Emergency protection unit for low-voltage radio equipment. 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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Many devices have fuses rated for the maximum current draw of the radio equipment. They burn out when the current in the circuit exceeds the nominal by 1,5 ... 1,6 times for a certain time required to heat the fusible link.
Different types of fuses have different response times (circuit breaks), but they are all rather inertial, and in the event of an overload, some elements of the circuit may fail much earlier. The fuse in this case protects the device only from fire of the structure, which, of course, is also necessary, but not from damage.
The above schemes work with the participation of a fuse, but allow you to more effectively protect radio equipment by accelerating its operation. This is achieved by forcibly increasing the current through the fuse (shorting the circuit after it with an electronic key) during overload.
Such circuits can also protect the powered device from the effects of short-term high-voltage surges that may appear, for example, in the on-board network of a car in case of electrical equipment malfunctions.
The electrical circuit of the set-top box (Fig. 1.25) is connected to a break in the power supply circuit of the device and in the normal state does not have any effect on operation. The protection is activated only when the input voltage rises or the polarity of the connection is reversed.
Rice. 1.25. Scheme of emergency protection of radio equipment from high voltage power supply
When the thyristor is opened, the power supply is shorted through the FU1 fuse, which leads to its rapid burnout, and if the voltage is connected in reverse polarity, the VD1 diode will open and the fuse will also burn out.
The indicator of emergency shutdown of the load is the glow of the HL1 lamp (any one for a current of at least 60 mA and the corresponding voltage is suitable).
A simpler diagram of the protective device is shown in fig. 1.26. It works in a similar way, but only in this case, the thyristor threshold depends entirely on the parameters of the VD2 zener diode used. In such a scheme, it is not possible to smoothly adjust the level of protection operation.
Rice. 1.26 The second version of the protection scheme
The third option (Fig. 1.27) is designed to protect the electrical circuit in case of damage to the integrated stabilizer DA1. In this case, the voltage is controlled at the output of the stabilizer and the protection threshold is determined by the stabilization voltage of the zener diode VD2.
Varistors or protective diodes (TRANSIL, TRISIL or TVS) can be used to perform faster protection of electronic circuit elements from the effects of short-term high-voltage voltage surges that occur during switching processes.
For automotive electronics, even special types of suppressors are produced that are capable of dissipating pulses with a power of up to 6000 W (for example, when starting an engine in a car, a short-term voltage surge can reach 300 V).
Rice. 1.27. Option for connecting emergency protection in a voltage stabilizer
In the normal state, the thyristor VS1 is closed and will open only if the input voltage increases higher than the threshold set by the resistor R4 (for ease of adjustment, it is better to use a multi-turn one). The threshold element is the transistor VT1. Due to the high gain of the transistor, an increase in the supply voltage by only 0,4 V leads to its opening and the supply of a control voltage to the thyristor. Any transistor suitable for conductivity, but it must have a gain of at least 400 (the use of a transistor allows you to set any protection threshold quite accurately).
Author: Shelestov I.P.
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