Clearing up switch terminology

Discussion in 'General Electronics Chat' started by JingleJoe, Jul 29, 2011.

  1. JingleJoe

    Thread Starter Member

    Jul 23, 2011
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    The terminology in question is SPST and DPDT (I know what they stand for, single pole single throw and the same but with "double")
    I was allways under the impression that the number of poles was the number of common connections and that the throw refered to the movement of the switch, single being one "throw" of the switch, into either an on or off position and double being "center off".
    Is this correct? I think I'm missing something as the double throw switches i just bought in lieu of single throw, are acctually on-off in switching motion, which is what i wanted. All this calls into question my knowledge of terminology, so I come unto you for clarification.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The number of "poles" does refer to the number of "common" terminals.

    In a "single throw", the switch will have two positions, where the common(s) will be connected to their respective contact(s), or open connection (on/off).

    In a "double throw", the switch will have at least two positions where a NO and a NC contact will connect with a common. There may or may not be a center off (on-on), (on-off-on).

    There are also specialty switches that have spring-loaded momentary contacts to one or both sides, there is an (on)-on-off configuration that might be used as an ignition off/run/start switch, and a number of other configurations.

    There are also multi-throw switches, but those are usually slide or rotary types.
     
  3. JingleJoe

    Thread Starter Member

    Jul 23, 2011
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    So a single throw would have just one connection besides the "common" and an off position?
     
  4. someonesdad

    Senior Member

    Jul 7, 2009
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    Here's my understanding; Wookie will correct me if I get something wrong... :p

    The picture shows a single pole switch on the top. Note the little circles -- they indicate where you can make an external connection to the switch (e.g., screw terminal, spade terminal, or solder connection).

    The double throw switch has the extra terminal for a total of three; they're usually called common, NO (normally open) and NC (normally closed). You'll also see this configuration called Form C. Wookie made a post somewhere else that explained these.

    The number of poles designates the number of "common" terminals. Thus, in the DPDT (double pole double throw), you see two commons besides the two NC and NO terminals. This extends to 3PDT, 4PDT, etc. switches.
     
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Correct.

    "Poles" count how many units of individual switches are in the assembly.

    "Throws" count how many connections each pole can make.

    I have seen this notation expanded for example: 4PDT for a switch with 4 units of a two position switch, 1P10T for a single deck 10 position rotary switch.

    Panel toggle switches have 2 or 3 positions AFAIK, as the Sarge mentions. A SPST switch could only be an ON-OFF type, while a SPDT can be ON-ON or ON-OFF-ON, indicating a third middle position where no contact is made.

    One of my favorite switches is the DPDT ON-ON-ON type; with this one then two end positions work as expected, but the center ON position switched one switch only from one connection to the other. It is quite useful when you really wanted a SP3T switch, or a switch that can connect 1 point to 3 different places.

    This also comes in a 4PDT ON-ON-ON configuration.
     
  6. bertus

    Administrator

    Apr 5, 2008
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  7. JingleJoe

    Thread Starter Member

    Jul 23, 2011
    185
    10
    Thankyou everyone, it is clear in my mind now like a pane of really really clean glass.
    Could you explain that again? it sounds like it would be incredibly useful if it does what i think you mean it does.
     
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