Device for automatically disconnecting household appliances from the mains. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Device for automatically disconnecting household appliances from the mains. 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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The proposed device automatically disconnects household equipment from the network after it switches from operating mode to standby.

Today, almost all household audio and video equipment equipped with a remote control, when turned off by a command from the remote control, goes into standby mode. This mode is very convenient in case of frequent use of household appliances. However, it does have its drawbacks. Firstly, this is the additional consumption of electricity by the equipment that is turned off, but in standby mode. Secondly, a relatively long (days, weeks) stay of the elements under mains voltage, which increases the likelihood of their failure in the event of an abnormal emergency increase.

Sufficient protection against these undesirable factors can only be a complete disconnection of household equipment from the mains after the end of work. Turning off the device with a standard switch is not always effective, since the switches used, as a rule, are installed in the gap of one network wire, and the second network wire is always connected. In addition, not all household appliances are equipped with a power switch. Disconnecting the equipment from the network, removing the mains plug from the outlet manually, is troublesome and inconvenient. The proposed device is able to perform such "work" automatically. The equipment connected to this device, after completing its work and transferred to standby mode, will be automatically disconnected from the mains.

Rice. 1 (click to enlarge)

The scheme of the device is shown in fig. 1. Its basis is relay K1, which, with its contacts K1.1, supplies power to the device, and with contacts K1.2 and K1.3 connects the load connected to the socket XS1 to the network. SCR VS1 performs the function of a voltage amplifier from the current sensor R6, proportional to the load current, as well as a switch in the power supply circuit of the relay K1 and the emitting diode of the optocoupler U1.

A timer is assembled on the unijunction transistor VT1, which, after a certain period of time, connects the charged capacitor C4 to the trinistor VS1 in reverse polarity, as a result of which the latter closes. Circuit R9, VD5 - starting, designed for the initial opening of the trinistor when the SB1 button is pressed. After turning on the device, the photothyristor of the optocoupler shunts the trigger circuit. The device is powered by a transformerless power supply with a ballast capacitor C1, assembled on rectifier diodes VD1, VD2, zener diode VD3 and capacitor C2.

With a short press on the start button SB1, the mains voltage is supplied to the device. A constant stabilized voltage of 26 V is formed at the output of the power supply unit of the device. Through the circuit r9, VD5, R8, R5, this voltage is supplied to the control electrode of the trinistor VS1. It opens and energizes the coil of relay K1 and the emitting diode of optocoupler U1. The relay is activated and shunts the closed contacts of the SB1.1 button with contacts K1. Contacts K1.2 and K1.3 connect the output socket XS1 of the device to the power supply. At the same time, the photothyristor of the optocoupler turns on and closes the starting circuit R9, VD5 to a common wire.

From this moment, charging of the capacitor C4 begins through the resistor R4 and an open trinistor. The charging time constant is chosen to be about 5 s. During this time, it is necessary to transfer the powered device to the operating mode. The trinistor remains open from the moment it is initially turned on by the starting circuit until the next closing after connecting the charged capacitor C4. To protect the relay from possible contact bounce at the moments of closing the trinistor, a capacitor C3 is connected in parallel with the winding. It creates a delay for the relay to turn off.

When the threshold voltage on the capacitor C4 is reached, the transistor VT1 opens and connects the charged capacitor to the SCR in reverse polarity. The trinistor closes, the relay power circuit opens. The relay is turned off, contacts K1.1-K1.3 open, the emitting diode and photothyristor of the optocoupler close. The device returns to its original state.

This is a normal device operation algorithm when there is no load on its XS1 output. The presence of a load connected to the socket XS1 creates an alternating voltage drop on the sensor R6, the positive half-wave of which, through the diode VD6 and resistors R8, R5, enters the control electrode of the trinistor and turns it on. Such an inclusion of the trinistor with a positive half-cycle will occur every time after its next closing by the voltage of the charged capacitor C4.

The arrival of positive pulses on the control electrode of an open trinistor between switching on the timer on the transistor VT1 does not affect the conductive state of the trinistor, and the device is in a stable operating mode.

Reducing the load current or turning it off leads to a decrease in the amplitude or the complete absence of control pulses and, as a result, to disconnecting the device from the network. The shutdown threshold is set by a trimmer resistor R5. After switching off the machine is immediately ready for a new cycle of operation.

The device uses silicon diodes KD105B (VD1, VD2, VD4, VD5) and germanium D7Zh (VD6). A replacement can be KD105V, MD226, KD221V. Zener diode D816B (VD3) will be replaced by two serially connected KS512A, KS515A, D815D. You can replace the unijunction transistor KT117V with an analogue on bipolar transistors (Fig. 2).

Device for automatically disconnecting household appliances from the mains
Fig. 2

Relay K1 - REK28 (version KShch4.569.007 with three groups of switching contacts, rated winding voltage - 24 V), in case of replacement, it is selected based on the required operating voltage and load capacity of the contacts (at least 5 A) capable of switching voltage 220 V.

The trinistor KU103A (VS1), despite the fact that its passport current value in the open state is 1 mA, switches the current flowing through the relay winding (30 ... 40 mA) and the emitting diode of the optocoupler (5.10 mA) without problems. It will be replaced by devices of the KU201, KU202 series. It should be noted here that the sensitivity of these trinistors is less, and for stable operation it is necessary to increase the resistance of the current sensor to 3 ... 4 Ohm or select instances of devices with a higher sensitivity. Optocoupler AOU103V (U1) will be replaced by AOU115B, AOU115V.

Capacitor C1 - MBGCH-1, it can be replaced by three capacitors K73-17 connected in parallel with a capacity of 0,47 microfarads for a voltage of 630 V. Capacitors C2, C3 - K50-29, it is permissible to replace them with any other oxide of the appropriate capacity for a rated voltage of at least the specified on the diagram, C4 - non-polar K50-6V or imported. Resistor R6 - C5-16MV with a power of 5 W, you can use PEV, R5 - SP3-4aM, we will replace it with trimming resistors SP2, SP3, PPB.

All elements are placed on a board measuring 155x75 mm and 2 mm thick, which is placed in a plastic case measuring 165x85x40 mm.

I use this machine when working on a home computer, a computer, monitor, modem and printer are connected to it. The computer turns off automatically after clicking the "Shutdown" screen button with the mouse, and the monitor, turning off, goes into standby mode. The machine is configured for this load (monitor and modem). After 2 ... 7 s, it disconnects from the network. When you turn it on again with the SB1 button, I turn on the machine, then I turn on the computer with the "Start" button, the machine goes into operating mode.

Functionally, the device is also applicable to loads with manual control. It can be configured so that when manually turning off one load, the rest will automatically be turned off, powered through the device.

Author: A. Kuzema

See other articles Section Clocks, timers, relays, load switches.

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Comments on the article:

Andrei
I used KU202 in this circuit. At the control electrode VS1 0 volts. There is about 5 volts on the right terminal of R8. There is 25 volts from the power supply. Thyristor checked and changed to others. What could be the problem?

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