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Since I heard two versions and also read two versions in manuals to various devices, I would now like to hear the community opinion.

How should we connect a power supply to an electronic device?

Option 1: Connect the PS to the device, then plug the PS into the mains

Option 2: Plug the PS into the mains, then connect it to the device

I have two mobile phones from different manufacturers, and their manuals say the opposite thing. Why? Hasn't it always been a golden rule of the radio electronics not to let a PS run without a load?

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5 Answers 5

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Option 3: It really doesn't matter, and people just pull things out of their fundaments to justify their existence in writing user manuals that are long on irrelevance and short on useful information.

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  • Good luck getting an appreciable arc out of a mobile phone charger.
    – womble ♦
    May 9, 2014 at 5:00
  • Well, when there's a question about plugging in a heater, you're all set.
    – womble ♦
    May 15, 2014 at 5:32
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Option 1 has the downside of the device being connected before the rail(s) settle, which with some (mostly early) switchmode designs could cause spikes of high voltage. Given there's always the chance of a power outage you'd hope this is safe.

Option 2 has the downside of running the PS with no load, something that any external PSU should be designed to handle.

So in short, either should be ok.

There is one exception I've found, in the EU or Australia always connect the device end of removable power cables first as the plugs are better at ensuring earth connects first then many IEC connectors.

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The game "The Dig" contains the quote:

"Always connect the wire to the dead battery first"

And I think it kind of explains it all...

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  • +1 - not particularly useful, but you mentioned "the Dig" :-)
    – Rook
    Jul 5, 2009 at 21:31
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IMHO it doesn't matter in most cases. Though I would suggest that you attach a PS of a PC first, and only then attach it to the mains. :)

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  • If it doesn't matter, why the suggestion?
    – tomjedrz
    Jul 5, 2009 at 18:49
  • Because you should always disconnect the power from the PC when you open it up. :P
    – Vilx-
    Jul 7, 2009 at 10:27
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I would be really surprised if it matters. The issue is that, because human beings only have two hands, one side or the other has to be plugged in first.

If you are concerned about it, I suggest the following.

1- Plug in a switched power strip. Switch it OFF.
2- Plug the transformer into the power strip.
3- Plug the transformer into the device.
4- Flip the switch, connecting the whole thing at one.

To take apart, reverse the sequence.

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