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I have a 220 V -> 110 V AC "wall wart" shaped step-down transformer with a plastic body. The input has an ungrounded three-prong plug (the ground is plastic) and the output is a polarized socket. The lowest resistance path between an input and output is the hot input being connected "neutral" output (fat side of a polarized US-style socket), with 0.2Ω resistance. My analysis is that the transformer's inputs are backwards. Is it worth plugging in the transformer backwards if it requires cutting and glue and makes a less secure physical connection afterwards?

Info about the application that may not be relevant:

I am using it to power a fan that has a fully plastic body. The fan has an internal DC adapter and is the internals run on 24 V DC. Is there anything unsafe about this?

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  • \$\begingroup\$ "wall wart" type? How much of a mains transformer can you even fit in a wall wart? \$\endgroup\$
    – Hearth
    Commented Oct 8, 2022 at 5:25
  • \$\begingroup\$ @Hearth It's low-wattage, nearly fist-sized, and the plug is a UK plug--fairly strong. \$\endgroup\$
    – piojo
    Commented Oct 8, 2022 at 5:26

1 Answer 1

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The question relates to a 24 V DC fan, along with its matching 110 V ~ to 24 V DC power supply, housed in a plastic enclosure.

110 V ~ is to be sourced from a wall wart housed in a plastic enclosure and having a 3-pin plug for 220 V ~ input. The ground pin is made of plastic which effectively makes it a 'no-ground' unit. It also has a 2-pin polarised socket for 110 V ~ output.

It is inferred that the wall wart is a step-down auto transformer.

The schematic of such a unit would be as follows:

enter image description here

There should be no problem provided that its rating is adequate for the fan.

However, it's understood that the connections have been interchanged as shown below.

enter image description here

In spite of that, there should be no risk of insulation failure, with the wall wart and fan being double-insulated and having no earth connection.

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  • \$\begingroup\$ Thank you. To be clear, the L and N of an autotransformer can be reversed with no issue, provided the insulation is reasonable? \$\endgroup\$
    – piojo
    Commented Oct 8, 2022 at 7:01
  • \$\begingroup\$ Since the 220V~ input plug and the 110V~ output socket are both polarised, there is no question of L & N getting interchanged. You may confirm that 'L' on the output side is live, using a line tester. \$\endgroup\$
    – vu2nan
    Commented Oct 8, 2022 at 7:53
  • \$\begingroup\$ No, the line tester says the N side is hot. \$\endgroup\$
    – piojo
    Commented Oct 8, 2022 at 8:10
  • \$\begingroup\$ Okay. However, that should pose no problems. My answer has been edited accordingly. \$\endgroup\$
    – vu2nan
    Commented Oct 8, 2022 at 11:34
  • \$\begingroup\$ Thank you! The output is reversed from your last diagram, but your point is taken. It's okay since the double insulation is acceptable quality, and there's no part that really needs to be not energized. There are many things I don't know about working with mains, so I appreciate the answer :) \$\endgroup\$
    – piojo
    Commented Oct 8, 2022 at 13:18

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