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Reading Relay Schematics

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One relay may appear in several places on the page

This is one of the first schematic conventions that can make a relay circuit look stranger than it really is.

SAME PHYSICAL RELAY — TWO DRAWING STYLES Grouped together K1 coil K1 NO contact K1 NC contact Dashed line = mechanical association Separated on schematic K1 K1 K1 The matching reference tells us these belong together.

The drawing does not have to resemble the physical package

A physical relay contains a coil and one or more contacts in the same device. A schematic may draw them together — or place the coil near one part of the circuit and the contacts wherever they are electrically useful.

The reference designator ties them back together. A coil marked K1 and contacts marked K1, K1-1, K1A, or similar notation may all belong to the same relay, depending on the drafting convention being used.

Names vary

Relay designators are not perfectly universal. K is common, while CR may mean control relay and other drawings may use R or manufacturer-specific conventions.

The important habit is: read the drawing's legend and follow the repeated reference marks.

What about a solenoid?

A relay coil is an electromagnetic coil that produces mechanical motion, so its operating principle is closely related to a solenoid. But in ordinary schematics the word solenoid often means an actuator whose motion does some mechanical work, while a relay specifically operates electrical contacts.

Because coil symbols can look similar, the device label and the contacts associated with it matter.

Very important: relay contacts are normally drawn in their normal, de-energized condition unless the drawing explicitly says otherwise.

Try these yourself

A relay coil marked K1 is drawn on one part of a schematic and a contact marked K1 appears elsewhere. What does the repeated reference usually tell us? SAME PHYSICAL RELAY — TWO DRAWING STYLES Grouped together K1 coil K1 NO contact K1 NC contact Dashed line = mechanical association Separated on schematic K1 K1 K1 The matching reference tells us these belong together.
Matching reference designators are used to associate separated relay coils and contacts.
Relay contacts on ordinary schematics are normally shown in what condition unless noted otherwise?
NO and NC are referenced to the device's normal condition, typically with the coil de-energized.
Do all schematics use exactly the same relay reference letters?
K, CR, R and other conventions are encountered. The drawing's own legend and consistent reference marks are the authority for that drawing.
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