Adjustable voltage converter for LDS

Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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When the mains voltage is turned off, say, in the country, they often use small-sized and low-power fluorescent lamps (LDS) powered by a rechargeable battery or a battery of galvanic cells. The proposed converter allows you to adjust the brightness of the lamp and set it so that the battery energy is spent more economically.

The converter consists of a master oscillator and a single-cycle power amplifier (Fig. 1).

The generator is made on the elements DD1.1-DD1.3 according to the scheme proposed in the book by S. A. Biryukov "Digital devices on MOS integrated circuits" (M.: Radio and communication, 1990). Such a generator allows you to change the duty cycle of the pulses (i.e., the ratio of the pulse repetition period to their duration) with a variable resistor R1, which determines the brightness of the LDS. A buffer element DD1.4 is connected to the generator.

The signal from DD1.4 is fed to a power amplifier made on transistors VT1, VT2. Amplifier load - LDS (EL1), connected through a step-up transformer T1. It is permissible to connect the lamp both with closed filament terminals (shown in the diagram) and with open ones. In other words, the integrity of the lamp filaments does not matter.

The converter is powered by a DC source with a voltage of 6 ... 12 V, capable of delivering current up to several amperes to the load (depending on the lamp power and the set brightness). Power is supplied to the microcircuit through a parametric stabilizer, in which a ballast resistor R4 and a zener diode VD3 work. With a minimum supply voltage, the stabilizer practically does not work, but this does not affect the operation of the converter.

In addition to those indicated in the diagram, it is permissible to use transistors KT3117A, KT630B, KT603B (VT1), KT926A, KT903B (VT2), diodes of the KD503 series (VD1, VD2). zener diode D814A (VD3). Capacitor C1 - KT, KM, K10-17, the rest - K50-16, K52-1, K53-1. Variable resistor - any design (for example, SP2, SP3), constant - OMLT-0,125. Lamp - power from 4 to 20 watts.

The transformer is wound on an armored magnetic core made of 2000NM1 ferrite with an outer diameter of 30 mm. Winding I contains 35 turns of PEV-2 wire with a diameter of 0,45 mm, winding II - 1000 turns of PEV-2 0,16. The windings are separated by several layers of varnished fabric. To increase the reliability, the winding II must be divided into several layers, laying a varnished cloth between them. The cups of the magnetic circuit are assembled with a gap of 0,2 mm and tightened with a screw and a nut made of non-magnetic material. With slightly worse results (brightness - current consumption ratio), a transformer made on a magnetic circuit from a line transformer of a TV will work.

The adjustment of the converter begins with checking the master oscillator with the output stage of the amplifier turned off. An oscilloscope is connected to pin 11 of the microcircuit and the pulses shown in the upper diagram of Fig. 2. Then set the variable resistor slider to the left position according to the diagram (resistance is entered). Measure the duration of the pulses and the period of their repetition. By selecting resistor R3, a pulse duration of approximately 20 μs is achieved, and by selecting resistor R2, a repetition period of approximately 50 μs is achieved. After moving the engine from one extreme position to another, they are convinced of the change in the pulse repetition period with their duration unchanged.

Next, the output stage is connected, the oscilloscope is connected to the collector of its transistor, and an ammeter with a scale of 2-3 A is placed in the power circuit. resistor. Observe the shape of the pulses on the collector of the transistor VT2 - in fig. 2 below, this form was obtained when the converter was working with the LB18 lamp. It may be necessary to more accurately select resistors R2, R7, and in some cases install a variable resistor of a different value in order to achieve the necessary limits for changing brightness and acceptable current consumption.

In the minimum brightness mode, which corresponds to a current of 250 ... 400 mA depending on the supply voltage and lamp power, it is more convenient to start the generator, and therefore turn on the lamp, by pressing the SB1 button. Sometimes it is useful to try to change the polarity of the lamp and check the reliability of its ignition in this mode.

You can evaluate the efficiency of the converter with different transistors, transformers, mode changes, etc. as follows. At a distance of about 0,5 m from the lamp, a photodiode or photoresistor is strengthened and an ohmmeter is connected to it. Measure its resistance with a burning lamp and a fixed current consumption of the converter. Next, the part is replaced, the previous current is set with resistor R1 and the resistance of the photocell is measured. If it has decreased, then the brightness of the lamp has increased, the result of the experiment is possible.

Author: V. Kobets, Feodosiya, Ukraine

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