Strange loop traces - Page 1
Forum section: electronics / general

author: dagema
author_rank: Contributor
timestamp: April 16, 2020, 04:11:32 pm
content: Hi all,I bought a defective PREMA 6000 6.5 digit DMM on eBay, and while attempting to repair it I noticed that there were 2 kinds of PCB traces on the bottom side of the main board: one type covered with green laquer and the other type without it, just exposing the unprotected, soldered traces. Looking closer, I saw that all of the unprotected type formed some sort of loop around other traces and/or pads. The loops do not really make any sense to me, so I wonder if anyone is familiar with this and can tell me the purpose or idea behind it. Roughly in the middle of the attached picture, a bit down from the center is, for example, a trace which simply circles around a pad, the trace itself not connecting to anythingCan anyone explain this?Thanks for any ideas.

author: cvanc
author_rank: Frequent Contributor
timestamp: April 16, 2020, 07:20:59 pm
content: "Guard ring"https://electronics.stackexchange.com/questions/106974/what-are-guard-rings

author: dagema
author_rank: Contributor
timestamp: April 17, 2020, 11:37:12 am
content: But the example I mentioned, and most of the other traces of this type, are not being "driven" by anything, other than maybe RF noise they might pick up as an antenna. So how could these be classified as guard rings?

author: thm_w
author_rank: Super Contributor
timestamp: April 17, 2020, 08:07:38 pm
content: Quote from: dagema on April 17, 2020, 11:37:12 amThe ones I see are all connected to through hole pins and are therefor being driven by something.Can you find any that are not connected to a pin? Other than maybe the trace around the screw hole, which is probably just for spacing.

author: David Hess
author_rank: Super Contributor
timestamp: April 17, 2020, 09:45:19 pm
content: The guard rings are not coated because they principally absorb leakage across the surface. There is leakage inside of the bulk of the printed circuit board but most of the leakage comes from contamination on the surface.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 21, 2020, 03:07:43 pm
content: Can you point me to one of the guard rings that is not driven by "something". I agree with thm_w. All the guard rings seem to be connected to a via.I've got a service manual of a 10 or so digit DMM, I think it's the HP23456 or something.The whole thing is an excellent tutorial in high precision design with lots of notes and explanations. To reach so many digits, they had to put a bunch of traces on teflon standoffs, and put guard rings around the teflon standoffs. It almost sounds reliculous.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 21, 2020, 03:17:04 pm
content: Hard to tell from the photo, but the trace around the mounting hole appears to be attached to a pair of pins.But a close up photo would be best.

author: dagema
author_rank: Contributor
timestamp: April 23, 2020, 10:39:06 am
content: After taking a closer look, I found that all of the loop traces appear to attach somewhere to a trace which could serve as a "driver".Thanks to all contributors for the enlightenment.