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Mains (230v) is connected to "L" (live) and "N" (neutral) and the voltage regulated device is connected to "1" and "2".

Because it's not documented if "1" or "2" is the live wire, a test, with a mains tester screwdriver, on transformer-level 5, showed that "1" is the live wire.

Out of curiosity I took a look inside the transformer. Here I couldn't figure out how the white wire ("1") is the live wire and the grey is the neutral.

How does electricity flow in this transformer?

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Documentation from the manufacturer:

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When the power switch is on, it will connect brown wire directly to white wire so "1" is "L".

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  • \$\begingroup\$ Gotcha. And black is neutral? \$\endgroup\$ Dec 21 '21 at 15:09
  • \$\begingroup\$ Yes, the switch will also connect the blue wire to black wires so they are neutral. \$\endgroup\$
    – Justme
    Dec 21 '21 at 15:15
  • \$\begingroup\$ So that means that the neutral wire is the one being voltage regulated. How does that work when the power has "left" the device? Shouldn't it always be the live wire being voltage regulated? \$\endgroup\$ Dec 21 '21 at 15:18
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    \$\begingroup\$ True, the gray output wire is either connected to black neutral wire or connected to one of the autotransformer taps depending on the rotary switch position. So yes, the regulation is on the "neutral" side of device. Usually devices don't care much on which side the regulation is, unless there is some special need depending on the device to have a definite neutral wire for some reason. It has no ground/earth either. And in many countries mains plugs are non-polarized so devices can't assume which wire is live or neutral to begin with. \$\endgroup\$
    – Justme
    Dec 21 '21 at 15:29
  • \$\begingroup\$ Thank you for the explanation! \$\endgroup\$ Dec 21 '21 at 16:10
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It looks like it is an autotransformer.

Taken from Wikipedia - https://en.wikipedia.org/wiki/Autotransformer

An autotransformer is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone, not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary winding and secondary winding sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which have no metallic conducting path between them.

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The white and black wires are the primary connections and the red wires are the secondary connections. The rotary switch connects one secondary to the grey wire output. In position 5, the transformer is by-passed to get full line voltage.

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  • \$\begingroup\$ So black is neutral? And the voltage regulation happens on neutral? If yes, shouldn't voltage regulation always happen on the live wire? \$\endgroup\$ Dec 21 '21 at 15:06
  • \$\begingroup\$ The grey wire is Neutral. It is worth noting that the white (L) wire is always at full line voltage and there is no electrical isolation from the mains supply. I don't know what standards will apply locally as to whether this is the correct way that it should be wired. \$\endgroup\$
    – HandyHowie
    Dec 21 '21 at 16:19

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