Protection of REA against high-voltage impulses in the network. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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The authors introduce a problem little known to most readers - the protection of household equipment from single high-voltage (more than 400 V) voltage pulses in the 220 V supply network, talk about options for its implementation, report on the components of protective devices produced by the industry.

The presence of voltage pulses reaching 220 V or more in the AC supply network 50 V x 1000 Hz is not new for specialists. For a wide range of electricity consumers, these impulses are a discovery. The article considers the possibilities of protecting equipment from pulses occurring in the network with a duration from tenths of a microsecond to a few milliseconds. Longer voltage surges - more than a half-cycle of a sinusoid with a frequency of 50 Hz - are eliminated in other ways that are not covered here. The reasons for the appearance of these pulses are different and are described in the literature, for example, in [1].

The energy of high-voltage pulses in the supply network can reach several kilojoules. Known and widely used methods for reducing impulse noise in power circuits using LC and RC filters, screens between the windings of network transformers, and other methods often do not provide the necessary reduction in the energy of pulses at the power supply pins of microcircuits. It is noted that pulses with energies up to a millijoule actually reach the microcircuits, which are quite capable of disabling the equipment.

Other well-known methods for limiting the level of pulses in various circuits of electronic equipment, in particular, on distribution network electrical panels, are associated with the use of gas-discharge and semiconductor devices. Gas-discharge devices, in practice often called spark gaps, do not always provide the desired result due to the relatively low speed and are rather bulky.

Semiconductor devices widely used to reduce transient noise include metal oxide varistors, general purpose semiconductor devices, and special semiconductor voltage suppressors. Varistors are resistors with a sharply non-linear current-voltage characteristic, their resistance decreases significantly with increasing applied voltage. General-purpose semiconductor devices mean zener diodes, diodes pulsed and with a Schottky barrier, defensors.

For special semiconductor voltage limiters, which will be discussed below, the current-voltage characteristic is similar to the zener diode. Their main difference from zener diodes and other general purpose semiconductor devices is the ability to dissipate a large pulsed power. Modern varistors, slightly inferior to the limiters under consideration in terms of response time, compete with them in terms of manufacturability and cost. However, the characteristics of the varistors deteriorate for some time after the impact of each interference pulse. Semiconductor limiters do not have this phenomenon. Considering that devices with maximum speed and stability of characteristics are needed to protect electronic equipment, they should be given preference.

In the early 90s, GSI (USA) produced over a thousand varieties of semiconductor voltage limiters with a maximum allowable pulse power of up to 60 kW and a limiting voltage of 0,7 to 3000 V. At present, similar limiters with a power of up to 30 kW are also produced in the CIS for voltage within 3 ... 1000 V.

The principle of operation of the limiter is to open its closed pn junction if the reverse voltage applied to it exceeds the threshold level. In other words, the limiter behaves similarly to zener diodes, however, the characteristic of the tunnel-lavion process in it is that only the majority carriers carry the charges, so there is no undesirable accumulation of minority carriers. This is mainly due to the high speed of the limiter.

The current-voltage characteristic (VAC) of the limiter is shown in fig. 1. Like the zener diode, it is asymmetrical.

To limit the pulses of both signs, it is convenient to turn on two limiters in an anti-sequential manner. The CVC of such a pair is symmetrical (Fig. 2).

Commercially available semiconductor voltage suppressors are usually evaluated according to the following characteristics:

• Rmp max - pulse maximum allowable dissipation power for a given shape and duty cycle (K3) of pulses and ambient temperature Tacr.av. imp max, with a front duration of 1 μs and K10 less than 3%, ensuring that the permissible average power dissipation by the crystal or device case is not exceeded;
• Iobr max - reverse maximum current flowing at the maximum reverse voltage;
• Urev max - reverse maximum voltage, which should not exceed the operating value (the operating voltage should not be limited in this case); the value of UoR max is usually taken equal to 0,8 of the opening voltage of the device;
• Uopen and Iopen - the voltage and current of opening the device, corresponding to the inflection point on the working branch of the current-voltage characteristic;
• Ulimit imp - limiting voltage - pulse reverse voltage at the maximum value of the limiting pulse current, depending on the maximum allowable dissipated pulse power;
• Ipr.imp.max - direct pulse maximum current - permissible direct current for a given shape, duty cycle and ambient temperature;
• Upr.imp.max - direct pulse maximum voltage drop across the limiter at current Ipr.imp.max.;
• Kogr - restriction coefficient equal to the ratio Ulimit. imp max / Uopen; Kogr varies from about 1,3 at maximum pulse power Rimp max to 1,2 at 0,5 Rimp max;
• ton - switch-on time during which the device opens in the opposite direction (for symmetrical limiters ton < 10-9 s).

According to the values ​​of these characteristics, the consumer can choose the voltage limiter necessary to protect electronic equipment. A symmetrical (two-arm) limiter is connected to the AC network in parallel with the payload. In the normal mode of the network, both of its arms are closed and only a very small reverse current flows through it at both half-periods. In other words, the limiter does not reveal itself in any way, consuming some - very small - power (hundredths of a watt).

As soon as a high-voltage voltage pulse appears in the network that exceeds Uopen of the limiter, both of its arms open, one in the forward direction, the other in the opposite direction. As a result, the pulse will be blocked, and the voltage at the load at this moment will not exceed Ulimit.

It should be noted that the value of Rimp max depends on the duration of the chi suppressed pulse and within τi = 0,1...10 ms is approximately proportional to the ratio 1/τi. As the ambient temperature rises, Tacr. cf from 40 to 100 CC, the dissipated power Rmp max must be reduced approximately proportionally to 0,024 Tacr. cf.

To reduce the amplitude of high-voltage pulses on the way from the 220 V network to the power supply terminals of microcircuits, it is most advisable to include limiters in the power supply [2].

If pulses appear in the supply network, the energy of which is greater than that allowed for the applied limiter, it, like the zener diode, with too much stabilization current, will overheat and fail. From this point on, the equipment connected to the network will be unprotected.

Therefore, a significant disadvantage of the use of limiters is the lack of information about their performance or failure after exposure to powerful pulses. To provide an indication of the correct state of the symmetrical limiter, it is made up of two single ones and a circuit of three s veto diodes and two current-limiting resistors is connected to it (Fig. 3).

A feature of the health indicator is the use of LEDs in a non-standard mode. With serviceable limiters VD1 and VD2 and a positive half-cycle of the mains voltage (plus - on the top network wire according to the circuit), the current flows freely through the limiter VD1, open in the forward direction, and through the HL1 LED. The VD2 limiter is closed at this time.

As a result, almost all mains voltage is applied to the HL3R2 circuit, and to the veto diode - in the opposite direction. Therefore, the HL3 LED opens in the opposite direction *; the current through it limits the resistor R2. Thus, a current of about 2 mA flows through the entire circuit from the positive wire to the negative one. This is enough to provide a noticeable glow of the "green" LED HL1. The HL2 LED does not light up, because too little voltage (less than 2 V) is applied to the HL1R3 circuit.

When the polarity of the mains voltage is reversed, the same processes occur, only VD1 and VD2, R2 and R1, HL3 and HL2 change places. That is, the serviceability of the limiters is confirmed by the green signal of the indicator. In some cases, the described indicator can simultaneously serve as an indicator of the presence of mains voltage.

It is easy to see that when the limiter VD1 fails (breaks), the "green" LED HL1 goes out and the "red" LED HL2 turns on, and when the limiter VD2 is damaged, the "red" HL3 turns on.

The described module, named ZA-0, was developed at Computer Engineering and Industrial Electronics OJSC (Moscow) together with NPK Quark (Tashkent) and mastered in mass production. The appearance of the module is shown in the photo (Fig. 4).

Main characteristics of the module

• Impulse maximum allowable power, kW, not less than, at an ambient temperature of 25°С......1,5
• The amplitude of the alternating voltage of the opening of the limiters, V, at an ambient temperature of 25 ° C (opening current 1 mA) ..... 400 ± 20
• Restriction coefficient,......1,2... 1,3
• Light intensity of LEDs, mcd, not less than ...... 0,5
• Power consumed from the network in the absence of high-voltage impulses, W, not more than ...... 0,5
• Housing dimensions**, mm, no more than......32x12x10
• Weight, g, not more than ...... 10

The body of the module is made of plastic by pouring into a mould. Climatic version UHL, placement category 4.2 according to GOST 15150. In terms of protection against electric shock, the product belongs to class II according to GOST 2757.0.

The ZA-0 module, in addition to installation in REA power supplies, is recommended for a wide range of users and radio amateurs for use in laboratories, offices and apartments to protect industrial and household electronic devices connected to 220 V AC power outlets. A variant has been developed for this purpose products, which received the name ZA-01. Here, the module case is equipped with standard pins that allow it to be plugged into any free socket in the room.

The development of the protective module ZA-0 was approved by the Scientific and Technical Fund "Energy Electronics", which assisted in the development of products in mass production.

Protective modules for 5 kW (ZA-1) and 30 kW (ZA-2), as well as variants of these products with plugs (ZA-11 and ZA-21) are in the process of being mastered in production. These modules should be used in cases where one and a half kilowatt ones cannot withstand high-voltage mains impulses. Modules have also been developed for the protection of DC networks, designed for impulse power from 1,5 to 30 kW and opening voltage from 6,8 to 450 V.

At the first stage of using ZA-0 protective modules and products based on them, the supplier will provide customers with a free replacement of failed ones with new ones. If the modules fail again, the consumer will be advised to purchase more powerful devices. If necessary, JSC "Computer Engineering and Industrial Electronics" (tel. in Moscow 330-06-38) will conduct a study of the consumer's network and give proposals for the protection of REA.

* This feature of LEDs (and a number of other electronic components) has long been noticed, researched and widely used by radio amateurs. See, for example, the article by I. Nechaev "LED as a zener diode" in "Radio", 1997, No. 3, p. 51.

** Without taking into account the length of the leads - 9 ... 12 mm and the height of the protruding LED housings - 3 ... 5 mm.

Literature

1. Cherepanov V. P., Khrulev A. K., Bludov I. P. Electronic devices for the protection of electronic equipment from electrical overloads. Directory. - M.: Radio and communication, 1994 (p. 17-21).
2. Kolosov V. A. Power supply of stationary REA. Theory and practice of design. - M.: Radio and communication, 1992 (p. 111, 112).

Author: V. Kolosov, Moscow, A. Muratov, Tashkent, Uzbekistan

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