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I am completely new to electrical engineering, and have trouble understanding the implications of the different switch types, or rather I think I understood them but some products don't seem to make sense.

The switch that I am looking for should be a rocker switch with 3 or 4 terminals and on-on switch function. Simply put it should either switch [1 & 2] / [2 & 3] (A) or [1 & 2] / [3 & 4] (B).

Firstly, I suppose variant A is called "SPDT", and variant B doesn't actually exist in the real world (or I have failed to find it). The closest I could find to variant B would be using a DPDT switch with 6 terminals and leaving one NO and one NC terminal of opposing circuits disconnected.

But since I don't really mind the common terminal, I am filtering for "SPDT" in online shops now. I am also setting the filter to "On-On" function. I think this should be the correct filters.

However I keep finding switches where the actuator marking is "0 I" or "On Off". This is what causes my confusion. I suppose if I do not connect either terminal 1 or 3, the switch could behave like a normal on/off switch. But why would you use an On-On switch then?

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I understand it to be a binary like thing: 0 is off 1 is on. The switch label does not always describe what the switch does internally, it could describe device functionality. If you wanted a device to be powered differently and not completely powered off, you could switch the power to a different terminal, sending the device into a standby mode. There are other uses but the outside of the switch faces the user and describes functionality of the device.

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    \$\begingroup\$ Aaah, so you mean the user switches the device "off" in his mind, but the On-On configuration of the SPDT switch allows some other consumer, for example a red standby LED, to be controlled with the same switch instead of needing additional circuitry. That makes sense. :D \$\endgroup\$
    – LWChris
    Apr 22 at 20:33
  • \$\begingroup\$ Yep, it's all in the label. \$\endgroup\$
    – Voltage Spike
    Apr 22 at 20:36
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I suppose if I do not connect either terminal 1 or 3, the switch could behave like a normal on/off switch. But why would you use an On-On switch then?

For use in switching device power, usually only one of the two poles is used. Many kinds of switches are only available in double pole variants, not single pole variants. Single pole switches are designed for "on-off" applications, obviously, but very many nice switches are targeting more generic markets and they are double-throw, and hence On-On etc.

But why would you use an On-On switch then?

Many beautiful switches are not available in a single-throw variant. The second pole adds little enough cost that it's not worth having both a single-throw and a double-throw variant of those switches. Those switches are not usually bottom-of-the-barrel in terms of cost. Applications where the last cent needs to be shaved off the cost will resort to often lower grade single pole switches.

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    \$\begingroup\$ Reminds me of Tesla car seats that all have seat heating built in, because it's cheaper overall to produce a larger quantity of just one kind of seat than having two separate smaller production lines to produce just slightly different seats. \$\endgroup\$
    – LWChris
    Apr 22 at 23:30
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An SPDT switch is an electromechanical "OR" switch—one or the other. For example, if you have a gadget with critical power needs, connect the gadget's power input to the common pole, utility on one pole, and inverter on the other for backup power.

An electrician uses them to wire two-way switches to control a lighting fixture or an outlet from two switches, like at opposite ends of a hall.

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