Device for protecting household appliances from mains voltage fluctuations. Scheme, description

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Device for protecting household appliances from mains voltage fluctuations. 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, uninterruptible power supplies

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The spread of new complex and expensive household and electronic equipment requires reliable means of protecting it from voltage fluctuations in the network. Many descriptions of devices for this purpose have been published on the pages of the magazine, but most of them are made on microcircuits that are still inaccessible to residents of rural areas remote from big cities. And it is they who suffer the most from sharp fluctuations in the mains voltage. The author proposes to assemble a protection device on widely used discrete elements.

When the mains voltage goes beyond the limits set during the adjustment, the device, the diagram of which is shown in fig. 1, disconnects the load from the network and turns it on again a minute after normal voltage is restored. Load power should not exceed 2 kW.

(click to enlarge)

With the help of a rectifier on diodes VD1, VD5 with a "quenching" capacitor C1, a constant voltage is obtained, proportional to the alternating mains. The output voltage of the second rectifier ("extinguishing" capacitor C2, diodes VD2 and VD3), stabilized by the zener diode VD4, feeds all the nodes of the device.

The engines of the tuned resistors R6 and R9 are installed in such a way that when the voltage in the network does not go beyond 180 ... 240 V, the voltage taken from the first of them is greater than the stabilization voltage of the zener diode VD6, and from the latter it is less than the stabilization voltage of the zener diode VD7 . As a result, the transistor VT1 is open, and VT2-VT4 are closed and no current flows through the emitting diode of the optocoupler U1.

If the mains voltage drops below 180 V, the transistor VT1 is closed and VT2 is open. At voltages above 240 V, transistors VT3 and VT4 are open. In both situations, current flows through the emitting diode of optocoupler U1.

The actuating element that connects and disconnects the load is the triac VS1. The dinistor of the optocoupler U16 is connected to the circuit of its control electrode through the resistor R8 and the diode bridge VD2, which opens under the action of pulses with a frequency of approximately 4 kHz, generated by a generator based on a unijunction transistor VT6, in the base circuit of which there is a emitting diode of the optocoupler U2. The generator works if the transistor VT5 is closed. Triac VS1 receives opening pulses, and the load receives mains voltage. Signaling this, the neon lamp HL2 is on.

The open transistor VT5, shunting the unijunction transistor VT6, disrupts the generation. In this state, the dynistor of the optocoupler U2 and the triac VS1 remain closed, so the load is disconnected from the network, and the HL2 lamp is off.

The neon lamp HL1 indicates the presence of voltage in the network and the health of the fuse-link FU1

When mains voltage is applied to the protective device, a short current pulse flows through the emitting diode of optocoupler U1. The dinistor of the optocoupler U1, having opened under the action of a pulse, remains in this state until the charging current of the capacitor C5 becomes less than the closing current of the dinistor. Transistor VT5 is open due to the discharge current of capacitor C5 through resistor R12. The discharge process takes 65 ... 75 s, after which the transistor VT5 closes, the pulse generator on the transistor VT6 starts working and the mains voltage is supplied to the load. This is the normal operating mode of the device.

When the mains voltage goes beyond the established limits, a current will flow through the emitting diode of the optocoupler U1 (as mentioned above) and the dinistor of this optocoupler will be opened. Capacitor C5 will charge quickly. This will open the transistor VT5 and disconnect the load from the network. This technical solution eliminates the problem of multiple false switching on and off of the load when the mains voltage fluctuates near one of the limit values. Capacitor C5 is fully charged at the very first very short output of the mains voltage beyond the established limits. Repeated (until the end of the discharge, which lasts, as mentioned above, about a minute) threshold crossings only lead to recharging the partially discharged capacitor and prolonging the exposure. This ensures reliable, without "bounce", switching the load.

The author's copy of the device is hinged mounted on eight mounting strips with ten double-leaf contacts each. It can also be assembled on a single-sided printed circuit board, shown in fig. 2.

(click to enlarge)

The VS1 triac is equipped with a 60x55 mm pin heat sink. Resistors R3 and R4 are soldered directly to the terminals of capacitors C1 and C2. The entire device is housed in a suitably sized case made of insulating material. HL1, HL2 neon lamp holders and FU1 fuse holder are installed on the front panel of the case.

Capacitors C1 and C2 - MBGCH, C3 - K50-24, C4 and C5 - K50-6; C6 - MBM. All fixed resistors are MLT, trim resistors are SPZ-38g. Replacing KD105B will be any rectifier diodes for a current of at least 0,3 A and a reverse voltage of more than 300 V (series D226 KD20b, KD109). The KTs407A diode bridge can be replaced by others that are close to it in terms of parameters, for example, the KTs402, KTs405 series, or assembled from separate KD105B diodes The KS515A zener diode is replaced by two D814A and D814B (VD6) and D814D (VD7) connected in series - other low-power ones with stabilization voltage, respectively 8...10 V and 12...14 V.

Instead of KT315V transistors, any of the KT503, KT3102, KT3117 series will do, and KT3102B (VT5) will replace KT3102V, KT3102D, KT3117A or a composite of two KT315V. AOU103B optocouplers can be replaced with AOU103V, or better, with AOU115G or AOU115D. With a load power of up to 1,4 kW, the TC122-25 triac can be replaced with a TC112-10 or TC106-10 voltage class of at least 4, and at 0,7 kW - with a KU208G.

To set up the protection device, you will need an adjustable autotransformer (LATR), an AC voltmeter and a load - a 220 V incandescent lamp with a power of at least 40 watts. At the time of tuning, it is advisable to install a capacitor with a capacity of 5 ... 1 microfarads as C2. This will reduce the turn-on delay of the load and make it easier to adjust the thresholds. Before starting the adjustment, move the sliders of the resistors R6, R9 to the lower position according to the diagram. This will disable the load.

Having set the input voltage equal to the lower limit (180 V) using LATR, move the slider of the resistor R6 until the load is turned on. It is usually possible to find a position in which the load, without outside interference, periodically turns on and off. Next, the input voltage is increased to the upper limit (240 V) and the protection is again activated, this time using the tuning resistor R9. It remains to replace the temporarily installed capacitor C5 with a standard capacitance of 200 microfarads and check the duration of the load turn-on delay.

Since the circuits of the device are under mains voltage, when adjusting it, it is necessary to follow the rules of electrical safety.

Author: A.Kuzema, Gatchina, Leningrad Region

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