Following is a diagram showing you different relay types.

 

electronic relay types
electronic relay types

 

 

The following animation shows a normally (NO) open relay in action

 

relay normally open animation
relay normally open animation

 

KEY

SPST (NO)- Single Pole Single Throw (Normally Open)

SPST (NO)- Single Pole Single Throw (Normally Closed)

DPST (NO) – Double Pole Single Throw (Normally Closed)

SPDT – Single Pole Double Throw (Normally Closed)

DPDT (NO) – Double Pole Double Throw (Normally Closed)

 

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