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Switch terminology?

Discussion in 'Electronic Basics' started by Esther & Fester Bestertester, Mar 15, 2007.

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  1. Maybe I don't understand switch terminology, but I thought SPST means single
    throw (2 positions, ie. on-off) and SPDT means double-throw (3 positions, ie.
    on-off-on).

    This catalog page show many switches with both terms, but mixing xST with 2-
    and 3-positions. Same with xDT term.

    <http://tinyurl.com/36petb>

    Is my understanding incorrect? Or is this page all screwed up?

    FBt
     
  2. BTW, I'm looking for a switch that has 3 pairs of terminals and 3 positions.
    Each position connects a pair of terminals together. Rocker preferred.

    FBt
     
  3. No momentary contacts.

    FBt
     
  4. Lord Garth

    Lord Garth Guest

    You're partially correct re SPDT excepting that if it has a center off
    position,
    the switch would be labeled as such. Example: Single Pole Double Throw
    center off.

    The poles are the armature or internal moving portion and a throw is the
    non-moving contactor that receive a pole.
     
  5. Lord Garth

    Lord Garth Guest

    You're going to have to be clearer here, 3 pairs of terminals means 6
    terminals.
    The assumption being that your signal goes into one terminal and out the
    other.

    You say you wish to connect pairs of terminals together so your 3 throws
    would
    connect terminal 1&2 then 3&4 and finally 5&6 for example.

    If that is what you desire, a double pole triple throw rotary switch would
    work if
    you short the two poles together. Rocker switches have only two positions
    plus maybe a center off so that would not be a reasonable choice.

    Another possibility is that you want two signals (stereo audio perhaps) to
    go to
    one of 3 destinations. A rotary switch would connect the poles to throw 1
    or 2
    or 3 for both the left and right signals. This would still be a double pole
    triple
    throw rotary switch but the poles would not be shorted together.
     
  6. The poles are the armature or internal moving portion and a throw is the
    So the term "throw" has nothing to do with the positions, or "stops" that you
    go through when moving the actuator (handle)?

    FBt
     
  7. Lord Garth

    Lord Garth Guest

    The throw has only to do with the number of positions where the pole can
    complete a circuit. The off positions are not counted.
     
  8. No, ON---ON.

    It's quite possible to have a SPDT switch with no center position - some
    microswitches and other sna switches are like that.

    You are describing SPDT center off.
     
  9. If that is what you desire, a double pole triple throw rotary switch would
    If rockers can have 3 positions:
    on off on

    why not:
    on on on

    each "on" shorting one pair of contacts.

    I'm sure I've seen these before... just can't find one now.
     
  10. Lord Garth

    Lord Garth Guest

    That would be a rotary switch turned on its side!
     
  11. You are describing SPDT center off.

    If I want:
    on - on - on

    double-pole with 3 stops, what switch am I describing now?

    2P3T?
     
  12. Lord Garth

    Lord Garth Guest

    Correct...this is most easily done with a rotary switch. Each pole
    is a separate wafer attached axially to the shaft.

    What is it you are trying to design? Maybe we can suggest some
    all electronic ways to switch the signals.
     
  13. Eeyore

    Eeyore Guest

    They're fabulously rare. Old style telephone swiches used to be able to do that.
    Check out http://www.switchcraft.com/

    Graham
     
  14. What is it you are trying to design? Maybe we can suggest some
    Small DC motor speed control. Switching resistors or diodes or...? to provide
    several speed choices.

    12v motor pulls 2-4 amps.

    Suggestions?

    Thanks,
    FBt
     
  15. Lord Garth

    Lord Garth Guest

    You'll need a pulse width modulator to control the motor speed yet maintain
    good
    torque. There are ways to create such a modulator with an LM555 timer IC.
    You
    use it output to control a power FET.

    FDB603AL is such a FET and I found this one on an otherwise dead
    motherboard.
    It is rated to handle 33A at 30V with a Vgs of 4.5 volts and having a
    Rds(on) of .036 ohms
    http://www.ortodoxism.ro/datasheets/fairchild/FDB603AL.pdf

    I'll drop copy of a modulator that ran in Electronics Design onto
    alt.binaries.schematics.electronic

    it's a Word doc...
     
  16. Yes.
     
  17. What sort of load? If your budget permits, a KB controller is a good choice.

    http://www.kbelectronics.com/

    Battery DC to DC: KBBC-24M 12/24 VDC, thru 40 Amps Continuous
     
  18. Phil Allison

    Phil Allison Guest

    "Esther & Fester Bestertester"
    ** Nope.

    SPDT = single pole, change over

    3 pos switches are specified as such.




    ......... Phil
     
  19. ehsjr

    ehsjr Guest

    One SPDT center off switch. Conceptually:

    V+ ---+---[R1]---+---[R2]---+----+
    | | | |
    | o | |
    +--------o-> | [Motor]
    o----------+ |
    |
    Gnd -----------------------------+

    Full speed with switch down, slow speed with switch
    off, medium speed with switch up.

    Realisticly, using resistors has problems. Reduces
    torque, and they need to dissipate a *lot* of heat
    at 4 amps (I^2R). Instead, use PWM - and then you
    can use a pot and get the widest possible range of
    speed available, avoid the heat problem, and have
    better torque.

    If you absolutely cannot use a pot, then you can
    still get 3 speeds using the scheme above with
    PWM where the resistors select the pulse width.
    They won't get hot that way and can be much smaller
    wattage.

    Ed
     
  20. You'll need a pulse width modulator to control the motor speed yet maintain
    Pretty nice specs for a $1 part, especially the Rds(on). What did it power on
    the moboard?
    Thanks. Without doing all the math, what R & C values would you recommend?

    I found a near-identical circuit here:

    http://www.discovercircuits.com/PDF-FILES/SIMPLEPWM2.pdf

    Any idea what function the diodes connecting to the pwm adjustment pot
    perform?

    Thanks again.

    FBt
     
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