Autotransformer automatic load switch. Scheme, description

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Autotransformer automatic load switch. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches

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If an autotransformer is used to regulate the voltage at the load, then with a sudden increase in mains voltage, the load may fail. The proposed device in such cases will automatically turn off the load.

A schematic diagram of the ATP protection device is shown in fig. 1. The scheme of the ATP itself is highlighted in bold lines, and the crosses indicate the chains that need to be broken. The device is powered by the ATP winding through a rectifier on the diode VD1. On capacitor C1, the voltage varies between 25 ... 55 V, depending on the network. A voltage regulator + 1 V is assembled on the transistor VT22.

Autotransformer automatic load break switch

Resistor R1 sets the device response threshold. If the voltage exceeds the set threshold, the zener diode VD4 will open, therefore, the trinistor VS1 will also open. Relay K1 will work, and contacts K1.2 will disconnect the load. In addition, contacts K1.1 will switch the input voltage to the highest output of the winding according to the circuit. This state of the ATR is not dangerous, since the upper winding output is designed for mains voltage up to 250 ... 260 V.

To return to the initial state, you must first set the required voltage at the ATP output by turning the adjusting ring, and then press the SB1 button. Thyristor VS1 will close, and at the moment the button is released, the relay will operate, connecting the load.

Relay K1 - REK28-1 UHL4 KShch4.569.007. It can be replaced by another with a winding voltage of up to 18 V, a trip current of up to 50 mA and two groups of switching contacts for a current of 2 A at an alternating voltage of 250 V, for example, REN-34 passport KhP4.500.001-01. Transistor VT1 can be series KT805, KT807. Diodes VD1 and VD5 - KD105V, KD105G or series KD209, D226. Zener diodes VD2 and VD3 can be replaced with one KS220Zh, and VD4 - KS175A, KS168A. Fixed resistors - MLT, trimmer R1 - SPZ-16, capacitors C1 and C2 - oxide K50-16, K50-35.

Trinistor VS1 - KU101 series with indices B, G, D, E or KU112A. It is possible to replace the trinistor with a node on two transistors, as shown in fig. 2. Here, transistors of the KT2, KT203, KT208, KT209 series are suitable as VT3107, and VT3 - KT315, KT3102. If the operating current of the relay is more than 50 mA, transistors should be installed more powerful - series KT814, KT816, KT837 (VT2) and KT801, KT807, KT815, KT817 (VT3).

Autotransformer automatic load break switch

The device is placed inside the ATP case. When setting up and operating the ATR, one must be very careful, since all circuits of the device are galvanically connected to the network.

Establishment comes down to setting the threshold for the operation of the device by resistor R1. First, the R1 engine is set to the upper position according to the diagram. By connecting a voltmeter in parallel to the load, the ATR is connected to the network. The control ring sets the output voltage to 230 V. With a screwdriver with a dielectric handle, slowly rotate the R1 engine until relay K1 is activated. The response delay (about 0,5 s) can be increased by using a larger capacitor C2. When using a relay with a trip current of more than 50 mA, the VT1 transistor must be installed on a heat sink to prevent overheating.

In conclusion, I note that the device can be used for any ATP if it has a tap for 25 ... 30 V AC. In self-made designs, you need to wind an additional winding for a voltage of about 25 V. At the same time, there is no galvanic connection between the device and the network.

If contacts K1.2 are excluded by connecting the load directly to the ATR output, in an emergency the load will not turn off, but the voltage will decrease on it, since the input voltage will be applied to the entire ATR winding.

For reliable operation, it is desirable to include a resistor with a resistance of 1 to 3 kOhm between the control electrode and the cathode of the thyristor VS10.

Author: I. Aliyev

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