Automatic protection of electrical devices against power surges. 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 disconnects the load from the network if the mains voltage goes out of the specified range.

The machine was developed as an integral part of the vibration pump control device. However, the device load can be any electrical device.

Similar devices are described in the literature [1, 2, 3]. This machine in all respects, with the exception of the number of parts used, is not inferior to the above, but surpasses in most. The machine has the following capabilities and features. Separate adjustment of the upper and lower voltage thresholds (within 170-260 V). Galvanic isolation of the control part of the circuit from the network; this allows the described device to be used to control a network with a voltage of 380 V and above.

Indication of the device status by means of a color-controlled LED. The device disconnects the load after the first half-cycle of mains voltage out of the specified range. Adjustable delay before turning on the device, and the time is counted not from the moment the load was turned off, but from the last "rejected" half-cycle of the mains voltage (voltage is also controlled during the delay). The machine has an open architecture, so it is easy to integrate it into other devices. The disadvantages include the irrational use of logic circuit gates.

The automatic machine works together with the pump "Strumok" of production of Open Society "Elektromashina" (Kharkov). When the voltage drops below 205 V, the water supply drops sharply at the pump, as a result of which it cools poorly and may burn out. When the voltage exceeds 235 V, the vibration of the pump becomes abnormal and the noise emitted increases by about two times.

The scheme of the circuit breaker is shown in Fig. 1.

(click to enlarge)

The input part is galvanically separated from the measuring circuit by means of a transistor optocoupler VE1. The mains voltage is limited by resistor R1 and creates current pulses through the LED of the optocoupler VE1. The VD1 diode bridge allows each half of the mains voltage to pass through the LED of the optocoupler in the forward direction. At point A, the voltage has the form shown in Fig. 2, a. Resistor R3 limits the current through the optocoupler transistor at an acceptable level. If the mains voltage is normal, then at the inputs of logic elements (LE) DD1.1 and DD1.2 - low logic levels and, accordingly, at the output of DD1.3 - log level. "0".

Consider the operation of a channel that responds to a decrease in the mains voltage. The channel is assembled on the elements DA1.1, R6, VD2, R8, C1. As long as the mains voltage is high enough, the voltage at point A in each half-cycle of the mains voltage drops below the voltage level set at the inverting input DA1.1 using the tuning resistor R4. Both gates of the DA1 chip are included as voltage comparators. Frequency correction capacitors can be omitted. In each half-cycle, negative voltage pulses appear at the output DA1.1 (see Fig. 2, b), which, through the chain R6, VD2, discharge the capacitor C1 almost to zero. Then, until a new pulse appears in the next half-cycle of the mains voltage, the capacitor C1 is charged through the resistor R8.

The value of R8 is chosen so that during the half-cycle of the mains voltage, equal to 10 ms, the voltage on C1 approaches the switching threshold of the trigger DD1.1, but does not exceed it (see Fig. 2, c). Resistor R6 limits the output current of the op amp. Diode VD2 prevents the charge of the capacitor by the output current of the op amp when its output is log. "1".

So, if the mains voltage does not drop below the level set by the resistor R4, then at the input of the inverter DD1.1 the voltage corresponds to the log level. "0", and therefore, the output will be a log level. "1". If the voltage in the network drops below the permissible level, then the signal at point A will not drop below the voltage set by the resistor R4, a negative pulse will not form at the output of the op-amp DA1.1, as a result, the capacitor C1 will charge to a voltage sufficient to switch the trigger DD1.1 (Fig. .2, b, c). Moreover, this switching will occur before the end of the current "defective" half-cycle of the mains voltage. The first next "normal" half-cycle of the mains voltage will return this node to its original state, since through the 270 Ohm resistor, the capacitor C1 is discharged almost instantly compared to the mains frequency.

The channel that responds to the excess of the mains voltage, set by the trimmer resistor R5, the level, is assembled on the elements DA1.2, R7, VD3, C2, R9. As long as the voltage in the network does not exceed the specified level, the signal at point A does not fall below the level set by the resistor R5 at the non-inverting input of the op-amp DA1.2 (Fig. 2, a). Since the voltage at the inverting input DA1.2 is greater than at the non-inverting one, the output will be a log. "0" (Fig. 2f).

Capacitor C2 is fully charged. At the input of the inverter DD1.2 - log. "0", and the output is a log. "1". For this channel, the task was to receive a constant signal during the period when the mains voltage is above the norm, which is necessary for the normal operation of the indication LED. As soon as the mains voltage exceeds the specified level, a positive pulse will be generated at the output of the comparator DA1.2. Capacitor C2 will be discharged through the chain R7, VD3 (Fig. 2, e, f). A log will appear at the input of the inverter DD1.2. "1", and its output is a log. "0", which corresponds to an increase in the mains voltage above the threshold. Until the next positive pulse appears at the output of comparator DA1.2, capacitor C2 will be charged through resistor R9. The value of the resistor R9 is chosen so that the voltage at the input of the trigger DD1.2 does not drop below the level corresponding to the log. "1", for a time of 10 ms, i.e. until the next half-cycle of the network (Fig. 2e). Thus, if several half-cycles of the mains voltage in a row exceed the specified level, then the DD1.2 output will have a constant log level. "0".

When the device is turned on, capacitor C4 does not charge instantly. Due to this, a positive pulse is generated at the output of DD6.3, which sets the trigger DD4.1 and the counter DD7 to the initial zero state.

The generator, assembled on the LE DD6.2, DD6.4, starts working immediately after the device is connected to the network and works constantly. While the mains voltage is normal, the trigger DD4.1 remains in the zero state. At both inputs DD5.1 ​​log. "0", its output is also a log. "0". As a result, at the input R of the counter DD7, the log level "1" is stored, and the counter does not respond to the pulse sequence at the input C. The level of the log. "1" from the output DD1.4 goes to the base of the transistor VT3, and the mains voltage is applied to the load. The logic of the automaton is given in the table of states of the elements DD5.1, DD6.1 (see Table 1).

Table 1

When the output of one of the elements DD1.1, DD1.2 log. "0", a log will appear at the output of DD1.3. "1" (Fig.2, d), which will transfer the trigger DD4.1 to a single state. In this case, the transistor VT3 will close.

Until the end of the current half-cycle of the mains voltage, there will still be current in the load, but in the next half-cycle, the triac VS1 will no longer open. Trigger DD4.1 remembers the state of the automaton. The DD7 counter generates a delay before the load is switched on to the network. Until the mains voltage returns to normal, both inputs of DD5.1 ​​will be log. "1", as a result, the DD7 counter will still not count the generator pulses.

When the mains voltage returns to normal, a log will appear at the input S of the trigger DD4.1. "0". Now the inputs DD5.1 ​​will have different logic levels, and the counter DD7 will start counting the pulses of the generator (see table). If at this time a power surge occurs again, this will cause a positive pulse at the input R DD7, returning the counter to the zero state.

Elements C3, R2 set the generator frequency to about 1 Hz. The delay time before turning on the load can be adjusted by selecting one of the outputs of the counter DD7. If output Q5 is selected, the delay is 32 s. Other outputs respectively decrease or increase this value by a multiple of 2 times. After the 7nd negative voltage drop arrives at the input C DD32, a high logic level will appear at its output Q5. Through DD3.1 this level will go to the input R of the trigger DD4.1 and set it to zero. After that, the transistor VT3 will open, and the mains voltage will be supplied to the load.

The three states of the circuit breaker are indicated by a color-controlled light emitting diode. When the machine is in the delay before switching on, the LED is orange because both transitions are lit. At the same time, at all four inputs of the LE DD2.1, DD2.2 there is a high logic level.

When the mains voltage becomes lower or higher than the permissible level, a log level appears at the input 8 DD2.1 or 12 DD2.2, respectively. "0", and one of the crystals stops glowing. Moreover, if the voltage is below the norm, then the red LED goes out and we have a green glow. If the voltage is high, then HL1 shines red. When the mains voltage is normal and the load is connected to the network, HL1 does not light, since the inputs 9 DD2.1, 13 DD2.2 have a log level. "0". The device uses an imported LED with a diameter of 10 mm with a milky-colored lens. The vast majority of imported LEDs with a lens diameter of 8 mm or more have a maximum constant current through one junction of 30 mA. In the described machine, the transition currents are limited to 20 mA by resistors R11 and R12. Transistors VT1, VT2 are amplifiers for the output currents of the LE DD2.1, DD2.2.

Load switching in the 220 V network is carried out by a triac VS1. Thyristor optocouplers VE2, VE3 are used for galvanic isolation from the network. When the load is connected to the network, a high logic level appears at the output of the LE DD1.4. The output current DD1.4 is limited by the resistor R14 and is amplified by the transistor VT3 up to 27 mA. When sufficient current flows through the LEDs of the optocouplers, the photothyristors open at the beginning of each half-cycle of the mains voltage. At the beginning of each half-cycle, the increasing mains voltage causes a current through the chain: contact 8, diode bridge VD4, photothyristors of optocouplers VE2, VE3, diode bridge VD4, R18, triac control transition VS1. The latter causes VS1 to open, as a result, the current continues to increase in the load and flows through the open triac VS1. In the next half-cycle of the network, the triac VS1 opens with a pulse of opposite polarity, however, the current flows through the photothyristors in the forward direction, thanks to the diode bridge VD4.

Resistors R16, R17 equalize the voltages on closed photothyristors. This must be done because the leakage currents of various optocouplers can differ by several times. When the load is disconnected from the network, the voltage is redistributed on the closed photothyristors so that the voltage is 250 V on one, and 89 V on the other (with an effective mains voltage of 240 V, the amplitude value is 240x2 = 339 V), while for this type of optocoupler the maximum output forward voltage in the closed state is 200 V. Because of this, two optocouplers also have to be used. The value of the resistors R16, R17 should be chosen so that the current through the resistors is approximately 10 times the current through the closed photothyristors (leakage current AOU103V is 0,1 mA).

Resistor R18 limits the current through VE2, VE3 and the control electrode of the triac. This is necessary because the triac VS1 opens only at a certain voltage between the anode and the cathode, at which the current passing through the optocouplers VE2, VE3 and the control transition VS1 can increase above the allowable value. Resistor R19 provides a galvanic connection between the control electrode and the triac cathode, which increases the stability of the triac when it is closed (especially at elevated temperatures). When using a triac TS106-10, the load power should not exceed 2,2 kW.

Another variant of a galvanically isolated load switch in a 220 V network can be made on the basis of an optothyristor module VS2 (see Fig. 1 in RE10). When current flows through the LEDs of the module, each half-cycle of the mains voltage passes through the load and the photothyristor, which is connected in the forward direction. In terms of price / quality ratio, both options for switching nodes are the same, but if we take into account the time for manufacturing, then the second option wins significantly. MTOTO80 modules are produced for currents of 60 A and above, so the switching power can be very large. Module size 92x20x30 mm. At a load of up to 1 kW without a heatsink, the module overheats by only 5°C relative to the ambient temperature.

Recently, a triac pulse control has been used to switch the load. This reduces the power consumption of the device. Such technical solutions unnecessarily complicate the circuit, since the energy saving is less than 0,5% at a load of 100 W (the worst triac consumes less than 0,5 W in the control circuit). As the load increases, the energy savings decrease even more. Before using the described automaton, as well as similar devices from [1-3], I recommend that you read the article in [4].

The described circuit breaker can be used to control a network with a voltage of 380 V and above. To do this, select the MTOTO80 module for the required voltage and current and select the resistance of the resistor R1.

To power the circuit breaker, a stabilized voltage source of 9 V at a current of up to 100 mA is required. You can use a source based on a microcircuit stabilizer KR142EN8A(G) in its standard inclusion [5].

Power is supplied to pads 10, 11 on the printed circuit board.

Details. In the described machine, fixed general-purpose resistors of the MLT, S2-23, S2-33 type are used. Trimmer resistors R4, R5 type SP5-14, SP5-22. Capacitors C1, C2 type K73-17 for a voltage of 63 V or more, C3, C4 type K10-17v or other ceramic of a suitable size. Microcircuits can be used from the K176, K561, KR1561 series. Transistor KT315 with letter indices B, G, E. Optocoupler AOT128 with any letter index. Diodes VD2, VD3 types KD522, KD521 with any letter index.

Device design. The device is assembled on a printed circuit board made of double-sided fiberglass. Figure 3-5 shows the layout of the elements on the printed circuit board, respectively, the conductors on the upper and lower sides of the printed circuit board.

The size of the board is 85x85 mm, there are 4 holes with a diameter of 2,8 mm for fixing the board. Power elements VS1 or VS2 are installed outside the board. They are connected to the circuit through contact pads 1, 8, 9 (VS1) or 6, 7 (VS2). In the manufacture of a printed circuit board, one-sided fiberglass can be used, while the connections from the top layer of the board are replaced with a flexible mounting wire, for example, MGTF. When designing a printed circuit board, the number of conductors on the top layer was minimized. Between the elements operating under mains voltage and low-voltage elements on the printed circuit board there is a safety gap that can withstand voltages up to 500 V.

Setting. To set up the circuit breaker, you need a laboratory autotransformer (LATR) and an AC voltmeter. Before setting the slider of the variable resistor R4 is set to the upper position according to the diagram, and the slider of the resistor R5 to the bottom. The machine, together with the load, is connected to the LATR output. It is not necessary to use a powerful device as a load - it can be a 100 W lamp. A voltage corresponding to the upper voltage limit is set at the output of the LATR. Then, by rotating the engine of the resistor R5, they ensure that the load is turned off. After that, by changing the "network voltage" with LATR, they check the correctness of the adjustment. The lower limit voltage is adjusted in the same way.

References:

1. Nechaev I. Automatic protection of network equipment from voltage "jumps" // Radio. -1996. -#10. -p.48 - 49.
2. Protective device for radio equipment from overvoltage // Radio. -1997. -No. 6. -S.44 - 45.
3. Zelenin A. Semiautomatic device for protection of radio equipment from network voltage fluctuations // Radio. 1998. -№10. -S.73 - 74.
4. Kvetkovsky V. Device for protecting radio equipment from exceeding the mains voltage // Radio. -1999. -#10. -p.39.
5. Shcherbina A., Blagiy S. Microchip stabilizers series 142, K142, KR142 // Radio. -1990. -No. 8. -S.89 - 90.

Author: A. A. Rudenko

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