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Continuity: So Easy to Check, Except When It's Not

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zeeglen
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Re: RF continuity tester
zeeglen   4/9/2014 2:23:57 PM
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@Bill the continuity threshold was about 5 ohms. Of course, your value may differ!

Our lab DMMs buzz at 33 ohms - useless!  So I built a bunch of "milliohm squawkers" for our techs to use.  This changes delta-R of a few milliohms into a variable pitch tone so a reverse-engineering tech can audibly distinguish correct PCB connections in spite of closed relay contacts or low resistance transformer windings.  Watch for it in upcoming EDN Design Ideas.


David Ashton
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Re: One wire try a thousand
David Ashton   4/9/2014 4:12:21 PM
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@Sheepdoll... you can get RS-232 cable testers which do 25 wires at a time.  Or there are companies which make bigger ones....try here....

http://www.cablescan.com/products/products.htm

The small 8-wire network cable testers usually use a CD4017 counter chip to put a + on one wire at a time and use any or all of the other wires for return...so you need at least 2 wires working.  You can use the same technique and a bunch of 74HC154 dedoders to expand that to 16 x n lines.    So you could build yourself a custom cable checker quite cheaply if you don't mind picking up a soldering iron.

Of course, I don't know if would be worth your while doing this - I'm not sure how many pipe-organ axe-murderers there are out there.....

Sheepdoll
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Re: One wire try a thousand
Sheepdoll   4/9/2014 11:58:44 PM
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 @David Ashton - I'm not sure how many pipe-organ axe-murderers there are out there.....

It is more a theoretical exercise. In practice, given the age of the wire it is easier to just make a new cable.

salbayeng
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Re: RF continuity tester
salbayeng   4/10/2014 3:38:31 AM
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Hi 

I just use RF out on the spec analyser, but it's kind of big to lug around. 

Years ago I remember there were hand made testers that generated swept signals around 455kHz or 10.7MHz or 45MHz. You could test all the way from the antenna through to the speaker / TV screen, (AM /FM didn't matter) They relied on leakage of the IF through the front end so you didn't need to tune the radio. (If you have a spec analyserswith a tracking generator you can make a similar test signal)

For locating the other end of a single wire , I just hook up a 20.00MHz crystal oscilator to 4.5v worth of battery and attach the crystal oscillator output to the cable. You can do all this with 3 jumper cables. Then just use a FM radio tuned to 100MHz to follow the wire. You can use any crystal oscillator that has an odd harmonic in the FM band. 

If I need a TDR , I just connect a Tee to the scope input, and put the "1kHz probe comp" signal into one end of the tee through say a 100ohm resistor and connect the other end to the cable. Do a calibration check on a known piece of cable and you are away!.

MeasurementBlues
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Followup story on continuity
MeasurementBlues   4/10/2014 10:24:08 PM
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See DMM Shows Differences in Real, Fake, and Good Substitute Apple Cables checking for continuity turned up some differences.

 

DCH0
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Tic Tracer
DCH0   4/11/2014 2:33:36 PM
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Commonly called a Tic Tracer, Triplett makes a version Sniff-It 2.  With a variable setting, it beeps and flashes a light near an energized circuit.  Beeping becomes more frequent the nearer you get.  Rated for 5V - 600VAC, it is a non contact device great for finding live wires behind dry wall.

I think your time domain reflectometer will bounce signals off of every wire nut connection.

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