Executive nodes of the schemes. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Executive nodes of the schemes. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Miscellaneous electrical devices

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At home, various energy consumers are actuating devices: lighting lamps, heaters, electric motors, electromagnets, sound signaling devices, and many others. Most often they are powered by a 220 V (50 Hz) network and well-known circuitry options can be used to turn them on.

Consider those of them that provide galvanic isolation from control circuits. Such decoupling is necessary not only for the purpose of electrical safety during the operation of the device, but often also provides the convenience of matching individual circuit nodes. To control any device that is powered directly from the network, you can use the diagrams on the following elements:

1) electromagnetic relays - this is the most ancient, simple and universal way, and also quite cheap. The latter circumstance contributes to the widespread use of relays not only in household, but also in industrial equipment. To control the relay, a direct current from 10 mA (for reed switches) to 300 mA (for power electromagnetic ones) is required from the source.

The relay has many disadvantages: relatively long response time (5...100 ms); bounce of contacts when switching; sparking of contacts at high switched currents, because of which they burn out and fail (small resource); create noise when switching and a number of other less important ones. In modern radio equipment, if possible, they try to do without them and use only electronic switches;

2) pulse transformers - for transmission of control signals to power switches. The simplest of these schemes are shown in Fig. 1.2 (elements marked with an asterisk are sometimes not used). Unfortunately, transformers are laborious to manufacture and have large dimensions;

Scheme executive nodes
Rice. 1.2. Schemes with transformer isolation of control signals from the power circuit: a - for thyristors, b - for triacs

3) elementary optocouplers (transistor, thyristor and triac) for controlling power thyristors or triacs, as shown in fig. 1.3, a-e. The EL1 lamp is shown everywhere as a load, although the winding of a heater, electric motor or transformer may be included instead. Due to the low control current (5 ... 15 mA - the current can be pulsed), high reliability and low price, these circuits are very widely used in radio equipment;

(click to enlarge)

Scheme executive nodes
Rice. 1.3. Optocoupler-Based Circuits for Network Load Management

To speed up the switching of the power switch, an RC circuit is sometimes used, shown in the diagrams by a dotted line, but even without it, the speed of such electronic switches is much higher than that of a relay (about 0,5 ms). When triggered, they do not spark, do not create acoustic noise and withstand short-term hundredfold current overloads, which often occur when switching reactive loads (electric motors, transformers).

Some optocouplers have a "null organ" inside - a special circuit that ensures the switch is activated only immediately after the mains voltage passes through zero (see the reference section), which reduces switching noise.

4) power optothyristors and symmetrical optotriacs (also called optotriacs). In them, the optocoupler is installed inside the body of the power element. These components have the same advantages as conventional thyristors and triacs - they can be used to switch any load, on alternating current, fig. 1.4, but such nodes require more current from the control circuit (up to 100 ... 150 mA - the current can be pulsed);

Rice. 1.4. Load switch circuits based on optothyristors (a) and optotriacs (b) (click to enlarge)

5) solid state relays - they are made on the basis of powerful field effect transistors or triacs. In terms of their capabilities, they are close to conventional relays (specially designed to replace them, which is the reason for the spread of such a name), only they are much more reliable and smaller in size, have greater speed than all types of other electronic switches, and require a small control current (1,25 .XNUMX mA). Most of these components can operate not only on alternating current, but also on direct current.

There is a wide range of solid-state relays of domestic and foreign production with different internal structure, designed for different currents, on the market. On fig. 1.5 shows a simplified internal structure and an example of an AC and DC load switching circuit for only some of them.

Scheme executive nodes
Rice. 1.5. Schemes of switches based on a solid-state optoelectronic relay: a - for alternating current; b - for direct current

What is better to use in the circuit as a switch depends on the required system speed. For slow processes, electromechanical relays are suitable in many cases. If high speed is needed, and dimensions do not play a big role, transformers are used. Where not only speed is required, but also increased reliability, optoelectronic switches are used for the corresponding rated current. The components of optoelectronics are described in sufficient detail in the reference section of the book, and, using this information, you can easily find the right one.

Author: Shelestov I.P.

See other articles Section Miscellaneous electrical devices.

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