# Grounding a switching PSU with AC mains neutral

I'm working on a circuit that measures mains current/voltage. I've seen the usual setup where a half wave transformerless simple circuit is used to supply couple of miliamps of power to run the IC that does the measurements and after that, an optional optocoupler is used to communicate with other ICs that run on isolated PSUs.

In my circuit however, the combined power consumption is in dozens of milliamps and the simple PSU above will waste too much power. I thought, why not connect the ground of a regular switch mode PSU ( e.g. a mobile phone charger adapter) to the mains' neutral lead and use it to power up my circuit. I've since been trying to find a problem with this solution and couldn't. Can anyone think of any problems or perhaps other solutions?

Note: I'm aware that such a circuit would be dangerous to humans without a proper enclosure (e.g. in case the live and neutral leads are swapped).

Edit: I didn't know how easy it was to add schematics here, so here's one to show what I mean:

simulate this circuit – Schematic created using CircuitLab

A switcher that runs from the AC can have its isolated output connected to the neutral. Make sure it is an isolating switcher though.

• And I assume it won't make a difference if the mains live is connected to the negative output of the PSU, for instance in case of incorrect wiring in the house, right? – Mansour May 30 '13 at 2:56
• Correct but do make sure you take adequate precautions when testing it. – Andy aka May 30 '13 at 6:59

It's not an answer to the question you're asking, but it is an answer to what you're trying to accomplish:

Have you looked at the LNK30x switching power supply ICs? They're intended for replacing a Linear, Resistive / Capacitive Dropper based PS, while providing higher output power.

I'm working on a project that I think is likely nearly identical to what you are doing. It's a CS5490 power meter IC, a nRF24L01+ for connectivity, and an ATxmega to glue it all together. It's open-source, too.

• Wow! Had no idea such a thing existed. But this might actually cost more to make than buying a, let’s say, phone charger that can supply 1A. Those two 4.7µ input stage caps are expensive ($5.50 each) and huge (27mm lead spacing, film cap). – Mansour Jun 30 '13 at 18:00 • Okay—I don't know why I was searching for film caps. The usual electrolytic ones are cheap and not too big. – Mansour Jun 30 '13 at 19:48 • @Mansour - The total BOM cost for the whole circuit should be in the ~1.50$ - 3.00\$ range, at least for quantities > 100. – Connor Wolf Jul 1 '13 at 12:35

Are you talking about the "zero" wire, or the "safety ground" wire? Any current through the safety ground can conceivably trigger a ground fault interrupter.

If you're just talking about the "zero" wire (white wire in standard US wiring,) then with your rectifier bridge, you already have a galvanic connection to that wire. That wouldn't change with the switch mode circuit, although that entire circuit would be exposed to live AC voltage and possible lightning induced spikes and whatnot, so safety precautions still apply as it is high-voltage circuitry.

Or maybe I don't entirely understand your proposal. Actual schematics / wiring diagrams would probably help!

• I'm talking about the "zero" wire (called the "neutral" here in Australia). Also, what I mean by the regular switch mode PSU is something like a mobile charger adapter which provides an isolated output due to use of a high frequency transformer. – Mansour May 30 '13 at 2:48
• Wiring a regular "plug in" power supply to the hot and cold wires is electrically no different from plugging it into an outlet with the same connection. Assuming the PSU is designed to be always-on, and does not build up heat if you close it up in a box or wall, that could work fine, engineering-wise. What your local building code thinks about this, I have no idea. By the way: Those assumptions are NOT typically true for cheap Chinese wall plugs that come with cheap Chinese electronics. They may overheat if not in free air, and may not be rated for 24/7 operation. – Jon Watte May 31 '13 at 6:00
• OK - I just edited the question with a schematic (didn't know it was so easy to add one). – Mansour May 31 '13 at 12:03