Remote switch for electrical appliances. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Clocks, timers, relays, load switches
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Sometimes there is a need for a remote switch for various electrical appliances, and the switch itself must have reliable galvanic isolation from the network, i.e. be safe. Similar requirements are valid, say, when controlling lighting lamps in rooms with high humidity. The use of electromagnetic relays in these conditions is undesirable due to their low reliability.
The way out is the proposed electronic switch. It uses a symmetrical thyristor - triac, which made it possible to switch alternating voltage without using rectifier diodes necessary for thyristor devices. The operation of the triac is controlled by the switch SA1 through a low-power transformer T1. This uses the property of the triac to open when a voltage of the same polarity is applied to its control electrode as on the anode. That is why the winding I of the transformer is connected between the anode and the control electrode.
If the switch contacts are open, a small no-load current flows through the I winding. The voltage across the resistor R1 is not enough to open the triac, the load - the lighting lamp EL1 - is de-energized.
When the switch contacts close, winding II is in short circuit mode. In this case, the current in the winding I increases sharply, which means that the voltage drop across the resistor increases. The triac will open at the beginning of each half-cycle of the mains voltage, the load will be energized.
The transformer in this mode will not be overloaded, since after the triac opens, the voltage across it decreases to 2 V and a small current flows in winding II.
In the device, it is desirable to use a transformer with a voltage on the secondary winding of 8 ... 10 V with a current flowing through the primary of 0,2 ... 0,4 A. The no-load current should not exceed 10 mA. A similar transformer can be borrowed from a network adapter used to power players, calculators, and transistor radios. It is preferable to use a transformer with windings wound in different sections of the frame - this will increase the electrical safety of the device.
The triac can be, in addition to that indicated on the diagram, KU208V. If the load power does not exceed 250 W, the heat sink is not needed, with a higher power, the triac will have to be installed on the heat sink. The maximum load power should not exceed 1,1 kW. Trimmer resistor - SPO, SP4 or other. They set the mode of reliable opening of the triac.
The triac, transformer and resistor can be located at a distance of several meters from the room in which the switch and the load (for example, lighting lamps) are installed. For the convenience of load control, several switches can be connected in parallel with the secondary winding of the transformer.
Author: I. Nechaev
See other articles Section Clocks, timers, relays, load switches.
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