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 So you have a connection & Musings

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Join date : 2010-11-22

PostSubject: So you have a connection & Musings   September 5th 2014, 8:20 pm

This is one of those " I shoulda realized that" type of hints.
OK so you have a bad wire, maybe in a cable bundle, or is it a single wire and you think the connection is intermittent.

In today's technology, the DVM's are great for measuring resistance or intermittents, or are they?
Lets see- Hmmm. The DVM puts out little voltage and a miniscule amount of current. The old VTVM's, 630's, etc. put out a fair amount of current and often a higher voltage. The older ones were much more suitable for putting a current thru an Xistor to find a bad junction, whereas the new ones will find a blatant problem, but not one that appears with higher current flow.

So what do you ask has that to do with bad wiring connections?

Well, a low current and voltage through a wire and the connection -might- find an obvious problem, but for many of the PITA connection problems, toss the new meters into the garbage. An older VOM might also find it- maybe.

But the sure fire method ( OK, well 95+ % of the time- satisfied?) is to put an automotive tail light bulb in the end of the wire to ground and run say, 12V through the wire. Ac or DC- no matter. ( No, a LED won't work. No current draw to speak of).

A bad connection will rapidly heat up with the high current the tail lite bulb needs and the connection will become pretty obvious if its in the end of a cable at the connector. You might even find the end of the wire HOT from the current trying to go through a high resistance connection. (Have you not seen a burned wire connection at a tail light housing where the lamp developed a corroded connection and the wire got too hot and burned off at the connector or melted the lamp socket?).
Think of all those RCA patch cables you tossed when you couldn't find the bad end, or the guitar cables.
This WILL find the problem.

What if the break is in the middle of the cable? Well, feel along the cable and "massage it" as you go. If the lamp lights, you are in the immediate area of the fault. If the cable gets hot, you are likely right on top of the break.
" Massaging" the ends of the cable with a lamp load will show up the fault at the end of the cable to the connector too. Many end and internal breaks will momentarily light up as you shove the wire ends together during "Massaging" the wire.

You need a lamp that will draw a high current to test with. I have even used an automotive headlight at times.
Just remember to see if the wire gauge will handle the current needed by the lamp.

This test is not 100% fool proof, but when you are trying to find the fault in coax,( Like corroded connectors or splices) and you are up in the top of a 150' fir tree, it is awesome. it beats climbing up and down several times.

This works on coax, wire pairs, multi-strand cables, etc.

Its cheap and easy. Go to the wrecking yard and get a used bulb and socket if you are too cheap to buy a new bulb and replacement socket. Hint- buy a "bulk" box of 10 bulbs at the auto parts store generally for less than you pay for a 2 pk. in blister packaging- this gives you spares to carry in your car too. Oreilly's Auto Parts is great for this sort of deal. Check around. (Some places won't sell you anything but a 2 pk and have the same pricing on a 10 PK as you pay for 5 blister packs of 2- so check before you buy.)
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Cliff Jones
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Cliff Jones

Join date : 2010-11-22

PostSubject: Re: So you have a connection & Musings   October 7th 2014, 2:03 pm

That is a very good method I haven't used (yet) and practical too!
1. Another way is to use a TDR (time domain reflectometer) which is ghastly expensive to own.

2. Another method for shorts, is to get a wire gauge chart showing resistance values per 1000 feet and using a Ohm-meter, some have a low ohms scale which can be of good help for larger diameter wires.

Assuming that the cable uses all the same gage wires, the resistance value read would be multiplied by 2 and since it would involve 2 wires (unless the cable were shielded and shorted to, then that's a different can of worms to tackle with) to give the distance by resistance reading.

3. There are a couple of other methods using an O'scope and/or/with  a signal generator to find the short like the first method I mentioned.

4. Also you can use a resistance metered bridge, these use a couple of different methods to find a short, one is called a  Murray and Varley Loop Test for convenient location of line faults in power, telephone, telegraph, and signal cables and such as "shorts" and "grounds"

5. Then you could also use a infrared scope detector to locate power shorts and High resistance connections in power circuits.

I have used resistance to check multi-connector military shipboard cables running upwards of thousand of feet long with multiple junction boxes in between and over 50 wires each.

6. There is a RF detector the size of a pen that can be used with a signal also.

7. I have an old two flashlight cell navy noise generator that was used to trace out unknown wires. It uses a chattering relay to send RF noise ( Called Hash) down wires for detection.
8. And lastly a couple of sound powered phones to listen to each wire until your work partner connects to the same wire on His end to identify. Then you tag each wire in turn with a alpha-numeric number and make a chart and diagrams for wiring or rewiring cable runs.

I'm a Science Thinker, Radio Tinkerer, and all around good guy. Just ask Me!
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Join date : 2010-11-22

PostSubject: Re: So you have a connection & Musings   October 8th 2014, 5:49 pm

If you want a TDR, I have a quad trace storage scope, the software, and the TDR for it.
Just let me know.
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Join date : 2012-07-06

PostSubject: Re: So you have a connection & Musings   October 21st 2014, 10:09 pm

A TDR, eh? Makes me wonder if you once worked at that rather large aircraft manufacturer here in Washington state. In my days of working liaison engineering, this was something that was hauled out, now and then.

Getting back to FrankB's OP, this makes me think of a time, long ago, when I worked as an auto mechanic. All manner of test equipment was available for diagnosing electrical trouble but the one, simple and convenient tool used the most was a high quality 12V test lamp.

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