inductive pcb tracks - Page 1
Forum section: electronics / projects

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 15, 2012, 08:56:27 pm
content: Hi all.Does anyone know much about pcb inductive tracks? I saw the following on the inputs to a alarm board and wondered how much noise filtering they gave?ThanksTrev

author: AndyC_772
author_rank: Super Contributor
timestamp: October 16, 2012, 06:52:20 am
content: How long are those tracks, about 40mm all folded up? In that case they're providing a series inductance of about 40nH give or take, bypassed by enough parasitic capacitance to all but negate whatever filtering effect they might otherwise have provided.If this was an rf application I might just about be convinced that they'd been added in order to achieve some very specific, defined behaviour at a given frequency, but as a general EMC filtering measure they're a waste of space. A few cm of wire does not constitute a useful filter, it constitutes an antenna!

author: bfritz
author_rank: Regular Contributor
timestamp: October 16, 2012, 07:26:00 am
content: They look like they are along the edge of the PCB, and they probably connect to a connector that goes to external door/window/smoke/etc sensors. Not really noise filtering or RF, but likely protection for lightning. One of the problems with alarm systems, is they have long lengths of wire attached to them, often going to many windows, easily in the 100's of feet. These wires do act as antennas, and can end up with very large induced voltages. The traces probably provide just enough inductance for the short lived lightning events to subside enough, to not damage the inputs to the micro attached to the other end of the traces.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 16, 2012, 07:33:13 am
content: There's no energy absorbing device there, though. How would a few cm of PCB trace provide inductance which is any more useful in that context than the inductance of the long wire that's already attached? It's just a continuation of the external wire; there's nothing special about the bit that happens to be on the PCB.In telephone applications we use transient voltage suppressors, and for external cabling, gas discharge tubes, MOVs and the like, to suppress induced spikes and protect the equipment. A little bit of PCB trace just won't do anything.

author: Codemonkey
author_rank: Regular Contributor
timestamp: October 16, 2012, 08:23:59 am
content: It would be worth looking at the other side of the board, maybe they're using them as current shunts to detect short circuit conditions on the external wiring, so using them as resistors, not inductors.

author: SeanB
author_rank: Super Contributor
timestamp: October 16, 2012, 03:44:45 pm
content: They are fusible tracks, designed to be sacrificed if there is a nearby lightning strike or mains is applied to the cabling. Terminal for the board, but they will open on overcurrent and prevent a fire in the cable. They rely on the transient suprtession on the board if fitted or the input diodes of the opamp comparators, and a PTC fuse to protect the board when the opamp latches up from the abuse.

author: Neilm
author_rank: Super Contributor
timestamp: October 16, 2012, 06:51:21 pm
content: I have used similar tracks when I needed some resistance in the circuit and did not want to use a resistor. It might be that the circuit requires a "minimum" resistance on the input.Neil

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 18, 2012, 07:39:14 am
content: Interesting stuff. These connections are the inputs of an alarm board so they will be coming from long pieces of wire. The inputs go via a resistor network to inputs of a processor. If the inductive traces do work to filter out rf maybe they are just helping stop any rf frequency signals getting into the processor and latching up the processor port.Trev

author: ColinB
author_rank: Contributor
timestamp: October 19, 2012, 06:44:32 pm
content: Quote from: AndyC_772 on October 16, 2012, 07:33:13 am"Never underestimate the power of human stupidity." - Robert A. HeinleinDon't discount the possibility that these tracks were designed to provide some kind of protection, but do not in reality. Maybe the designer though it would be a good idea, and the result was never tested.It is an interesting pattern though. Can you provide a picture of the other side? As a previous poster said, maybe something interesting is there.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 19, 2012, 07:15:10 pm
content: Believe me, I've come across some train wrecks before now - designed with the best intentions, but completely inept or misguided.Just occasionally, though, I come across a board which looks like a disaster at first glance, but then starts to look more and more sane the more I study the design and come to understand it. That's why I don't like to just say "that's a bad design" in front of other engineers who might know more than I do - however sure I am that I'm right!

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 22, 2012, 12:38:31 pm
content: Sorry, I do not have the board so I cant get a picture of the other side.

author: Mangozac
author_rank: Frequent Contributor
timestamp: October 24, 2012, 04:32:37 am
content: Is there any chance it's a Digiflex alarm panel? I know that they (at least used to) include such trace structures on their inputs but I can't remember the reason I was told. It was however something they had tested and found to be beneficial. It's not for lightning/spike suppression: they use a zig zag pattern with no solder resist closely spaced to ground to act as an arc gap for that.I'll see if a colleague of mine remembers and get back to you...

author: JoeyP
author_rank: Frequent Contributor
timestamp: October 26, 2012, 12:14:39 am
content: I'd bet they are there as an attempt at a safety "fuse". The reason for the zig-zag is to increasing heating effect. I did some tests a while back and was surprised at just how much current a straight trace could withstand without opening up. I was using 20 mil traces with 1/2oz copper, and found they could easily handle 20 amps continuous current for quite a while. Eventually the PCB would burn and the trace would give up. A zig-zag pattern concentrates the heat in a smaller area, and the traces open up *much* quicker.

author: trevwhite
author_rank: Frequent Contributor
timestamp: October 26, 2012, 07:07:35 am
content: I am pretty sure the tracks are not for fusing. They would need to be a lot smaller and there are input resistors anyway.I do not know the make of the alarm box. I will see if I can find out.Trev

author: Big_Al
author_rank: Contributor
timestamp: October 26, 2012, 06:50:03 pm
content: Probably just enough Inductance to get it through its EMC Immunity tests.You can bet its followed by an R/C Filter, R divider, and then fed straight into a Micro pin.