Power amplifier for dynamic lighting installation. Encyclopedia of radio electronics and electrical engineering

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Power amplifiers of modern light-dynamic installations (SDU) are performed either on trinistors or on transistors. Both solutions have their advantages and disadvantages, however, when the total power of the lamps in the screen device is up to 100 W, preference should be given to transistor amplifiers, since they are easier to set up, do not require scarce parts and are safe to operate, since they operate at a relatively low voltage.

Since the output transistors of the SDU power amplifier usually operate in a linear mode, significant power is released on their collector, commensurate with the maximum power of the lamps. This necessitates the use of powerful transistors equipped with heatsinks, which complicates the design.

The transistor pulse-width power amplifier described below is free from many of the disadvantages of conventional amplifiers. It is in good agreement with the rest of traditional SDU units: its input can be connected directly to the detector output. The principle of operation of such an amplifier is to control the power released in the load by changing the duty cycle of the supply pulses under the action of a control signal.

The amplifier (see diagram in Fig. 1) is a kind of asymmetric multivibrator made on transistors of the same structure. The control signal of negative polarity from the output of the SDU detector is fed to the base of the transistor M2 through the resistor R5, the resistance of which determines the equivalent input resistance of the amplifier.

In the absence of an input signal, transistors V2, V0,3 are closed, there is no generation, the power in the load is zero. When the input voltage is more than 2 V, the multivibrator starts to generate pulses, the duration of which depends on the parameters of the C2R1 circuit. The duration of the pause between pulses (it depends on the parameters of the C5R1 circuit and the voltage at the collector of transistor VXNUMX in cutoff mode) decreases with increasing input voltage, due to which the average value of the current in the load increases accordingly. The law of output power change depending on the input voltage is close to logarithmic, which makes it possible to do without an additional compression device.

To control the sensitivity of the multivibrator, a variable resistor R2 is used, which allows you to change the voltage at the collector of the transistor V1 in cutoff mode. In the middle position of the slider of the resistor R2, the sensitivity of the amplifier is 4 ... 5 V (at maximum power in the load). The maximum sensitivity of 1,5...2 V corresponds to the lower position of the slider.

The generation frequency of the multivibrator at an average power in the load (pulse duty cycle 2) is approximately 1 kHz; the maximum frequency corresponding to the maximum power is about 2 kHz.

In another version of the amplifier (Fig. 2), to increase the sensitivity to 1 ... 1.5 V, a silicon diode V1 is included, which is included in the parametric voltage regulator 0,8 ... 1 V on the collector of the closed transistor V2. With a low collector voltage of the transistor V2, which is necessary to obtain high sensitivity, such a circuit solution provides higher generation stability and pulse edge steepness than in the first version of the amplifier.

In this option, it is possible to control the level of the initial glow of the lamps. This mode of supplying the lamps reduces sudden current surges due to the low resistance of the cold filament of the lamp (and, in addition, it makes it possible in some cases to abandon a separate pause illumination channel).

In the backlight mode, transistor V4 is heated. Since the amplifier implements the principle of pulse-width power control, which involves the operation of the output transistor in the key mode, in the ideal case, power is not dissipated on it at all. But in real conditions, due to the non-ideal characteristics of electronic elements, some power is released on the V4 transistor, and the transistor heats up most strongly at a certain average power value in the load. The main reason for this phenomenon is the operation of the power transistor in an unsaturated mode and the low steepness of the pulse fronts.

The heating of the transistor V4 in the backlight mode can be reduced by selecting transistors VЗ, V4 with the highest possible current transfer coefficient, disconnecting the capacitor C2 from the collector VЗ and connecting it to the collector of the transistor V4 (in this case, it is desirable to connect an oxide capacitor with a capacity of 500 in parallel to the lamp supply circuit. .. 1000 uF, designed for a voltage of at least 16 V), reduce the resistance of resistors R2, R4, R5 by 3 ... 4 times, increasing the capacitance of capacitors C1, C2 by the same amount. It is also desirable to exclude the resistor R15 and use the PPB-100 adjusting resistor with a maximum resistance of 200 ...

At an elevated temperature of the case of the V4 transistor, it is recommended to connect between its base to the emitter (that is, parallel to the base-emitter junction) a constant resistor with a resistance of 0,3 ... 1,0 kOhm of any power

The maximum load current for the transistors indicated in the diagram is 1,2 A. At the same time, the high efficiency of the amplifier, reaching 90%, makes it possible to completely abandon radiators with lamp power up to 15 W. If more power is required, transistors from the P403-P213 series with any letter index, also without radiators, should be used instead of the GT217B. MP42B transistors can be replaced by any low-power germanium transistors with a h21E coefficient of at least 50.

The power supply circuits of the multivibrator and lamps are separated, which allows you to power the lamps directly from the rectifier, and to power the multivibrator, use a low-power stabilizer rated for current up to 50mA, and both the lamps and the stabilizer can be powered from one secondary winding of the mains transformer. The power supply circuit is shown in fig. 3. Transformer T1 is made on a magnetic circuit with a section of 19x38, the network winding contains 1400 turns of PEL 0,27 wire, the secondary winding contains 100 turns of PEL 1,0 wire. At the same time, up to six MH13,5-0,16 lamps connected in parallel can be used in each channel of the three-channel SDU.

A. Belousov

Another version of the amplifier was proposed by V. V. Chernyavsky (see figure below). The sensitivity of this amplifier is 0,1 ... 0,2 V, which allows you to connect it to the linear output of a tape recorder or player. The operating voltage of the H1 lamp is 12V, the power is 30 watts.

Publication: cxem.net

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