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I am making a PCB that uses AC Mains power where 120VAC is converted to 5VDC with the help of AC/DC converter. The 5V output is then supplied to my digital circuitry. The main confusion I have is to whether to connect the neutral point of the Mains power to my common ground of my PCB or not. In theory, it shows no harm. I have shown the example circuit below where I=0 between mains's circuit and my digital circuit but I am still not sure if its still a safe thing to do.

If I did not connect my common ground to the neutral point, would there be any EMI/crosstalk issue on my digital circuitry?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ From what I understand it's common to use a small value capacitor to tie the two grounds together for EMI suppression, but if you're using a pre-built AC/DC converter unit that's probably already inside it. Otherwise, unless you know you need an un-isolated system I think the best strategy is to not tie the two grounds together. \$\endgroup\$ – helloworld922 Oct 25 '15 at 7:52
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    \$\begingroup\$ Imagine there is a break in the neutral line returning to the breaker panel. Then your entire circuit becomes hot and potentially deadly. \$\endgroup\$ – The Photon Oct 25 '15 at 8:50
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    \$\begingroup\$ @ThePhoton If there's a break in the neutral, his circuit isn't powered. \$\endgroup\$ – Nick Johnson Oct 25 '15 at 9:59
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    \$\begingroup\$ @Nick : true, but you'd better not touch it while trying to work out why it's not working! This is a fail-dangerous arrangement unless the digital circuit is completely inaccessible to the user - the only valid use-case for a transformerless AC/DC converter. \$\endgroup\$ – Brian Drummond Oct 25 '15 at 11:35
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    \$\begingroup\$ He was asking about connecting his circuit's digital ground to mains neutral, as show in the diagram. Which means, if the neutral return line breaks, his whole circuit becomes hot. That is not something you want to happen. \$\endgroup\$ – The Photon Oct 25 '15 at 16:07
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Let me give you the simple answer. No, do not connect any current carrying conductor from AC mains to your low-voltage circuit. Regardless of whether it is "hot" or "neutral". Don't connect it. Some people are suggesting that in some situations you might have to connect it for the power supply to function. If that is the case, do not use that power supply.

Also, you would be better off whether you are a professional or an amateur, if you used external AC-DC converters (agency approved) for all your projects.

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When thinking about if this is a good idea (it isn't) you MUST keep in mind that for plug-in kit in vast chunks of the world, you can never know which conductor is 'live' and which is 'neutral'.

For this reason safety standards make little distinction between the two wires, both are treated much the same.

So if you've somehow convinced youself that something about 'neutral' isn't very dangerous, then ask yourself if your reasoning still applies when the wires are swapped.

There are supply techniques which legitimately play non-isolated games, and contrary to much of what you read they are not dodgy or dangerous at all, in their proper place. However, they ARE completely unsuitable for hobbyists or test equipment or anything like it. If you have to ask, then the answer is definitely not.

In summary:

  • Both mains conductors are live
  • Don't connect either of them to anything other than your psu primary.
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As Will Dean noted, it is quite common for neutral and hot to be reversed, but even if they are not, neutral usually does carry some voltage on it relative to earth ground, and this tends to cause problems like ground loops when you connect to some other device using a proper ground, and also can cause GFCI circuit breakers to trip. To be UL listed, your device must not bleed current onto its ground pin or its chassis even if the neutral is cut.

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  • \$\begingroup\$ What are reasons for neutral and hot to be reversed? \$\endgroup\$ – anhnha Sep 19 '17 at 4:15
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    \$\begingroup\$ @anhnha, lazy/dumb contractors installing it in the wall usually. When I was a kid I found a power strip and was enjoying sticking a paper clip into the "safe" side until one of them was backwards. That was a painful lesson. \$\endgroup\$ – psusi Sep 19 '17 at 22:21
  • \$\begingroup\$ " lazy/dumb contractors installing it in the wall usually". What do you mean by "it" here? \$\endgroup\$ – anhnha Sep 20 '17 at 19:04
  • \$\begingroup\$ @anhnha, the outlet of course. \$\endgroup\$ – psusi Sep 20 '17 at 22:23
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    \$\begingroup\$ @anhnha, as I said before, the person installing it simply screws it up and connects it wrong. It's just a simple mistake. \$\endgroup\$ – psusi Sep 21 '17 at 2:29
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Connecting your circuit ground to mains neutral is probably a bad idea. First, for safety reasons, you'd have to make sure that the chassis of your device is connected to earth ground so it doesn't become live if there is a ground fault. Second, if your circuit is to be connected to other devices, you must ensure that they have the same ground reference. At best it would create a ground loop and at worst it could blow something up. The usual way of doing this is to use an isolation transformer and make your circuit floating ground.

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  • \$\begingroup\$ At worst it would electrocute someone. \$\endgroup\$ – mkeith Oct 26 '15 at 0:40
  • \$\begingroup\$ Yes, I suppose that would be worse \$\endgroup\$ – Robert Stiffler Oct 27 '15 at 17:08
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This depends on the architecture of your AC-DC converter. If it's transformerless, your circuit must share a common ground (AC neutral) in order to function.

If you're using a transformer (followed by diode rectifier, capacitors, and regulator), your low voltage side is galvanically isolated from mains and is thus floating. You can connect DC ground to neutral, but absent a concrete reason to do so I'd avoid it - having a floating ground can be useful if you want to interface with other systems that might have different ground references.

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  • \$\begingroup\$ Here is the AC/DC converter I am using: ca.mouser.com/Search/… \$\endgroup\$ – dr3patel Oct 25 '15 at 15:22
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    \$\begingroup\$ @dr3patel That uses a transformer, so as I said - don't connect input neutral and output gnd unless you have a compelling reason to do so. \$\endgroup\$ – Nick Johnson Oct 25 '15 at 15:52
  • \$\begingroup\$ You may want to tie AC ground (mains side) to DC ground (power supply output side), but it is NEVER a good idea to tie AC neutral to DC ground. This is a SERIOUS SAFETY ISSUE. If the input side doesn't have a separate ground contact, then it had better be an isolated supply, in which case you still don't connect AC neutral to anything on the DC side. While AC neutral is usually tied to earth ground at a building power entrance, the neutral AC wire is a current carrying conductor, and thus is not necessarily at ground potential. Due to common miswiring AC neutral may actually be hot! \$\endgroup\$ – Eric Smith Oct 12 '16 at 7:10

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