I've never worked on power supplies. What is the purpose of the pattern in this image (yellow) ? I don't have this PCB, I only have this photo.
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4 Answers
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The pattern is a pair of spark gaps, a kind of low-cost voltage limiter. The closely-spaced pointy conductors favor the formation of a spark, and the soldermask is removed (the tan rectangular area) so that they are exposed directly to the air.
The whole idea is that if anything is going to arc over, it will happen preferentially here, protecting other areas of the circuit.
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That looks like an area with two pairs of pointy “teeth” with a small air gap between them, and no solder mask over that area. Most likely this is part of the board’s EMC protection, providing a place for extreme overvoltage (like lightning) to arc across those gaps instead of finding a path through the board’s circuitry.
There are other, more commonly used forms of overvoltage protection, like transorbs / zeners which may also be on the board somewhere nearby. Often EMC protection has multiple layers to address multiple vulnerabilities.
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According to this NXP document, PCB spark gaps have an approximate breakdown voltage of V = 3000pd + 1350 where p is the pressure in atmospheres and d is the gap in mm. So a 0.5mm spark gap would have a breakdown voltage of about 2,800V at STP.
The CM choke has a relatively high CM reactance (because that's what it does) in order to reduce EMI. Often something like 1mH for common-mode inputs.
However that means a high voltage can appear right across the CM choke and it might cause arcing to some other less desirable place. At least that's my assumption.
answered Jul 9 at 21:23
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Those are spark gaps.
You sometimes see them on things like mains inputs on power supplies.
Usually under the mains input common mode choke.
In this picture they also seem to be under the common mode choke.
So in case there is some large overvoltage surge event, the arcing is engineered to happen there, at the common mode choke.
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\$\begingroup\$ Why do you suppose it is usually under the common-mode choke, as opposed to anywhere else on the line? This looks like it would protect arcing across the common-mode choke, and no place else. It's basically a "series" spark gap, across each coil in the choke, rather than HOT-NEUT or HOT-GND or otherwise. \$\endgroup\$– rohmeoooCommented Jul 9 at 20:56
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\$\begingroup\$ @rohmeooo Becaue it usually is, when you look at things like power supplies with mains input. The PCB component placement is surely not 100% known but the placement does not give any indication that it would be under some other component than the CM choke, rather it supports it. Yes exactly, it is a bit weird but that's how they are - over the coils of CM choke. \$\endgroup\$– JustmeCommented Jul 9 at 20:59