What does a relay do?
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.