ABSTRACT
Let us remember that relays are usually defined as devices that can only be in extreme stable states and can switch from one state to another, stepwise, even when the input actuating quantity varies smoothly. In that definition, there is not a single word about the character of the actuating action. Most often, such actuating action is current, which is why relays that are energized by electrical current (voltage) are the most widespread. The bulk of this book is devoted to this type of relays, but this is not the only type of relay that exists. There are relays responsive to light, temperature, location in space, air or liquid pressure, air or liquid speed, etc. Obviously, it is impossible to consider all known types of relays in detail within just one book, but in order to get a complete picture one should be familiar with at least some of them. One of the most widespread types of relays (after electrical relays) is ‘‘time relays.’’
Usually these are relays operating with a certain delay with regard to the signal applied to the relay input, which is why frequently the term ‘‘time-delay relay’’ is used. As the change of state of a relay is accompanied by a certain delay with regard to the signal applied to its input, one can say for sure that, apart from its other functions, every relay also functions as a time relay. Sometimes standard electromechanical relays are used to enhance stability of complex automatic control systems. Their only function is to provide a certain signal delay, the value of which equals its own make delay. In terms of engineering, ‘‘time relays’’ or ‘‘time-delay relays’’ are usually defined as relays in which the time-delay function dominates, and in which the characteristics of that function are enhanced, by one means or another.
