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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Universal powerful power supply, 220/3-20 volts 500 watts. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The power supply unit (PSU) is assembled from available elements. It requires almost no adjustment, operates in a wide range of input AC voltage, and is equipped with overcurrent protection. This power supply differs from previously known designs in simplicity and reliability, as well as the ability to remotely turn on and off the stabilizer using an external control signal. This simple circuit allows you to get a good stabilization factor and a large output current, which depends on the number of control transistors connected in parallel.

Technical capabilities

Adjustment of output voltage within 3...20 V. Fixed voltage 13,8 V with overvoltage protection.

The instability of the output voltage in the control range when the mains supply voltage changes by 10% of the nominal value at any allowable load current does not exceed 0,03%.

The instability of the output voltage when the load current changes from the maximum allowable value to zero does not exceed 0,1%.

The output voltage ripple amplitude does not exceed 1 mV effective value in the regulation range at any allowable load current.

The temperature coefficient of the output voltage over the entire regulation range at any allowable load current when the ambient temperature changes from 5 to 40 ° C does not exceed 0,02% / deg.

Protection of the power supply against overloads and short circuits. Grounding of output circuits of positive or negative polarity is allowed, as well as parallel and serial operation of two identical power supplies. It is possible to connect and disconnect loads without removing the voltage.

Universal powerful power supply, 220/3-20 volts 500 watts
Fig.1. Schematic diagram of the power supply (click to enlarge)

The circuit diagram of the power supply is shown in Fig.1. The power supply is assembled according to the classical scheme of a serial compensation voltage regulator. The device consists of two functional parts: the voltage regulator itself and the protection unit. The stabilized power supply consists of a step-down transformer T1, a powerful diode rectifier VD1-VD4, filter capacitors C1-C3 and a DC voltage regulator on the DA1 chip. Smooth adjustment of the output voltage is carried out by potentiometer R5.

The K142EN3 chip allows you to significantly simplify the design of the power supply, improve its quality characteristics, increase reliability, and reduce dimensions. This microcircuit is an adjustable voltage stabilizer with a protection system against overcurrent and short circuits in the load circuit, provides an output voltage of 3 to 30 V at a current of up to 1 A, and also allows an external control signal to turn the stabilizer on and off remotely. If the thermal protection system is triggered, the stabilizer can be switched on again only after the microcircuit has cooled down. The electrical circuit of the microcircuit is much more complicated compared to the circuit of stabilizers K142EN1, K142EN2 due to the introduction of a two-stage differential UPT with current-stabilizing two-terminal circuits, which significantly increased voltage stability, and the presence of a powerful pass transistor provided a load current of up to 1 A.

The purpose of the pins of the microcircuit: 2 - input of the protection system; 4 - feedback signal input; 6 - shutdown circuit; 8 - common output, electrically connected to the flange; 11, 17 - correction; 13 - exit; 15 - entrance.

To increase the output power of the integrated circuit, an npn transistor is used, the collector of which is connected to the output of the power supply, and the emitter is connected to the output of the rectifier. The base of the transistor is connected to the output terminal of the stabilizer. When the overcurrent protection system is activated, the output voltage drops to almost zero.

Operating principle

The current control circuit works as follows. When current flows through resistor R3, the voltage drop across it affects the input of the microcircuit protection system and closes the regulating transistor VT1. In order to put the PSU back into operation after the cause of the overload has been eliminated, it is necessary to turn off the PSU from the network for a short time using the SA1 toggle switch. The output voltage and current are controlled by instruments.

The thyristor included in the rectifier circuit reliably burns the fuse if the output voltage for some reason becomes higher than the permissible one. The overvoltage protection trip voltage depends on the zener diode. When the protection is triggered, the LED lights up, indicating that the fuse has blown. This node can be excluded if desired.

Design

The whole device is housed in a metal case measuring 250x170x180 mm. Holes with a diameter of 4 mm are drilled on the top and bottom covers (on the side of the rear wall of the radiator) to improve cooling. On the bottom cover, small legs are strengthened, which can be used as caps from tubes.

On the front front panel are located: toggle switch for switching on the network SA1; sockets for fuses FU1, FU2 (fusible inserts are located on the front panel of the power supply for easy replacement); voltmeter RA1 and ammeter RA2 (not shown in the diagram); potentiometer R5; LED HL1; control lamp EL1; output terminals 3 ... 20 V and a 24 V connector. The latter is used to power electronic devices with unstabilized voltage. On the rear panel there is a rubber grommet through which a power cord of the required length with an X1 plug at the end is brought out.

The power supply is mounted on a printed circuit board made of one-sided foil fiberglass. It is possible to use resistors such as MLT, S2-33, S1-4. Oxide capacitors C1, C2 type K50-46 or imported. If necessary, their number or capacity can be increased. Capacitors C3, C7, it is desirable to use tantalum, for example, K521B or the like. Blocking and correcting capacitors C4-C6 type. KM, soldered directly to the pins of the microcircuit.

Regulating transistors and an integral stabilizer are mounted on a radiator located on the rear wall of the case. They should be reliably isolated from the radiator with mica gaskets 0,05 mm thick, previously lubricated with heat-conducting paste. KPT-8, or put the radiator itself on the insulating racks.

Diodes VD1-VD4 are installed on heat sinks and isolated from the case. This PSU uses diodes of the KD2999 type, two in parallel. Diodes KD2999 can be replaced by KD213A (with more parallel connection) or any others, so that the allowable forward current is at least 20 A. Instead of a VD5 thyristor of the KU202 type, thyristors T4-10, T10-16 can be used.

Potentiometer R5 type SP-1 or any other, convenient for installation on the front panel of the power supply. Current-leveling resistors of the C5-16 type are installed next to the transistors by surface mounting on mounting racks isolated from the case.

Measuring instruments RA1 and RA2 are any with a total deviation current from 0,05 to 1 mA and a convenient scale. The scales are graduated through 1 V and 1 A. Microammeters of the M4248 type with a measurement limit of 100 μA can be used. In this case, the resistance of the additional and shunt resistors should be selected.

The power of the transformer T1 must be greater than the power consumed by the load. Approximate power 450 ... 500 watts. The primary winding has several taps to select the optimal voltage on the secondary winding. The inclusion of a larger number of turns of the primary winding allows you to reduce the power dissipation on the transistor VT1 while maintaining the basic parameters of the power supply. The secondary winding of the transformer produces a voltage of 2x17 V. To reduce the size of the PSU, you can use a transformer with a toroidal magnetic circuit.

Switch SA1 type TV1, it is even better to use imported network switches that have appeared on the market with a built-in lamp that indicates the switching mode. Resistor R3 type C5-16 or a piece of nichrome wire with a diameter of 1 mm and a selected length. The resistance of this current protection adjustment limiting resistor is calculated by the formula:

Universal powerful power supply, 220/3-20 volts 500 watts

Before connecting the PSU to the network, check the correct installation. The PSU is connected to the network and the voltage across the capacitors C1-C3 is measured. It should be about 24 V. The PA1 and PA2 scales are calibrated using standard instruments, while selecting additional and shunt resistors.

If necessary, you can increase the output current of the source by parallel connection of the required number of control transistors. At the same time, current-leveling resistors with a resistance of 0,1 Ohm should be included in the transistor emitter circuit, as well as a higher power transformer should be used and the number of diodes in the rectifier arm should be increased.

With two KT819 transistors in parallel, the PSU “holds” a current of 22 A at a voltage of 13,8 V for a long time. With a well-executed installation, the “drawdown” of the output voltage does not exceed 0,2 V.

Universal powerful power supply, 220/3-20 volts 500 watts
Fig.2. Parameters and pinouts of transistors

It is permissible to replace the VT1 KT819 transistor with any of the KT802, KT803A, KT805A, KT808A, KT809A, KT812, KT827, KT908 series or other powerful ones with a permissible collector current of at least 5 A and a permissible collector-emitter voltage greater than the supply voltage. The parameters and pinouts of transistors are shown in Fig. 2. Diodes VD1-VD4 are any rectifiers with a permissible forward current of more than 5 A and an appropriate voltage. The LED can be used in any type. The current circuits are made with a stranded mounting wire with a cross section of 4...6 mm2.

This PSU can also be used as a charger if you equip it with a timer that would turn off the unit after a specified time required to charge the battery.

See other articles Section Power Supplies.

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