Protection of radio equipment from surges. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Protection of radio equipment from surges. 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

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If both linear and switching DC voltage regulators made on transistors or microcircuits fail, the output voltage can become almost equal to the input (rectified) voltage, usually taken from the power filter capacitor installed at the output of the diode bridge. For example, when KR142EN5A, which is usually used in the power circuits of digital devices, burns out, a voltage of 7 ... 15 V can be supplied to the power buses instead of the prescribed 5 V. This is already dangerous for the vast majority of devices.

Sometimes, to protect overvoltage-sensitive nodes of radio equipment, a powerful zener diode with a stabilization voltage slightly higher than the rated supply voltage is used. The disadvantage of this protection method is that many zener diodes have a sufficiently large differential resistance, and the protected device can continue to work for some time, receiving a voltage that is 0,5 ... 1,5 V more than the nominal one. A zener diode that is very hot at this time can "go on a break", and protection as such will not work.

To protect individual components and blocks of radio equipment from increased voltage in case of damage to the stabilizer or improper connection to the power source, you can assemble a simple adjustable protection unit (Fig. 1). It is included in the open circuit between the output of the power supply and the load.

Protection of radio equipment from surges. Schematic diagram of a simple protection unit
Rice. 1. Schematic diagram of a simple protection unit

This block works as follows. With an increase in the input voltage, the current through the zener diode VD1 increases sharply, respectively, the current in the control electrode circuit of the thyristor VS1 also increases, the thyristor opens and shunts the load power until the fuse FU1 trips. A powerful wire resistor R3 prevents breakdown of the thyristor due to a strong inrush current that occurs if high-capacity oxide capacitors are installed in the power circuit. The zener diode VD1 is selected for a voltage approximately 0,3 ... 1,5 V less than the rated supply voltage. The choice of its type depends on a number of factors, so it is better to determine the best option for each specific case experimentally. By adjusting the resistor R1, you can accurately set the voltage at which the thyristor will open. Capacitor C1 prevents false operation of the protection unit during short impulse noise that may appear in the power circuit. Resistor R2 protects the zener diode and thyristor when the trimmer slider is in the upper position. At the time of setting up this unit, it is advisable to replace the fuse with an incandescent lamp, by ignition of which one can judge the moment the thyristor is turned on.

A more advanced protection unit can be assembled according to the scheme shown in Fig. 2.

Protection of radio equipment from surges. Schematic diagram of the protection unit with relay
Rice. 2. Schematic diagram of the protection unit with relay

With an increase in the input voltage, the load power is stopped due to the opening of the contacts of relay K1. The R3-VD2 circuit is designed to reduce a short-term voltage surge at the output of the unit, which may appear due to the inertia of switching the relay contacts.

To protect the radio-electronic devices installed in the car, for example, a car radio or an alarm against overvoltage in the on-board network, you can assemble a protection unit according to the diagram shown in fig. 3.

Protection of radio equipment from surges. Schematic diagram of the protection unit of automotive radio-electronic devices
Rice. 3. Schematic diagram of the protection unit of automotive radio-electronic devices

Here, the slider of the resistor R1 is set to a position in which the protection is triggered at an input voltage of 15 ... 16 V. When the thyristor opens, the relay contacts open, the load is turned off, the HL1 LED flashes. Capacitors C1...C3 increase noise immunity. By pressing the SB1 button (without fixation), you can return the protection unit to standby mode.

Trimmer resistors can be taken with a resistance of 150 ... 470 Ohm types SPZ-38, RP1-63M, SP5-16VA, SP4-1 or, better, multi-turn - type SPZ-39. Wire resistors - type C5-16MV or self-made from a short piece of thick high-resistance wire. Capacitors - types K10-17, KM-5. Thyristors will fit any of the KU228, KU201, KU202, T122 series. The KD213A diode is replaced by a powerful diode from the KD202, D242, KD2999 series. The flashing LED is red. It can be replaced by any of the L56, L36, L799, L816 and other similar series. An electromagnetic relay of the RMU type (certificate ChP4.523.332) can be replaced by any one that reliably operates at a rated input voltage and has sufficiently powerful normally closed contacts.

In the nodes according to the diagrams in Fig. 2 and 3, you can install a REK29 type relay from the remote control systems of old domestic TVs by unwinding the required number of turns from its coil. Automotive relays of suitable design can also be adapted.

Zener diodes KS297V, D814D, KS213Zh, KS508A, 1N6003B can be used to protect automotive equipment in the voltage sensor circuit. For designs on low-voltage digital microcircuits, zener diodes of types KS126G, KS126D, KS139A, KS147A, KS407B, KS439A, 1 N5991 V are suitable. , 561N564B, 1561N215A.

Literature

P.Horowitz, W.Hill. The art of circuitry. - M.: Mir, 2001, p.335.

Author: A.Butov, s.Kurba; Publication: cxem.net

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