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I bought an Old porcelain round rotary switch which I thought was single pole i.e. every time I turned the knob on top, the light would go either ON or OFF.

There are 4 terminals. When I connect 2 terminals (opposite each other) the switch behaves like single-pole as described above.

But this switch has 4 terminal, so I connected a second circuit to the other 2 terminals (double pole?). The results are when I turn the knob, one circuit goes ON and the other goes OFF. And each time I turn the knob the circuit that was ON goes OFF and the circuit that was OFF goes ON. There is no position when both circuits are ON or OFF together.

So, is this DPDT or DPST? also, does anyone know what a switch like this would be used for?

enter image description here

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  • The fact that two circuits operate independently would seem to indicate that it's double-throw. Why not connect both circuits to the same two screws? You could bring them together with crimp-on fork connectors.
    – isherwood
    Commented May 20, 2017 at 20:52
  • It seems that can be a single pole or 3-way switch. Jumper two adjacent terminals and the remaining 2 become travellers, do the same with a second switch at the other end. Neither DPDT (that would need 6 terms) or DPST really fit perfectly, more like "Douple pole, Single Opposite Throw.
    – Tyson
    Commented May 20, 2017 at 23:48
  • How did you determine which terminals to connect to what in the first place? If you haven't already, I would recommend taking the switch out of any circuit and measure the resistance between all terminals in both positions before trying to come to conclusions about its behavior and how it could be used in home wiring.
    – statueuphemism
    Commented May 21, 2017 at 11:36

3 Answers 3

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This is a DPST switch, but with one of the contacts normally open, as you'd expect, and the other contact normally closed. It's a bit of an oddity as a result -- it could be used as a 3-way switch, but that'd be silly compared to a normal (SPDT) switch.

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    It's definitely not a DPDT, but it could be a 4-way. Just saying
    – Retired Master Electrician
    Commented May 21, 2017 at 16:40
  • It wouldn't be silly in the era the switch was used. Why wouldn't one want a single device that could be used as either a single pole or 3-way, and operated identically in both situations? I tried last night to figure out when and where these may have been used, I had no luck with my normal vintage hardware resources.
    – Tyson
    Commented May 21, 2017 at 16:47
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Thanks for the responses. I tested it as a 3-way & 4-way switch and it cannot be done (I'm sure of this). So, I can say it is definitely a double-pole. The switch cover is also porcelain, so I'm wondering if was used on some Vintage appliance (washing machine,stove, etc.), instead of it being a wall mounted light switch.

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Sounds like a DPST rotary switch. Basically it's "On/Off/On/Off" but for a 240v motor that only goes one direction (think big attic fan). When used in conjunction with a capacitor (aka "converter") you can use single phase power to run 3-phase motors. Simple, yet brilliant! I'll try to sketch out a schematic to clarify:

enter image description here

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    Commented Jul 27, 2019 at 11:14

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