Block of protection against voltage fluctuations in the mains. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Block of protection against voltage fluctuations in the mains. 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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Modern electronic equipment with switching power supplies can operate within a very wide range of mains voltage instability. At the same time, appliances such as a refrigerator, air conditioner, washing machine are more demanding on the power supply. Deviation of the mains voltage from the norm even by 10% is highly undesirable for some of them. And this applies not only to increasing the voltage, but also to lowering it. For example, when the supply voltage is low, the refrigerator compressor motor may stop.

In this case, the current through its winding will increase sharply, which can even lead to a fire. Extremely undesirable and excess voltage, as well as its sharp fluctuations.

The figure shows a diagram of a protective device that monitors the voltage level in the network, and if its value goes beyond the limits specified during configuration, the circuit disconnects the load.

Voltage fluctuation protection unit in the mains

It is noteworthy that the load is turned on not immediately after the mains voltage returns to normal, but a few seconds after that. This delay prevents transients that occur in the network from adversely affecting the equipment.

The circuit is constantly powered by the mains, regardless of voltage fluctuations in it. The load is turned on and off by means of a relatively powerful relay K1. The electronics and relays are powered by a transformer power supply at T1. The supply voltage of the D1 chip is maintained stable at 5 V using the A1 stabilizer. The rectifier on VD4 and C3, as well as R1 ... R4, serves as a sensor for the magnitude of the mains voltage.

It may seem strange that the entire circuit is powered by a transformer, and the control voltage is taken directly from the network. This decision had to be made after testing the initial version of the circuit, in which the control voltage was taken from the secondary winding of the transformer.

The reason for the failure was that when the relay is turned on and off, a significant change in voltage occurs at the output of a low-power transformer and rectifier. This is due to the relatively large current consumption of this type of turnip. When the relay turns on, the voltage at the output T1 decreases, and when the relay is turned off, it rises.

Even if you feed the relay through the stabilizer, this does not change the essence of the matter, since the voltage on the relay will be stable, and the voltage on the secondary rectifier will change. Therefore, the voltage level sensor is connected directly to the network.

The sensor works as follows. VD4-C3 is a rectifier. Its output will be a constant voltage proportional to the alternating voltage in the network. Resistors R1-R4 are two adjustable voltage dividers. The elements of the D2 chip form a kind of sensor signal amplifiers.

The elements of the K561LA7 microcircuit do not contain Schmitt triggers, therefore their threshold levels are the top of zero and the bottom of one, almost at the level of one voltage. Elements of the K561TL1 chip with Schmitt triggers. and the thresholds of zero and unity are very different. Resistor R4 sets the lower threshold for the mains voltage, and resistor R3 sets the upper one. When the voltage in the network is below the set threshold, the voltage at input D2.1 slides towards logic zero. The voltage at the output D2.1 starts to rise and the element D1.1 switches to the zero state at the output. This causes element D1.2 to switch to a single state. Capacitor C4 is rapidly charged through VD5 and R5. Zero occurs at the output of D1.3. Transistors VT1, VT2 turn off and relay K1 disconnects the load. When the voltage returns to normal, the reverse process occurs and zero is set at the output D1.2. In this case, the discharge of the capacitor C4 occurs through a relatively large resistance R8, so it takes several seconds to turn on the load (until C4 is discharged to the logic zero threshold).

If the voltage in the network exceeds the maximum limit set by resistor R3, then element D2.2 is activated. At its output, the voltage decreases and this leads to switching element D1.2 to the state of unity at the output. Then everything is as in the case of lowering the voltage.

When establishing, you need to use an autotransformer of the LATR type or similar, which allows you to adjust the alternating voltage. With its help, the voltage is lowered and increased to the required limit values, and the actuation points are adjusted accordingly by resistors R3 and R4.

Details. Capacitor C4 must be for a voltage of at least 360 V. The remaining capacitors must be for a voltage of at least 16 V. Transformer T1 is Chinese with a primary winding of 220/110 V (the tap is not used) and a secondary winding of 9 + 9 V, and a current of 300 mA.

The type of relay K1 depends on the maximum load power. With a power of up to 200 W, you can use the KUTs-1 relay from old domestic TVs. In this circuit, a HJQ-13F relay with a 12 V winding is used. With this relay, the maximum load power is 2600 W.

Author: Merzlyakov A.K.

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