Check Wiki for "switch" for some pictures, but the basics are this:
SPST - Single Pole, Single Throw; simple on-off switch, like a household light switch, line supply is switched between a purely open condition and closed to supply power to the load. Say you want a motor to run; motor will run when relay is energized, motor will not run when relay is de-energized.
SPDT - Single Pole, Double Throw; simple load switch, like a household 3-way light switch, line supply is switched between two different loads, no open position. Say you want a light to indicate when your motor is not running; motor runs when relay is energized, light comes on when relay is de-energized.
Do you have a particular application in mind?
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What's the Difference: SPST vs. SPDT Relays
Re: what is the SPST and SPDT relay difference?
