I have a wall-wart charger that put out 4.2 V DC on a figure-8 cable just over 1 metre long. There seems to be a weak spot or intermittent break in one of the wires --- sometimes it doesn't work but jiggling the cable makes it work.
Is there any non-destructive way to find the position of the break? (I know I could cut it in half, test both halves, repeat as necessary, ... solder all the good bits back together; but I'd rather not make a big mess.
-- A lot of people never use their intiative because no-one told them to. --- Banksy
A> with the break broken measure capacitance from both ends.
or
B> put a noisemaker on it such that a working wire makes noise and manipulate it until you have located the break.
or
C> in my experience 80% peobability it's near tip 18% near rhe box 2% elsewhere.
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And, besides, the tortoise might win. (*)
More seriously, I'd think that the simplest approach, based on the information provided, would be to replace the cable and see if the problem goes away.
If there are no further "hiccups" you can assume the cable was at fault and pitch the old cable as scrap.
If the problem recurs, it probably wasn't the cable, and you can concentrate on other possible causes.
Frank McKenney McKenney Associates
(*)
--
...I am convinced that scientists have an obligation to communicate
with the public, people who do not know the difference between an
anaysis of variance and a factor analysis but who are eager to read
about current research findings and the hot controversies in our field.
The trick is to inform readers about the latest results in a way that
is interesting, but at the same time respect the complexity of the
issues and be honest about what we know and what we don't.
-- Hal Herzog / Some We Love, Some We Hate, Some We Eat
A thermal imaging camera? Might work if the intermittent connection is generating enough additional heat to be visible.
A TDR? Depending on what you have around the shop, you might be able to throw one together. A one meter cable is kind of tight for a "what's in the junk drawer" project, though.
As Jason mentioned, a "fox and hounds" cable tracer might do the trick.
Take a fine sewing needle and poke it through the insulation. This is non-destructive because that small hole does not matter. The plastic insulation is flexible enough to close the hole completely again.
w.
Way before anywhere else, the problem is at the connector end, or where the cable comes out of the AC adapter. Those are the two stress points, and anywhere else will be way down in terms of chances of a problem.
Cut off the connector, and cut a bit of the cable off, and resolder. Chances are good you've fixed the problem, unless it was at the other end.
Someone suggested just changing the cable, that's not only cleaner (except for having to open up the AC adapter) but put things in a clean slate to begin with.
Michael
Yes, those are exactly the numbers I have. Push pins* into the wire 3" and 4" from the tip, and measure your 4.2 volts, now see is you can wiggle the wire and lose your 4.2v. If you don't lose it, the problem is at tip.
*just enough to make the connection, somewhat angled lengthwise to the wire. Clipleads from the pins to the meter. Mikek-- This email is free from viruses and malware because avast! Antivirus protection is active. http://www.avast.com
If it was a high value cable, you could use time domain reflectometry to pinpoint the break.
Usually, passing the current for a car headlamp bulb through the cable will finish off any of the broken strands that are making intermittent contact - if you're lucky the heat will deform the plastic covering and reveal the break.
IME - most wall warts have cheap wire leads, often the copper wire wasn't annealed properly and is brittle - if you fix one break another will be along soon.
On some, you may be able to exorcise the old wire from the strain relief gland and glue a new one in.
You can probably get the replacement leads with a molded power jack on the end - but they're probably same quality as the original!
...unless there is some physical damage along the cable - was the cable squeezed in a door jamb or drawer?
Feel along the cable to see if there are any kinks or unevenness that may be the source of the problem.
-- Peter Bennett, VE7CEI Vancouver BC peterbb (at) telus.net Vancouver Power Squadron: http://vancouver.powersquadron.ca
That's a point. But it isn't likely to happen except under certain circumstances, so it likely still comes third.
Michael
---
I have a similar problem, but it's with the mate between two connectors going hi-Z just sitting there with signal going through them.
My solution, since it's easy to get to and there's nothing critical affected by the break, is just to give the cable connector a good wiggle while it's in its receptacle.
Works every time. For a time...
Good one. Thanks to everyone else who replied too. I'm still fiddling with this.
The problems are: this is the charger for a really good bike headlight; I think I've had it too long to justify taking it back to the shop & complaining; one end of the cable is sealed into the wall-wart; the wall-wart itself is sealed up (no screws) so opening it up & getting it back together again will be difficult at best.
-- You know, there are many people in the country today who, through no fault of their own, are sane. Some of them were born sane. Some of them became sane later in their lives. --? Graham Chapman
I've tried to be pretty careful with it (& generally I'm pretty good about not leaving cables where they can get pinched that way).
-- I look back with the greatest pleasure to the kindness and hospitality I met with in Yorkshire, where I spent some of the happiest years of my life. --- Sabine Baring-Gould
Slightly tricky, they almost never glue those things closed such that they can't be opened non-destructively*. The preferred method is stressing the seam until it releases. eg squeeze it in a vise, clamp, or multi-gips, tap it with a small hammer, as a last resort chuck it at a concrete floor.
it should come open mostly intact and be able to be glued shut (eg: cyanoacrylate) with only a little damage visible. (or taped shut while you test the verair)
If the fault is at the other end those soft molded shells come of fairly easy with pincers and/or a good sharp knife. (this is destructive, you'll need to use heatshrink or similar to finish the repair|)
(*) Commodore PSU's for C=64 etc were an exception, with the case entirely filled with a thermoset potting compound. This made repairs destructive to the case and tricky.
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I don't remember a problem getting that case open (but perhaps I've forgotten) but I certainly remember the potting once it was open.
Michael
Sometimes I've got lucky whacking it sharply with a heavy plastic handled screwdriver, you go around the seam increasing the amount of force in small increments until a gap starts to part.
These days many aren't glued - they're plastic welded.
A jig is used to support the case halves together and ultrasonic vibration applied at either 20kHz or 40kHz, the movement is just a few microns but it rubs the plastic parts together so they get hot and weld.
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