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I have a fused power inlet with a voltage selector (Schaffner FN1394-10-05-11). I want to connect it through a separate DPDT switch to a transformer with dual 115V primary windings, and make everything work on either 110-120V or 220-240V grids.

Here is the wiring diagram for the inlet and voltage selector:

Schaffner voltage selector diagram http://schaffner.com/uploads/pics/FN390_Typical_electrical_schematic.png

Top position on the voltage selector is parallel wiring for the four outputs (for 110-120V), and bottom position is series wiring (for 220-240V). The four outputs on the inlet are numbered 2-4-3-1 from the top down in the diagram.

The DPDT switch is pretty simple, it just has two sets of terminals, each consisting of an open-closed-common triple. When the switch is open, both open terminals short to common, and when it's closed, both closed terminals short to common. (Two separate common terminals.)

The transformer is also simple: two 115V windings, four wires.

What is the correct way to wire the inlet to the switch to the transformer, assuming P is the mains hot wire?

PS: I know that the inlet has a built-in switch. I want a separate switch because it has a nice LED on-off display and will be on the front panel of the device, rather than its back.

PPS: When connecting mains wires to the primary windings, does polarity matter?

(I think that wires 1 and 4 should run to the two closed terminals on the switch, and the common terminals on the switch should run to matching wires on the transformer. Wires 2 and 3 should go directly to the transformer. That way, when the switch is open, the hot wires terminate at the switch, no matter what the voltage selector switch is set to.)

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The four outputs on the inlet are numbered 2-4-3-1 from the top down in the diagram.

Your link to the diagram is broken, I will assume it's the same as the diagram in http://www.farnell.com/datasheets/1511815.pdf

I think that wires 1 and 4 should run to the two closed terminals on the switch, and the common terminals on the switch should run to matching wires on the transformer. Wires 2 and 3 should go directly to the transformer.

What you propose will work but your double pole switch will effectively only provide single pole switching. If you wanted full double pole switching you would need a four pole switch.

That is probablly not a problem for a functional switch on a transformer powered device but it's something to be aware of.

PS: I know that the inlet has a built-in switch. I want a separate switch because it has a nice LED on-off display and will be on the front panel of the device, rather than its back.

You might want to think about how the indicator light (it's probablly a neon not an LED) will end up connected in both 120V and 240V modes

PPS: When connecting mains wires to the primary windings, does polarity matter?

It matters that the two windings are connected with the same polarity. Otherwise they will "fight" each other resulting in an effective short circuit.

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  • \$\begingroup\$ Thanks for your comments, Peter. I had finished building the device I was asking about. The external switch light is powered by DC from the power supply downstream from the transformer, so it doesn't care about input AC voltage. I never tried it in 240V mode, but 120V works fine. (Was planning to borrow a step-up transformer at some point to double-check, but considering it's an audio amplifier for my own use, it doesn't really matter right now.) \$\endgroup\$ – gcv Dec 9 '15 at 23:24
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You may be ill advised to place your planned switch on the load side of the power inlet. There could be a potential safety hazard if someone went to service the unit and mistakenly thought that your planned switch cut all power from inside the box.

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  • \$\begingroup\$ Wouldn't the same safety hazard exist if the switch were on the neutral side of the power inlet? The load side would still be powered, just running to the transformer instead of the switch. \$\endgroup\$ – gcv Jun 1 '15 at 16:08
  • \$\begingroup\$ I didn't say that the switch would be the hot or neutral side. No such distinction. I was saying that the switch you propose leaves some connections inside the box with power applied. Instead if you stayed with a switch integrated in the power inlet module, especially a fully integrated one, the output side of the module gets unpowered when the switch is off. \$\endgroup\$ – Michael Karas Jun 2 '15 at 4:29
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    \$\begingroup\$ I don't think this is a reasonable concern. Generally the assumption of safety standards is it's not possible to protect idiots with screwdrivers from their own stupidity. \$\endgroup\$ – Peter Green Dec 9 '15 at 19:15
  • \$\begingroup\$ @PeterGreen - There are many many cases of safety policy that make less sense than common sense. Take a look at a lot of OSHA guidelines. \$\endgroup\$ – Michael Karas Dec 9 '15 at 21:19

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