Uninterruptible power supply. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Uninterruptible power supply. Encyclopedia of radio electronics and electrical engineering

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

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When operating communication equipment, sometimes there is a need for an independent power source (for example, during power outages). If the equipment works without human intervention, then the charger must be automatic. In such cases, uninterruptible power supplies are used. One of these blocks will be discussed in this article.

The proposed uninterruptible power supply unit (UPS) is designed for automatic power backup of radio equipment at remote sites that do not have permanent service personnel (for example, repeaters). It can also be used for other 12V DC equipment. The BBP provides two modes of operation: the main one, when the load is powered from the mains voltage of 220 V AC, and the emergency one, when in the absence of mains voltage the load is powered from a backup battery with a nominal voltage of 12 V.

Structurally, the device is a single housing, which houses a stabilized power supply with a voltage of 13 V, capable of delivering a current of 1 ... 1,4 A to the load; Charger; battery that provides power to the load for 6...8 hours; control system.

The control system automatically performs:

- indication of operating modes (power supply from the mains, charging, emergency power supply from the battery);

- inclusion of the BBP into operation when voltage appears in the mains;

- charging (recharging) the batteries with a stable current;

- control of the degree of charge of the battery by the voltage at its terminals;

- switching the load to autonomous battery power when the mains voltage fails;

- emergency shutdown of the battery in case of its malfunction or deep discharge due to a long absence of mains voltage for more than 6 ... 8 hours.

In manual mode, it is possible to force the power on from the battery.

As a backup power in the BBP, various variants of both domestic and imported acid batteries were used. Lead-acid batteries manufactured by YACHT BATTERY CO, LTD (type Y7-12) and YUASA CORPORATION (NP7-12) with a rated voltage of 12 V and a capacity of 7 A==dot==h proved to be reliable in operation. They do not need periodic electrolyte replenishment and constant maintenance, there is no “polarity reversal” effect, they can be stored in a charged state for a long time (up to a year).

According to the passport data, the charging voltage of the battery in the standby mode is 13,5 ... 13,8 V (at a temperature of 20 ° C), and according to the discharge nomogram, with a 6-hour discharge with a current of 1,4 A, the threshold critical voltage is 11 V, below which there is a steep decline - a section of the curve corresponding to complete discharge. More details about the battery parameters can be found in the article "Wide-use lead-acid batteries" ("Radio", 2000, No. 12, pp. 43, 44).

Based on the foregoing, the thresholds for the operation of automatic control were chosen: the upper threshold (charging shutdown) - 14 V (charging voltage 13,8 V plus voltage losses in the supply wires and contacts from the battery terminals) and the lower threshold (emergency shutdown of the battery to prevent deep discharge ) - 11 V.

The scheme of the device is shown in fig. one.

(click to enlarge)

When the SA1 toggle switch is turned on, the voltage from the secondary winding of the transformer T1 is supplied to the rectifiers VD1-VD4, VD5. Relay K1 is activated, and its contacts K1.1 turn on the automatic control circuit. The rectified voltage is stabilized by a stabilizer on the DA1 chip. To obtain the required value of the output voltage, a zener diode VD1 is included in the common wire circuit of the DA6 microcircuit. To increase the load capacity of the stabilizer, an emitter follower on the transistor VT1 is used. LED HL2 green indicates the presence of output stabilized voltage.

The device automatically starts charging the battery every 12 hours. If it is charged, the charging process will stop quickly as soon as the voltage reaches 14 V. This mode allows you to keep the battery charged at all times.

The timer consists of a multivibrator element DD1.1 and counter DD2. 12 hours after the start of operation of the device, a high level will appear at the output of the M counter, and a low level will appear at the output of the element DD1.2. The trigger on the elements DD3.5, DD3.6 will switch to a state in which the output of DD3.6 is high. At the same time, a pulse will appear at the output of the element DD3.1, which will reset the counter DD2.

A high level from the output of the element DD3.6 opens the transistor VT3. The charging current stabilizer on the transistor VT2 is turned on. When current passes through the HL1 LED, the voltage drops on it, which is used as a reference. The stabilized charging current is supplied to the battery pack GB1. Glowing yellow LED HL1 serves simultaneously as an indicator of the charging process.

The comparators are made on the op-amp DA2.1 and DA2.2. A reference voltage source for comparators is assembled on resistor R8 and zener diode VD9. It does not depend on battery voltage. The thresholds for the operation of automation are set by trimming resistors R10 and R13 (lower and upper thresholds, respectively).

When the battery voltage is 14 V, a low level occurs at the output of the op-amp DA2.2. The trigger on the elements DD3.5, DD3.6 is reset, and a low level also appears at the output of DD3.6. Transistor VT3 closes and the battery stops charging.

If the mains voltage fails, the K1.1 relay contacts will open faster than the voltage at the stabilizer output disappears. A positive voltage drop will go to the differentiating circuit C7R17, and a low-level pulse will appear at the output of element DD1.4. The trigger on the elements DD3.3, DD3.4 will switch, and a high level will appear at the output of DD3.3. Transistor VT4 will open, relay K2 will work and its contacts K2.1 will connect the battery GB1 to the load. LED HL3 red indicates the transition to emergency power supply from the battery.

When voltage appears in the mains, the contacts of relay K1.1 will close again. A low level through the VD15 diode will switch the trigger DD3.3, DD3.4 in such a way that the output of the element DD3.3 will be low. Transistor VT4 closes, relay K2 switches to its original state, and the device switches to the main mode. At the same time, the differentiating circuit C6R16 will generate a low-level pulse at the input of element DD1.3. This pulse, passing through the elements DD1.3 and DD3.2, will switch the trigger (DD3.5, DD3.6), a high level will appear at the output of the DD3.6 element. The VT3 transistor will open and the battery charging process will begin until a 12-hour cycle is reached.

In the emergency mode of the UPS operation, the control system protects the battery from complete discharge, when, as a result of a long absence of voltage in the mains, the battery is discharged and the voltage on it drops to 11 V. In this case, the comparator is triggered by the lower threshold, the output of the op-amp DA2.1 goes low level, which through the diode VD16 affects the trigger DD3.3, DD3.4. Transistor VT4 closes, and the contacts of relay K2.1 go to their original state. The load supply is completely de-energized. When the mains voltage appears, the load will be powered by the stabilizer.

Diodes VD1 -VD4 can be replaced with any of the KD202 series, as well as the KD226, KD228 series, etc. for a current of 2 ... 3 A; diode VD8-KD202A or similar. Diodes VD11-VD17 - any universal, for example, series KD522, D220, D310. Transistor VT1 can be used in the KT817, KT819 series, and VT2 - in the KT818 series.

The DA2 chip will be completely replaced by two general purpose op amps, for example, K140UD708. Instead of the DD3 microcircuit (six inverters with increased load capacity and strobing), K561LN2 can be used, also taking into account the differences in the pinout.

Relay K1 - reed relay RES64A (passport RS4.569.724) with one make contact You can use almost any reed relay by choosing a resistor R1 to dampen excess voltage. Relay K2 is an imported small-sized relay for a voltage of 12 V and a trip current of 30 mA.

You can use a relay with a voltage of 9 ... 12 V, a trip current of up to 50 mA and with a contact breaking capacity of at least 3 A, for example, RES9 (passport RS4.524.200, RS4.524.201), RES32 (passport RF4.500.341), RES47 (passport RF4.500.409).

Transformer T1 must provide a voltage on the secondary winding of 13 V at a current sufficient for the load.

The BBP is mounted in a plastic rectangular case measuring 95x135x305, including a rectangular cover 40 mm high (Fig. 2). Ventilation holes are drilled in the sides of the lid.

Uninterruptible power supply

A common ribbed heat sink with an area of 100 cm² is fixed on the outside of the rear end of the case. Transistors VT1, VT2 and integrated stabilizer DA1 are installed on the heat sink on insulating gaskets made of PTFE tape. The battery is located in the front of the housing and is separated from the adjacent transformer by an elastic rubber gasket. All other radioelements, including relays, are mounted on a 75x250 mm foil fiberglass circuit board attached to the inside of the cover. The foil layer is divided by a thin cutter into isolated areas 5x5 mm for mounting radioelements (the dimensions of the areas for microcircuits are 2,5x5 mm). Connections between elements and platforms are made by conductors.

When establishing a BBP, it is recommended first of all to set the output voltage value by selecting the VD6 zener diode. Then set the charger current by selecting resistor R2. The current through the HL1 LED should not exceed the maximum allowable (selected by resistor R4). Experience in the operation of batteries has shown that the optimal charging current is a current sufficient for recharging. It is numerically equal to 0,05 of the battery capacity, i.e. 0,35 A.

It is convenient to set the comparator thresholds using a digital multimeter and an oscilloscope. To do this, you need to temporarily disconnect the connection point of the resistors R8, R9 and R12 from the device and connect it to an external regulated power source. Then we turn on the BBP and set the external source voltage to 14 V (according to the digital multimeter). By controlling the output of the op-amp DA2.2 with a voltmeter or oscilloscope, by rotating the engine of the tuning resistor R13 we achieve a low level. Similarly, by setting the voltage of the external source to 11 V (corresponding to the lower threshold of the comparator), we achieve a low level at the output of the op-amp DA2.1 by adjusting the resistor R10. After setting the thresholds, we restore the original connection.

For reliable stable operation of the reed relay K1, a capacitor C2 is connected in parallel with its winding (selected empirically).

Authors: V.Lavrinenko, F.Rotar, Volzhsky, Volgograd region.

See other articles Section Protection of equipment from emergency operation of the network, uninterruptible power supplies.

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