Wrong. Be sure you know the correct answer before you give a "quiz".SUbuggy wrote: very nice pic, but all relays in diagram are SPST.
Both types of relays have terminals 85/86 "coil" or trigger circuit and terminal 30, the main power feed.
the key difference is a SPST has termainal 87 and 87a. a SPDT has TWO 87 terminals and NO 87a terminal. So what makes them different? when you apply your main power feed (single pole circuit) to the relay at terminal at terminal 30, you have a normally closed (N.C.) connection at 87a (ie 87a will become hot). when you energize the relay via +12 and ground across 85 and 86 terminals, the normally open (N.O.)terminal 87 becomes "hot".
but a SPDT relay has TWO OUTPUT terminals (87). the idea is to terminate power for two different devices using a single feed....
SPDT has two outputs, 87 and 87a. Depending upon the state of the relay (energized or not) one or the other is "hot" (closed with 30).
SPST may have a single 87 or a dual 87, closed to 30 when the relay is energized. When the relay is not energized, there is no "hot" output. If there happens to be a second Term 87, it's there for connection convenience - electrically it's identical to the other (if it were not, it would not have the same terminal number). Another type of SPST relay has one Term 87 and one Term 87b - each having its own set of contacts which work in unison (close to Term 30 when energized). Externally you would not otherwise be able to tell it apart from a SPST relay with dual 87 terminals (unless one set of contacts were to go bad) but each set of contacts can handle the rated load rather than having just one set for both outputs.
DesertGuy has it right. "DT" means double throw, not dual terminal.
