Indicator for connecting electrical appliances to a 220 V network. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Indicators, sensors, detectors

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I write in the footsteps ARTICLES in No. 12 "Electrics" [1]. I realized the importance of this topic recently, when my family forgot to turn off the electric stove in the morning, and by the evening, the electricity meter "twisted" energy for an extra 3 UAH. The scheme in [1] is very simple, but has raised such objections.

1. In modern homes, electrical wiring is hidden in the wall. Where is this entrance to the apartment located? Most likely in the most inconvenient place.

2. Well, if there is a transformer in the house. If not, you need to buy it, and this part is not cheap (and not every transformer will do).

3. The apartment has appliances that are constantly on. Some of them turn on from time to time (refrigerator), others work constantly (electronic clock, electronic thermometer). What to do with them?

4. If you forgot to turn off the light bulb in the pantry with a power of 25 W, then by the evening the additional expense will be a few cents. Do I need to set an indicator for this?

5. Radio equipment that is not turned off reminds of itself with a sound, so it's hard not to notice it.

6. The only electrical installation that needs to be equipped with a connection indicator is an electric stove. This is where you need to put the connection indicator.

The simplest connection indicator is a neon light bulb or LED indicator connected to the network wires after the switch. If the switch is off, the indicated devices do not light up. But there are many such switches in the electric stove and they are installed in hard-to-reach (from the inside) places. Therefore, it is necessary to install a current consumption sensor. Usually this is a low resistance resistor (so as not to take a lot of power from the network), which is included in the break of one of the network wires.

Now let's do some calculations. At a minimum power (about 100 W), the electric stove consumes a current of 0,5 A from the network. When using a 1 Ohm resistor, 0,25 W of power is released in it. But with a maximum electric stove current of 30 A (all burners are on), 900 W of power will be released on this resistor at a voltage of 30 V across the resistor! And this is a solid part of the consumption of the stove, wasted. Thus, you need to somehow limit the voltage across the resistor.

Powerful diodes VD1, VD2 are perfect for this purpose, shunting the resistor R1 in the forward and reverse directions (Fig. 1).

With a current through the resistor of 0,5 A, the voltage drop across it is 0,5 V, and at this voltage, the silicon diodes VD1 and VD2 are locked. As the voltage across the resistor increases, the diodes gradually open and enter saturation at a forward voltage of about 0,8 ... 1 V (Fig. 2).

Power begins to be released on the diodes, they heat up and, as can be seen from the characteristics of Fig. 2, the voltage across them decreases. Thus, diodes become ideal voltage limiters. Together with the diodes, the resistor R1 also heats up. The thermistor R2 is electrically isolated from R1, but mechanically connected to it, and therefore also heats up. A communication line (telephone wire) is stretched from R2 to the actual indicator (highlighted by a dash-dotted line in Fig. 1).

The divider R4, R2, R3 in the base circuit of the transistor VT1 is designed so that at a normal temperature of the thermistor R2, the transistor VT1 is locked and the HL1 LED is off. When the thermistor R2 is heated, the transistor opens and the LED lights up, indicating that the load is on. A galvanic cell is used as a power source.

If the LED just glows, then this may not attract the attention of a person leaving the apartment. In the circuit of Fig. 3 (only the indicator itself is shown), a low-frequency generator is installed on CMOS digital elements AND-NOT DD1.

At a normal temperature of the thermistor R2, the divider R2R3 provides the voltage at input 1 of the DD1.1 element below half the supply voltage, therefore this element is closed, at its output 3 there is a log. "1", respectively, at the output 4 of the element DD1.2 - log. "0 ". Transistor VT1 is closed, and LED HL1 is off. When the thermistor R2 is heated, the voltage at the divider R2 / R3 exceeds half the supply voltage, the generator starts at a frequency of approximately 1 Hz. At this frequency, the LED starts flashing.

With a heavy load (load current up to 15-20 A), about 1 watts of power begins to be released on the diodes VD2, VD10. Therefore, diodes must be placed on radiators, unfortunately, each on its own radiator. Each transistor can be turned into a diode by shorting the collector and base. Using transistors of different types of conductivity (as shown in Fig. 4), the same pair of diodes can be realized, but since the collectors of the transistors are connected together, one heat sink can be dispensed with.

The simplest calculation of a radiator for a power of 20 W can be made according to the method [2]. In addition to thermal coupling between the measuring element R1 and the indicator, optical coupling can also be used. But for the operation of the light-emitting element, a voltage of the order of 1 V, which is released on the measuring element, is not enough.

It is necessary to increase the resistance of the resistor R1 to at least 5-6 ohms, so that at a current of 0,5 A the voltage drop is 2,5-3 V. But then, to limit the voltage on R1, it is necessary to install two branches of three diodes. Instead of diodes, thyristors can be used (Fig. 5).

The thyristors VS5, VS1 of the KU2 type indicated in Fig. 202 are triggered at a voltage on the control electrodes of the order of 4 ... half-cycle of the mains voltage, "flashes" of 8 ... 2 V are formed. These "flashes" start the transmitting diode of the transistor optocoupler UB1. The receiving transistor of the optocoupler opens and the HL2 LED lights up (in dynamic mode).

In all the schemes described above, the indicator was powered by a galvanic cell. If the element is "hungry", then the indicator may not work. Fig. 6 shows the direct connection of the indicator to the measuring element R1 (for the circuit of Fig. 5, for other circuits this connection does not work). In this case, the indicator is under mains voltage. To reduce the danger, the measuring element must be included in the break of the neutral wire of the network.

References:

1. Rybak P.D. Indication of the connection of electrical appliances to the 220 V network //Radioamator-Electric. -2000. -#12.
2. A simple calculation of the heat sink area for high-power transistors and thyristors//Radioamator-Konstruktor. -2000. -#4.

Author: I.N.Proksin

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