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Relays

Objectives

  • Explain the difference between a relay and a switch.
  • Demonstrate the mechanics of a relay.
  • Analyze a relay schematic symbol.
  • Predict the behavior of a circuit that utilizes a relay.
Relay
Switch

Relay Vs. Switch

A relay is a specialized version of a switch that is electromechanically triggered, instead of manually operated. The ability to operate a switch electrically instead of manually offers all sorts of automation opportunities.

In addition, a very small voltage can be used to control a large voltage, or a small current can be used to control a large current. The coil that triggers or controls the relay contacts is electrically separate from the switch contacts themselves.

Schematic Symbol

Relays are always shown in a de-energized condition on a schematic. Touch the glowing dots on the symbol shown here to learn more about what it represents.

Throw 1a (N.C.)
Throw 1b (N.C.)
Throw 2a (N.O.)
Throw 2b (N.O.)
Pole a (COM)
Pole b (COM)
+
-
Spring Bias

The spring always tries to pull the poles to Throws 1a & 1b.

Coil

This symbol represents the coil. Expect poles to snap in its direction when the relay is energized.

Mechanical Linkage

This dashed line indicates that the two poles are mechanically linked - one can't move without the other. Many schematic standards do not include this line.

Pin

The lines exiting the relay symbol represent the pins on the relay. Each pole and each throw will have a pin.

This "block" contains one complete set of contacts. You can stack as many blocks as needed on top of the coil to accurately represent the relay. There is no connection from this set of contacts to the one above - you could run completely different currents through each set of contacts in the relay.

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