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What does a relay do?

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2007-06-03 23:37:40

A relay is an electrically actuated switch. There are a number

of contacts actuated by a soleniod. When the solenoid is energized

the position of the contacts changes, "throwing" the switch. The

terminology of switches and relay contacts is as such:

(number)p-(st(no/nc)/dt) The number followed by p is the number of

poles. This is the number of independent "switches" in the unit.

The (number)p may be replaced with sp (single pole) meaning 1p or

dp (double pole) meaning 2p. ST stands for single throw, there is

only two contacts for each pole. ST is usually suffixed with NO or

NC in relays standing for normally open and normally closed.

Normally open contacts conduct when the solenoid is energized.

Normally closed contacts conduct when the soleniod if off. DT

stands for double throw, meaning that there are 3 contacts per

pole: a common contact, a NO contact, and a NC contact. So: a 3p-st

NO relay has 3 switches that close when energized. a sp-dt relay

has one 3 contact switch. An electrically actuated 3-way switch, if

you will. a dp-st NC relay has one switch that opens when the

solenid is energized.

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