Relay Element

a minimal assembly of parts and interconnections that has a relay characteristic—that is, the action the assembly produces at its output (or outputs) changes abruptly when a certain action is applied to its input (or inputs). Often called simply a relay, a relay element is treated as the simplest component part in the design of numerical control devices.

A relay element can be characterized by its operating and release thresholds. The operating threshold is the minimum absolute value of an increasing input action at which the relay element changes its state and, at the same time, changes its output action in accordance with the relay characteristic. The release threshold is the minimum absolute value of a decreasing input action at which the relay element returns to its initial state. Some relay elements, however, have a holding property: after the input action is removed, they remain in the state they were occupying. In this case, the relay element returns to its initial state usually after an action is applied to another of its inputs (or an action of opposite sign is applied to the same input). Hold relay elements are used in, for example, the construction of memory units for computing and control devices. A relay element is also characterized by its speed, defined as its operate time or release time. In present-day contactless relay elements, the operate and release times are as low as several nanoseconds. Other important characteristics of a relay element are its energy consumption, mass, and size.

There are many different types of relay elements. On the one hand, for example, power relays switch currents of —10—100 amperes (A) at voltages of ~10⁴-10³ volts (V) in ~0.1 sec. On the other hand, contact and contactless relays for automatic control and monitoring devices react to currents of ~ 10^-4. A at voltages of ~0.1 V in ~ 10^-4 sec.

From the design point of view, a relay element consists of three parts: a sensing part, which reacts to the external action; an actuating part, which transmits an action to the outside from the relay element; and an intermediate part, which processes and transmits actions from the sensing part to the actuating part. These parts may be distinct units or may be combined. Relay elements can be classed as either contact or contactless, depending on the form of the actuating part. In the contact types, the actuating part consists of electric contacts that switch electric circuits. In the contactless types, such as electrical and pneumatic contactless relays, the output action is in the form of a change in various parameters of the output circuits—for example, resistance, capacitance, or inductance—or a change in, for example, the voltage or pressure in these circuits. The relay characteristic in contactless relay elements either is inherent in the element or is obtained by an appropriate combination of electrical elements that do not themselves have relay characteristics. Examples of the former case are a relay element with a rectangular hysteresis loop, a glow-discharge tube, a thyratron, and a cryotron; flip-flop relay elements are examples of the latter case. Contactless relay elements are generally much smaller than contact relay elements. Modern technology permits the fabrication of up to 10⁴ semiconductor relay elements on a thin silicon wafer 4 mm X 4.5 mm in size. In addition, contactless relays are more reliable, consume less power, and have a higher speed of operation.

Relay elements can also be classified according to many other criteria. The classifications encountered most often depend on the types of physical phenomena the relays make use of, the nature of the physical quantities to which the relays react, the functions the relays perform in a relay system, and the purpose for which the relays are constructed.

Figure 1. Classification of relay elements according to the type of physical phenomena on which the operation of the relay elements is based

The physical phenomenon that is made use of in a relay element determines the element’s principle of operation, construetion, and chief characteristics. From this point of view, relay elements can be divided into such classes as the following: (1) electrical relays, whose operation is based on phenomena that result from the flow of an electric current or the presence of an electric field or that are associated with the electrical conductivity of a solid; (2) mechanical relays, in which, for the most part, use is made of a change in the dimensions of a solid body under the action of certain factors (hydraulic and pneumatic relay elements are also usually included here); (3) chemical relays, which primarily make use of chemical transformations that occur under the action of an electric current; and (4) optical relays, which make use of processes that occur under the action of light. This classification is presented in greater detail in Figure 1.

Relay elements are classified according to the type of physical quantities to which they react as electrical, mechanical, thermal, optical, magnetic, and acoustical relays (Figure 2). Relay elements that must react to nonelectrical quantities are often provided with measuring transducers for the quantities in

Figure 2. Classification of relay elements according to the type of physical quantities to which the relay elements react

question. Depending on the nature of the change in the physical quantity, the following classes of relay elements are distinguished: directional relays, which react to a certain value and sign of the quantity; over-and-under relays, which react to an increase or a decrease in the quantity; limiting relays, which react to a change in the given quantity when its value goes beyond specified limits; and relationship relays, which react to, for example, the sum, difference, ratio, product, or integral of two or more quantities acting on the inputs of the relay elements. A special place is occupied by pulse relay elements, which have come into widespread use with the development of pulse engineering; they react to various pulse parameters, such as duration, steepness of leading or trailing edge, shape, or off-duty factor.

Depending on their position in the relay equipment and the functions they perform, relay elements are classified as sensing, actuating, or intermediate. Sensing relay elements that receive input actions from communication lines or channels are often called line relays.

Relay elements perform extremely diverse functions, and the purposes for which they are constructed in different fields are also quite varied. Consequently, they are often classified differently in each field. Several general groups, however, can be identified: protective, control, monitoring, and logic relays. Protective relays form a large group; they are designed to disconnect industrial or other equipment or to change the operating conditions of the equipment when the conditions become dangerous. The groups of control and monitoring relays are used in automatic control systems. Logic relays perform the functions of logic transducers in, for example, computing devices, control devices, and numerical control systems.

Relay elements are used most widely in automatic-control and communications engineering. They permit the control of high-power outputs of devices or systems by small-magnitude input actions and make possible the performance of logical operations. By combining different relay elements, complicated multifunctional relay devices containing tens or hundreds of thousands of relay elements can be easily constructed. Devices and systems of many kinds are wholly or partly based on the use of relay elements. Examples are digital computers and controllers, digital remote-control equipment, automatic telephone switching systems, transmission systems for digital information, and protective-relay devices.