Protection for devices powered by 220 volts. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network

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The maximum voltage that stabilized switching power supplies, which most household imported radio equipment is equipped with, can withstand is 272 V (this is the current value according to the technical documentation of manufacturers). But the network sometimes experiences spikes and higher levels of interference, which can lead to damage.

To protect equipment from overvoltage, it must be taken into account that the following types of impulse noise can act in the supply network:

• increased voltage between line wires;
• increased voltage between ground and one of the line wires.

These interferences can be present at the same time, so it is necessary that the protective unit and the devices connected to it have good grounding, otherwise it will only be possible to provide protection against increased voltage between the line wires.

Network protective devices are characterized by the use of all the components discussed above, while the circuits can be one-, two- and three-stage. Fragments of the most common protection schemes are shown in fig. 1.15 and 1.16.

A single-stage protection node usually uses a three-pole arrester or varistors (Fig. 1.15). If the specified breakdown voltage is exceeded on any conductor, both "halves" of the three-pole arrester are triggered and the voltage surge is discharged to the ground. This allows you to get protection from common mode interference (it can be much more powerful than differential pickup).

. Rice. 1.15. Unit of single-stage surge protection, made on: a - arresters; b - varistors.

In two-stage circuits, varistors and diodes are usually installed at the same time, fig. 1.16. Since varistors are capable of absorbing more pulsed power than diodes, they are used as primary protection, but are complemented by faster elements - suppressors.

Rice. 1.16 Options for two-stage surge protection schemes.

For electronic equipment, the greatest danger is not the current, but the voltage in the circuit, therefore, TRANSIL diodes are now increasingly used. To protect devices in a 220 V network, bidirectional diodes are usually used that allow operation on alternating current, for example, 1.5KE440CA or P6E440CA (the last letters - CA are often used in the designations of other types of elements to indicate. Symmetrical structure and voltage tolerance limits). If dual ones cannot be purchased, then you can take two unidirectional diodes and connect them in series with the same polarity.

The operating voltage of the protective diodes is selected taking into account the maximum voltage amplitude in the line. So, for example, according to the domestic standard, it is considered normal if the effective voltage (U) of the network has a deviation from the nominal value of 220 V -15 or + 10%, i.e. it can be 242 V, while its amplitude will be:

.

Taking into account the possible technological spread of the limiting voltage that protective diodes have, according to the recommendations of the developers of these elements, the operating voltage is selected with a margin of 10 - 20%, i.e., it must be at least 400 V.

Current fuses must be installed at the input of the protection circuit - in case of a short-term overload, the fuse will not have time to operate (it has a circuit break time of at least 0,05 ... 0,1 s), but flow protection is needed to limit the operating time of the remaining protective elements and exclude the allocation of high power to them during prolonged exposure to overload.

The most reliable protection of radio equipment can be provided by three-stage circuits, two typical variants of which are shown in Fig. 1.17.

Rice. 1.17. Three-stage surge protection circuit for 220 V mains

Three-stage protection is installed in cases where a direct hit of a lightning discharge into an overhead power line is likely. In this case, in the worst case, only the protection unit may fail, but the radio equipment will remain.

We considered the construction of universal network protection schemes, but in some situations simpler nodes can be dispensed with, for example, when it is necessary to protect a transformer-powered circuit. For this case, options for connecting suppressors are shown in Fig. 1.18.

Rice. 1.18 Option for connecting protective diodes to the transformer: a - in the primary circuit; b, c - in the secondary circuit

When installing protection elements, you should be aware that in the case of jokes by domestic power engineers, when 220 V operates in the mains instead of 380 for a long time, such elements will fail (they cannot dissipate high power for a long time), but at the same time they will still save radio equipment from damage .

Author: Shelestov I.P.

See other articles Section Protection of equipment from emergency operation of the network.

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