Whats the science behind locating wire breaks inside (metal) vehicles? - Page 1
Forum section: electronics / rf-microwave

author: Merlysys
author_rank: Regular Contributor
timestamp: September 19, 2020, 03:21:35 pm
content: How do those wire tracers work? I guess when you put power at one end it radiates like an antenna. Since the wire break is inside a metal box (vehicle) all kinds of things will happen to this radiated signal.This started when my hobby mechanic friend asked for a gadget to locate where a wire break is.Real mechanics may have dedicated devices for this. They are known by several terms...wire tracers is one.I found several but they vary so much in price range its hard to decide on one. I feel this is one tool where the user will end up with lots of frustration. He connects the transmitter part of the gadget to the vehicle's mystery wire, moves the receiving wand along the vehicle and expects to magically find the break. Maybe it works that easily.

author: bob91343
author_rank: Super Contributor
timestamp: September 19, 2020, 04:33:24 pm
content: Yes it works that easily. There are issues of course and you need to play with frequency and such to get the best results.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 19, 2020, 06:46:58 pm
content: There are 2 other techniques that are useful: one is simply measuring wire capacitance. This give a surprisingly good estimate of the length of the wire. Another method to measure the length is via the run time of a sharp edge / pulse. This method would also locate the distance to a short and possibly a poor joint / partially broken wire.

author: radiolistener
author_rank: Super Contributor
timestamp: September 20, 2020, 05:12:27 am
content: Quote from: Kleinstein on September 19, 2020, 06:46:58 pmThird is to measure reflection coefficient S11across frequency range. By applying some math it can be transferred from frequency domain to time domain, where you can see chart of impedance/time delay. The time delay depends on the distance. If you know velocity factor of wires, you can calculate chart impedance/distance.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 20, 2020, 06:06:04 am
content: Since it hasn't been mentioned, the usual technique is called TDR - Time Domain Reflectometry. It works as described above, but while it's sort of a specialty complicated measurement (usually requires pretty high frequency generation and receiving), there are much more affordable dedicated tools for it than the generalist options available with pulsers and fast oscilloscopes or the more complete package that's just vector network analysis.

author: borjam
author_rank: Unknown
timestamp: September 20, 2020, 11:44:13 am
content: Quote from: DaJMasta on September 20, 2020, 06:06:04 amA member of this forum designed an awesome (and indeed affordable!) pulse generator you can use to experiment with TDR.http://www.leobodnar.com/shop/index.php?main_page=index&cPath=124

author: filssavi
author_rank: Frequent Contributor
timestamp: September 20, 2020, 12:38:46 pm
content: My guess is that for cars the only thing that could be even remotely close to cost effective, and usable by a regular mechanic is one of the tone generators/detectors they use in phone networks you plug the signal generator in one end and the detector beeps if the signal is present when you get close to the cable, this means you have to have physical acccesd to the cable along the route (or at least in few places) but it is the only thing that will even workTDR is useful with coaxial cables and the likes since their impedance is somewhat constant over the length, a car/truck wiring looms are all but that making TDR hard to pull off (maybe not impossible but enough that a simple car mechanic will not be able to do it without some very serious training). Even worse i doubt you will find a cheap enough TDR that is useful for such small lengths now if you were on a cruise ship that would be different

author: filssavi
author_rank: Frequent Contributor
timestamp: September 20, 2020, 12:43:19 pm
content: Quote from: borjam on September 20, 2020, 11:44:13 amWhile that is remarkable generating the step signal is the easy bit, the receiver is where the real magic happens, especially if your wire is 5/10 meters long and not few hundredsAlso how do you attach a BNC (let alone sma or smaller) cable to a car’s wiring in a way that does not presents such a massive impedance discontinuity to make TDR almost impossible

author: borjam
author_rank: Unknown
timestamp: September 20, 2020, 01:56:37 pm
content: Quote from: filssavi on September 20, 2020, 12:43:19 pmI saidexperiment, and the original poster asked about the principles behind it.

author: Shock
author_rank: Super Contributor
timestamp: September 20, 2020, 02:12:42 pm
content: I brought a cheap secondhand Fluke Networks PRO3000 tone and probe set while back I assume it can be used somewhat similar to the auto cable testers. You can use vampire tap clips as well to pierce cable insulation (or improvise them). The Fluke one uses what looks like an inductive probe, the tone frequency (one of them at least) seems is an audio signal as it can also be used to identify speakers. Not had the chace to play with it.

author: filssavi
author_rank: Frequent Contributor
timestamp: September 20, 2020, 02:38:36 pm
content: Quote from: borjam on September 20, 2020, 01:56:37 pmThat does not change thefactthat TDR will not work outside of very specific settings for theoretical reasons.Namely if the phase velocity of the cable is not constant, and in cars it is most certainly not since the return path is not even near the signal most of the time, being the vehicle chassis.Also any number of issues like poorly mated connectors, oxidised pins, corroded pins, splices, connectors not specifically designed to have a constant and specified impedance, etc will show up in the TDR results just as much as a broken cable, especially if it has not been completely torn through (like by an excavator) as capacitive coupling might mean that the break is only seen as a series load and not as a break

author: ahbushnell
author_rank: Frequent Contributor
timestamp: September 23, 2020, 04:06:58 am
content: Quote from: filssavi on September 20, 2020, 12:43:19 pmA pulse generator and Oscilloscope will do the trick. The delay is 1.5 ns/ft. So double that for a reflected signal. 3 ns/ft.

author: SilverSolder
author_rank: Super Contributor
timestamp: September 23, 2020, 03:14:03 pm
content: The Fluke toners work well with car wiring.e.g.https://www.amazon.com/Fluke-Networks-26000900-Pro3000-Generator/dp/B000FTADX0The science behind them appears to be capacitive coupling. A sine voltage is put into the wire you want to trace which acts as one plate of a capacitor, while your body & the environment you are in (including ground, and the metal chassis of the car) acts as the other. When you put the probe tip close to the wire, you are making the distance between the two capacitor "plates" very small, increasing the capacitance enough that a tiny AC current can flow through space, which is then amplified by the probe and sent to its speaker. By moving the probe around, you are essentially moving that "capacitor" around until you find something interesting.