Relay Characteristic

a characteristic curve having a piecewise-linear shape and corresponding to the conversion by a device (or system) of a continuous input variable x into discrete values y_n of an output variable y, where η is the number of possible values or levels of the output variable; usually n is equal to 2 or 3. Thus, the curve characterizes the operation of a relay.

Figure 1,a and b shows the characteristics for ideal two-step (n = 2) and three-step (n = 3) relay elements. Figure 1,c and d

Figure 1. Relay characteristics: (a) and (c) two-step relay elements, (b) and (d) three-step relay elements

shows the characteristics for actual relay elements. Such characteristics of actual relays have a hysteresis loop (zone of ambiguity): when x varies within the region x, ≤ x < x₂ (Figure 1,c) or the regions x₁ ≤ x ≤ x₂ and x₃ ≤ x ≤ x₄ (Figure 1,d), the behavior of the function y(x) depends not only on the value of x but on the direction of the change in x. A value of x for which y jumps from one value to another is called an operation threshold. Figure 1,c illustrates the characteristic of, for example, simple two-step electromagnetic relays. The characteristic in Figure 1,d is exhibited by three-step polarized relays. Devices that have relay characteristics are widely used in amplitude quantizers and in relay automatic control systems.