7

enter image description here

I found this tooth-like non-masked, solder-covered trace pattern on modern single board HDTV.

It's in the 120VAC "HOT" section of the switch-mode PS.

Looks like some sort of spark gap or protection feature. There are two of them, one with 3 "teeth" and the one pictured with 4 "teeth".

Comes right after 4 large rectifier diodes that are connected to the ac line and before the switching transformer.

I'm pretty sure somebody here knows the function of the feature and would be interested to know what it is. Why 3 teeth? Is there some formula to determine the number of teeth? Is it a fuse like component? There is a fuse in another part of the HOT section.

Christopher Allen
  • 71
  • 3
  • I dont think there are reliable ways of determining the amounts of teeth/distance because it will vary a lot depending on humidity and other environmental factors. Its more like a ballpark protection feature (which is fine, since its basically free). Its not like a "fuse", maybe more like a shunt if you want to use analogies - (to discharge excess voltage). – Wesley Lee Jan 28 '21 at 13:04
  • Yes, it's a spark gap. – user253751 Jan 29 '21 at 00:05

2 Answers2

8

You can get rough calculations of the breakdown voltage from calculators. This paper has some measurements from an etched 0.55mm tooth gap:

Wan, F., Pilla, V., Li, J., Pommerenke, D., Shumiya, H., & Araki, K. (2014). Time Lag of Secondary ESD in Millimeter-Size Spark Gaps. IEEE Transactions on Electromagnetic Compatibility, 56(1), 28–34. doi:10.1109/temc.2013.2275922 

As a serious discharge will tend to vaporize the tip of the tooth, the number of teeth is probably an indication of how many events the designer wants it to survive before the circuitry is on its own.

Many products, even consumer products, use commercial GDT (Gas Discharge Tube) devices, which have more predictable characteristics. Of course they cost a few cents more than zero.

Spehro Pefhany
  • 376,485
  • 21
  • 320
  • 842
5

In effect it's an over voltage spark gap. If a line surge occurs, it will cause breakdown across the shark's teeth and (hopefully) offer some over-voltage protection to other parts of the circuit.

Andy aka
  • 434,556
  • 28
  • 351
  • 777
  • I'd guess the purpose isn't to deal with a line surge, but rather a situation where a component that fails open-circuit might allow a voltage to rise until something arcs over somewhere, and would serve to ensure that the "somewhere" that arcs over first won't be fail in a dangerous fashion. – supercat Jan 28 '21 at 22:07
  • @supercat not according to the limited facts revealed in the question. – Andy aka Jan 28 '21 at 22:32