Network disconnection signaling device. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network

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Unexpected blackouts of mains voltage have become another (in addition to the two well-known) Russian disaster. The main thing in such a situation is to find out about the shutdown in time and take action - switch to backup power (if possible) or simply remove perishable food from the non-working refrigerator. This is easy to do in the evening, when the absence of voltage is signaled by the extinguished lamps. During the day or late at night, there are no obvious signs of a shutdown and it can be overlooked in a timely manner. The device described in the article beeps for more than a minute a few seconds after a power failure in the network.

The proposed signaling device (its diagram is shown in Fig. 1) is built on the same principle as that described in the article by A. Dolgoy "Watchdog for a computer" ("Radio", 2000, No. 2, p. 27).

Rice. 1. Schematic diagram of the signaling device

Mains voltage is supplied to two rectifiers. At the output of the first (VD1C1) - a voltage of positive polarity, the second (VD2C2) - negative. The values ​​of the resistors R2 and R3 are chosen in such a way that the field-effect transistor VT1 is closed, the ringing circuit of the electromechanical alarm clock is broken. When the mains voltage fails, capacitors C1 and C2 begin to discharge. But since their capacity is different, the discharge occurs at a different rate. The voltage of negative polarity (Fig. 2, curve 1) drops faster than positive (curve 2), so the voltage at the gate of the transistor VT1 (curve 3) is growing rapidly. As soon as it (at the moment t1) exceeds the threshold value (Up), the transistor VT1 opens and closes the bell circuit.

Rice. 2. Timing charts

Thanks to the Zener diode VD4, the voltage at the gate of the transistor is limited to 0 and Ust, which are safe for the latter (without a Zener diode, it could reach 100 V or more, as shown by the dashed line). At the moment t2, when the capacitor C1 is almost completely discharged, the transistor VT1 closes, turning off the bell. With the values ​​​​of resistors and capacitors indicated on the diagram, the duration of the sound signal is more than a minute.

The main function of the resistor R1 is to limit the current if you accidentally touch the wires from the signaling device to the alarm clock, or connect them to a grounded object. This resistor will protect against serious consequences in the event of a breakdown of one of the capacitors.

The task of the VD3 diode is to prevent the "polarity reversal" of the voltage across the capacitor C2. In the absence of a diode, this can happen as a result of the redistribution of charge between the capacitors after the mains is turned off.

The signaling device is assembled in the case of a cell phone charger. The printed circuit board present in it is replaced by the one shown in fig. 3 with signaling device parts installed on it. Capacitors - imported, resistors - MLT-0,5 or others with a maximum operating voltage of at least 350 V. Zener diode - any low-power with a stabilization voltage of 5 ... 15 V (no more than the permissible gate-source voltage of the transistor VT1). Diodes 1N4007 can be replaced by domestic KD105G or other rectifiers with a permissible reverse voltage of at least 600 V.

Rice. 3. PCB

Unfortunately, it was not possible to find an equivalent domestic replacement for the BS170 field-effect transistor. You can try to install a bipolar transistor instead of a field effect transistor, as is done in the device described in the note mentioned above. However, in this case, you will either have to use a transistor with a very large (several hundred) static current transfer coefficient h_21Э, or reduce the values ​​of the resistors R2, R3, which will lead to a proportional reduction in the signal duration. The use of a composite transistor is not recommended, since too much voltage drop across it in the open state can lead to the failure of the alarm clock.

In the alarm clock, you need to find the ones shown in Fig. 1 points A and B (S1 - bell switch, SF1 - clock mechanism contacts that close when the alarm goes off). It's a good idea to make sure that connecting them with a piece of wire results in an audible signal. It remains to determine the polarity of the voltage between these points with a voltmeter and, observing it, connect the signaling device.

The assembled device is plugged into any free outlet. The alarm clock can continue to perform its main function - to ring at the set time. An alarm will go off at an odd hour as a signal for a power outage.

Of course, not only an alarm clock can be used as a sound signal source, but also, for example, a piezoelectric emitter with a built-in generator and an autonomous power source, an electronic assembly from a sounded children's toy, etc.

Author: A. Sergeev, Moscow; Publication: cxem.net

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