possible from transformer T2 and inductor L2, and oxide capacitors C2, C3 should not be located in close proximity to transistors VT1, VT2 and resistor R5.

The converter uses capacitors K73-17 (C1, C4, C5) for a voltage of 63 V, K50-35 (C2, C3) for a voltage of 25 V and K15-5 (C6, C7) for a voltage of 1,6 kV. Transistors KT803A can be replaced by KT908 with any letter index. It is desirable to choose them with the same base current transfer coefficient. The KD105 diodes used in the device can have any letter index. Other low-frequency diodes with a permissible forward current of at least 0,5 A are also suitable. Diodes KD212 (VD3-VD6) can also be with any letter index. They can be replaced by other silicon diodes capable of operating at frequencies up to 50 kHz and allowing a forward current of at least 2 A and a reverse voltage of at least 50 V.

Chokes and transformers are wound on ring magnetic cores made of M2000NM-1 ferrite. The windings of the chokes L1, L2 are placed on the magnetic circuits K7x4x2 and K40x25x11 and contain 5 turns of PEV-2 wire with a diameter of 0,63 mm and 140 turns of PEV-2 wire with a diameter of 0,41 mm, respectively. The windings of transformers Tl, T2 are wound on magnetic cores K20x12x6 and K40x25x11, respectively. Windings I, III and PG of transformer T1 contain 3 turns of PEV-2 wire with a diameter of 0,63 mm, and II and IF - 12 turns of PEV-2 wire with a diameter of 0,41 mm.

Each of the windings I and I` of the transformer T2 consists of 11 turns of PEV-2 wire with a diameter of 0,8 mm, and winding II consists of 140 turns of PEV-2 wire with a diameter of 0,41 mm. The windings I and I` of the transformer T2 are wound simultaneously in two wires over the winding II. Lakotkan should be laid between the windings. The windings of the transformer T1 must be arranged in accordance with the diagram shown in fig. 3.55.

Electronic ballast for fluorescent lamps LBU 30 with a power of 30 W
Rice. 3.55. The layout of the windings of the transformer T1

Winding I must be placed symmetrically with respect to the other windings in order to ensure the symmetry of the half-cycles of the output voltage and to exclude one-sided saturation of the transformer magnetic circuit, which leads to an increase in energy losses. Choke L2 must have a non-magnetic gap. To do this, in its core, before winding, you need to make a cut with a width of 0,8 mm.

At the time of adjustment Converter instead of the lamp EL1 and capacitor C7 in series with the inductor L2 include a resistor with a resistance of 1 kOhm and a power of 5-10 watts. First, check the reliability of starting the converter. To do this, a supply voltage of 5 V is applied to it and, if it does not start generating rectangular pulses with a frequency of 20-25 kHz, the resistance of the resistors R1, R2 is reduced, but not more than three times.

Next, control the generation frequency of the converter. To do this, it is supplied with a nominal supply voltage of 13,2 Vis using an oscilloscope or frequency meter to determine the frequency of the alternating voltage on the windings of the transformer T2. If it goes beyond 20-25 kHz, change the number of turns of the inductor L1. To increase the frequency, the number of turns of the inductor L1 is reduced, and to reduce it, it is increased.

After that, the output circuits of the converter are restored and, in series with the inductor L2, a resistor with a resistance of 10 ohms and a power of 0,5-1,0 W is connected. Then, the nominal supply voltage is applied to the converter, and after the EL1 lamp lights up, using an oscilloscope, the voltage shape on the newly installed resistor is monitored: it should be close to sinusoidal.

The current through the inductor L2 should be about 0,22 A. When power is applied to the converter, the lamp should light up after 1-2 s. In addition to the LBU 30 lamp, others designed for the same voltage and current can work together with the described converter.

Author: Koryakin-Chernyak S.L.

See other articles Section Ballasts for fluorescent lamps.

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