While looking at how other circuits bring AC mains into their circuits, I came across this design where there are 3 differently shaped vias (from top to bottom):

  1. Figure of 8
  2. Octagonal ring
  3. Ellipse

Why are the vias shaped differently?

At the bottom, why are the vias for the HOT and NEUTRAL vias completely surrounded by the trace (pads?), while the AC-OUT and AC-IN vias do not?

enter image description here

  • 1
    \$\begingroup\$ Wow, forget about the pads, that layout is scary! Where did you find it? \$\endgroup\$
    – Matt Young
    Nov 25 '14 at 16:17
  • \$\begingroup\$ @MattYoung instructables.com/id/Hack-a-Toaster-Oven-for-Reflow-Soldering/… I'm looking for a design reference to bring AC mains into a AC-to-DC transformer, any recommendations? I'm guessing the HOT trace is too close to the NEUTRAL? \$\endgroup\$
    – Nyxynyx
    Nov 25 '14 at 16:22
  • 7
    \$\begingroup\$ I don't know what SSR relay that is, but based on its footprint, I'm guessing it's control pins are isolated. The designer of that board took the hot and neutral right through the middle of them. There's no scale, but I looks like there can't be more than couple mm of clearance. If the circuit ground is tied to a metal chassis, this thing could easily become deadly! \$\endgroup\$
    – Matt Young
    Nov 25 '14 at 16:30
  • 2
    \$\begingroup\$ A 170V input needs >1.25mm according to IPC-2221B. A 340V input needs >2.5mm. Allowing for 1000V transients increases this to 5mm. It looks like there's a DIP package in the bottom middle, with presumably 0.1" pin spacings (2.54mm), which appears marginally greater than the creepage distance. Probably passable on 120Vac, but I don't have a lot of confidence especially taking tolerances into account. \$\endgroup\$ Nov 25 '14 at 17:26
  • 1
    \$\begingroup\$ You're probably better off taking apart consumer electronics to look at their layouts (carefully, unplug and don't touch the big caps). \$\endgroup\$
    – mng
    Nov 25 '14 at 22:24

The top eight pads are octagons, the top four just happen to be very close together. None of the pads are vias, per se, they are landing pads for through-hole parts. The top four for one, probably a connector for AC input, the middle four for another, a solid state relay, the bottom two are likely for soldering wires to directly. You can see the component outlines for part placement.

The octagons are fairly normal for Eagle layout, I believe they're actually the default shape. The long pads are commonly used for soldering wire to, probably because the wire will often stick through the hole then be bent over parallel to the board. The long pad (and in this case additional SMT pad) of exposed copper allows for more solder to be applied between wire and copper to act as a strain relief.


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