Relays & Switches


 
  • Pole = A movable arm in the switch.
 

 
  • Throw = The number of contacts that are affected by a single movement of a pole.



  • SPST (Single-pole, single-throw): Turns a single circuit on or off.



  • SPDT (Single-pole, double-throw): Controls the flow of current to two different circuits. When one contact is open the other is closed and vise versa.

  • DPST (Double-pole, single-throw): Allows simultaneous opening or closing of two sets of contacts. Both contact arms are mechanically linked so that they move with a single switch action.



  • DPDT (Double-pole, double-throw): Connects one set of contacts to either of two other sets of contacts.




        Relays


  
  • NO = Normally Open, COM is connected to this when the relay coil is on.
  • COM = Common, always connect to this, it is the moving part of the switch.
  • NC = Normally Closed, COM is connected to this when the relay coil is off
            if you want the switched circuit to be on when the relay coil is on, connect NO & COM
            if you want the switched circuit to be on when the relay coil is off, connect NC & COM
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