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I'm using this AC-DC converter (120VAC-5VDC) as a power source for an Arduino-based device.

https://www.digikey.com/product-detail/en/recom-power/RAC01-05SGA/945-3043-ND/6677085

The AC-DC converter has input pins labeled Neutral and Live. I'm wondering how essential it is to get the polarity right. A quick test showed no difference, and since the output 5VDC is totally isolated, it doesn't seem like it should be affected by the input polarity.

Does anyone know why these are labeled specifically, instead of just "AC in"?

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  • \$\begingroup\$ Rules vary from country to country but this one is sold pretty much globally. There are most likley several countries where you would be allowed to just put a sine wave symbol for each input terminal. \$\endgroup\$
    – winny
    Jul 2, 2018 at 21:05
  • \$\begingroup\$ Why the difference? Are some regulations intended to clarify that the part needs one Live and one Neutral, even with the downside of saying that it matters which is which when it doesn't matter? \$\endgroup\$
    – Vulcan
    Jul 2, 2018 at 21:12
  • \$\begingroup\$ Pretty much. Look up autotransformer saftey. Pretty much banned everywhere now but where are a few historic reasons for labling it so still. Outside of the consumer world, you may run into some practical differences too here. \$\endgroup\$
    – winny
    Jul 2, 2018 at 21:20

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I would guess that there is a fuse or other kind of overload protection on the input, and for safety reasons you would want that to be on the line input. The neutral connection, at least in the U.S., is connected to ground at the breaker box, so the line connection is the more dangerous of the two. If something goes wrong, you would like to isolate as much of the power supply circuitry as possible from the high-voltage line input and not worry as much about the neutral input.

So, it doesn't matter electrically but the power supply might be a bit safer to use if you connect line and neutral as they are marked.

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Study the data sheet. It appears that there may be an internal fuse in the live line. They also show EMI filter capacitors from -Vout to neutral.

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In America, 1 fuse needed for Line, as Neutral is earth bonded at source.
In EU , it wont remove voltage from both inputs

Note7: Refer to local wiring regulations if input over-current protection is also required
Note8: For repeat Hi-Pot testing, reduce the time and/or the test voltage
Note9: For operation at 230VAC, an external MOV is recommended. The Varistor should comply with IEC-61051-2. e.g. EPCOS S14 series

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