Low voltage uninterruptible power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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The proposed device is produced in a series of 350 pieces. It is used at housing and communal facilities in Moscow to power a voice communication system with elevators and other similar equipment that must operate regardless of the presence of voltage in the power supply network.

The Uninterruptible Power Supply (UPS) is designed for unattended use in unheated areas (electric room, elevator machine room, garage, basement, etc.). The UPS is protected against short circuits in the output circuits. It has shown high reliability in operation since 2002. A simple circuit design allows it to be repeated by a wide range of radio amateurs.

Main Specifications

• Output voltage, V nominal ....... 13,5
• minimum when the mains is turned off ....... 8,5
• Maximum output current, A ....... 2
• ambient temperature, ^оFrom .......-10...+40
• Maximum air humidity at 20 ^оС,% ......80
• Dimensions, mm.......235x165x80

Rice. 1 (click to enlarge)

The UPS diagram is shown in fig. 1. It contains a step-down transformer T1; two diode bridges: VD1 -VD4 and VD1, VD2, VD5, VD6 (diodes VD1, VD2 are common for both bridges); smoothing capacitor C4; voltage regulator on the chip DA1, transistor VT2; gel lead-acid battery (battery) GB1 with a nominal voltage of 12 V; node for controlling its discharge on the transistor VT1; mains voltage control unit on relay K1. LED HL1 indicates the presence of mains voltage, and HL2 - output voltage.

In the presence of mains voltage, an alternating voltage of 18,5 V from the secondary winding II of the transformer T1 is supplied to the diode bridges. The rectified voltage from the output of the first bridge - the connection point of the cathodes of the diodes VD3 and VD4 - smoothes the capacitor C4. This voltage is used to power the discharge control unit and the voltage regulator. It is greater than the battery voltage GB1, so the VD7 diode is closed.

The voltage regulator on the DA1 chip and the power transistor VT2 is assembled according to a typical scheme. The output current is limited to a value that can be approximated by formula I_Max = 0,6/R8. Capacitors C2, C3, C5 prevent self-excitation of the voltage regulator.

The rectified voltage from the output of the second bridge - the connection point of the cathodes of the diodes VD5 and VD6 - is used to power the relay K1. Resistors R1 and R2 limit the current through its winding, and capacitor C1 smooths out voltage ripples on it. Relay contacts K1 are intended for use in external devices, including automation systems.

In the absence of mains voltage, the VD7 diode opens and the voltage regulator receives power from the battery. Diode VD8 is closed, as a reverse voltage is applied to it. The output voltage is less than the battery voltage by about 1,3 V. The duration of the UPS operation in the absence of mains voltage is determined by the battery capacity and the power consumed by the load. Deep discharge of the battery does not occur, because when the output voltage drops to 8,5 V, the transistor VT1 closes, a high voltage level appears at pin 14 of the DA1 chip, which turns it off. Transistor VT2 closes, LED HL2 goes out, the output voltage is turned off. Of course, the battery continues to discharge through the VD7 diode and resistors R4, R5, but with a low current (mA units), and irreversible processes in the battery can occur if the mains voltage is absent for a long time. So, with a residual capacity of 100 mAh, this will happen no earlier than in a day.

Most of the parts are mounted on a printed circuit board with dimensions of 75x55 mm and a thickness of 1,5 mm made of fiberglass. Network transformer T1 - any with a secondary voltage of 18 to 24 V and a current of 2 A, for example TP-50-5, in which two secondary windings are connected in series. Transistor VT2 is mounted on a heat sink with a cooling surface area of ​​400 cm2. Relay K1 - RES15, version RS4.591.001. Fixed resistors R1-R4, R6, R9 and R10 - C2-33N (analogue of MLT), R8, R11 - C5-16MB; trimmers R5 and R7 - SP3-19A. Capacitors C1, C4, C5 - imported CD295 (similar to K50-68), C2 and C3 - KM5B.

At the stage of device development, the author used a transformer, a heat sink, a fusible link and a BPP-20 uninterruptible power supply unit case, as well as a 7 Ah battery. The appearance of the device in the case is shown in fig. 2.

Fig. 2

To set up the UPS, you need a laboratory power supply (hereinafter LLP) with an adjustable voltage of 9 ... 20 V and a DC voltmeter, as well as a resistive load of 7 ohms 30 W or equivalent. The UPS is disconnected from the mains and the battery, and the trimmer resistors R5 and R7 are set to the upper position according to the diagram. A voltage of 20 V is set at the LIP output, it and a voltmeter are connected to the UPS instead of the battery, observing the polarity. By moving the engine of the tuning resistor R7, a voltage of 13,5 V is set at the UPS output, then the output voltage of the LIP is gradually reduced until the voltage at the UPS output drops to 8,5 V. After that, the engine of the tuning resistor R5 is smoothly moved down according to the circuit, until the output voltage drops sharply to a value close to zero.

Then turn off the LIP and connect the UPS to the AC mains. The voltage at its output should be 13.5 V. Close the output for 2.3 s, and after opening, check the restoration of the normal operation of the UPS.

Finally, the battery is connected and a 7 ohm resistive load is connected to the output for 2.3 hours. In this mode, the output current is 1,93 A. After this time, the output voltage should remain equal to 13.5 V. The VT2 transistor should not overheat. At the end of the adjustment, with the network disconnected, the presence of supply voltage at the load is checked.

In most cases, the UPS was installed in an electrical panel, which usually has free space where additional measuring instruments can be placed. To extend the battery life, it is advisable to control the UPS output voltage with a voltmeter, and its recharging current with an ammeter, which is included in the VD8 and R11 circuit breaks.

The load connected to the UPS output must be designed for the supply voltage, which can vary in the range of 8,5 ... 13,5 V.

To increase the reliability of the UPS in case of short circuits of its output, it is advisable to include a 2 Ohm 1 W resistor in the output circuit of the 240 DA0,25 microcircuit.

Author: I. Korolev

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