Watchdog time relay. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Watchdog timer. 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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In order not to forget to turn off the TV, tape recorder, radio in time, and when leaving the house to de-energize the electric heaters, assemble the proposed watchdog time relay. Its peculiarity is that the time delay is constant and equal to one hour. If during this time you do not switch the toggle switch on its body for a moment, then the relay will de-energize electrical or radio devices. If you manage to do this, a new countdown will begin.

The watchdog timer (see figure) consists of two blocks. One of them includes a mains plug X1, socket X2 "Load" for connecting the load and fuse F1. Another node is the time relay itself, assembled on the thyratron MTX-90 (V2). The load is connected to the network through the switching contacts K.1.1 of the relay K1. The units are connected by a long three-wire cord, resulting in remote control of the load.

Fig.1. Watchdog Timer

When the watchdog timer is connected to the network for the first time, switch S1 must be in the position indicated in the diagram. The mains voltage will be applied to the load, the capacitor C1 will quickly charge through the diode V1 and the voltage divider R1R2 to a voltage of 160 ... 170 V, and the capacitor C2 will start charging through the high-resistance resistor R3. After about an hour, the voltage on the capacitor C2 will increase so much that the thyratron will light up. Capacitor C1 will discharge through it and the winding of relay K1. The relay will work, disconnect the load from the mains with contacts K1.1 and get on self-locking, closing the outputs of the anode and cathode of the thyratron with the same contacts. A holding current will flow through the relay winding, determined by the resistance of the resistor R1. In this state, the watchdog relay can be indefinitely long.

To prevent disconnection of the load from the network, it is necessary, without waiting for the relay to operate. move the switch knob S1 to another position, and then return it back. Capacitor C2 will be discharged through resistor R5 and after the switch is returned to its original position, it will start charging again. Periodically rearranging the switch handle informs the watchdog relay to keep the load on.

If you need to turn off the watchdog relay, just set the switch knob to the right position according to the diagram. Then the power will not be supplied to the relay, and the load will remain connected to the network through the relay contacts.

It is convenient to make the first block from a "tee" plug, which you often use to turn on several consumers in one outlet. Two sockets are removed from her, and a fuse is installed in their place.

The second block can be mounted in any case of suitable dimensions.

Capacitor C1 - electrolytic of any type. Capacitor C2 must be made of paper. If its capacitance is less than that indicated in the diagram, the resistance of the resistor R3 should be proportionally increased (it can be made up of series-connected resistors of the highest possible resistance).

Relay K1 - RKM-1, passport RS4.500.892. The resistance of its winding is 3300 ohms, the operation current does not exceed 7,2 mA. Two switching groups of relays are connected in parallel for increased reliability.

If, during the operation of the device, at the moment of ignition of the thyratron, the relay is activated and immediately releases, the capacitance of the capacitor C1 is increased. You may then have to pick up the resistors R1 and R2.

When setting up the device, care should be taken (its elements are under mains voltage!), All soldering should be carried out when the "tee" is turned off from the mains socket.

Author: A.Aristov, Pervouralsk, Sverdlovsk Region; Publication: N. Bolshakov, rf.atnn.ru

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