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Looking for special SPDT switches

Discussion in 'Beginner Electronics' started by Mike Kaplan, Oct 26, 2005.

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  1. Mike Kaplan

    Mike Kaplan Guest

    I'm looking for 2 SPDT toggle or battery switches. The switches'

    85 Amp
    Break before Make

    The switches will reverse current to a motor. I don't want to use
    relays, solenoids or contactors - I want a mechanical switch.

    The two switches will provide Forward, Reverse and a third option where
    the motor leads are shorted together to provide resistance to rotation
    - braking.

    I could use a standard battery switch except the battery switch
    provides a fourth option that connects both "throws" to the single
    "pole". The intention here is to connect two batteries together, but in
    my situation this would be catastrophic.

    A household 3 way switch is basically a SPDT switch rated 15 Amp at
    120v. I don't know if this rating is sufficient for 85 Amps at 24v but
    I suspect not.

    Any ideas?


  2. Alexander

    Alexander Guest

    The household switch is not suited for 85 Amps.
    It probably won't do 15 Amps continues either.

  3. Mike Kaplan

    Mike Kaplan Guest

    I should add that the switches will not be opened or closed while
    current is flowing. After the switch is in position, then current will
    flow, so my main issue is whether the switch can handle 85 Amps at 24v
    without arcing or heating.
  4. Mike Kaplan

    Mike Kaplan Guest


    Thanks for your response.

    I thought about knife switches but couldn't find them on the internet.
    Can you tell me where to find SPDT knife switches that will handle 85
    Amps at 24v?

    My post refers to SPDT switches. You questioned my use of SPST
    switches, but I'm not considering SPST switches. I'm looking for SPDT

    A sketch would be the best way to describe the configuration of the
    switches, but here goes:

    Each switch has three connections, a common and two terminals labelled
    A and B. On each switch terminal A is connected to 24+ and terminal B
    is connected to ground. The common terminal on switch 1 is connected to
    one motor lead; the common terminal on switch 2 is connected to the
    other motor lead.

    There are 4 possibilities:

    Switch 1 A, Switch 2 A - Motor is shorted to 24+.

    Switch 1 A, Switch 2 B - Motor runs forward.

    Switch 1 B, Switch 2 A - Motor runs reversed.

    Switch 1 B, Switch 2 B - Motor is shorted to ground, provides braking.

  5. Jasen Betts

    Jasen Betts Guest

    +---------------------+-- +V
    | |
    +-o-_ + _-o-+
    +-o 0-+
    | fwd rev |
    +---------------------+-- 0V

    braking by shorting the motor...
  6. Guest

    Not for very long, no.

    Why not use copper washers? Theyre an easy way to make 1 pole 2 way
    switches, and are press button as well.

    You have 3 copper washers, all threaded loosely over a thin bolt. The
    middle one is bolted to the bolt, the outer 2 fixed to frame with a
    small gap between them and the middle moving one. Now, add a spring to
    push the plunger upward. To use, press bolthead down (add knob since
    you dont want to come into contact with those inductive kickbacks). The
    one I had had washers around 2cm across, handled around 200A, and
    lasted about 15 years.

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