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4-Wire Cable Tester

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Frank Koenig:
Everyone should know how to make a 3-wire cable tester with 2 LEDs. (DC is applied to wire 1 at end A. An LED bridges wire 1 to 2 at end B. Another LED bridges wire 2 to 3 at end A. Wire 3 at end B connects to the DC return.) Both LEDs illuminate only if there are no opens, shorts, or any of the 5 possible miswirings (3! - 1) that exclude connecting a wire directly back to the same end of the cable from which it came.

I was trying to extend this idea to 4-wire cables so that I could make a speaker cable tester (all my speaker cables are 4-wire). There are 23 (4! - 1) possible miswirings that fit the criterion above. The obvious thing to try is to use 3 LEDs and bridge pairs of wires at each end so as to connect all wires in series. It turns out that while this approach detects opens and shorts it will not detect all miswirings, usually missing one of the 23 depending on the specific topology. I believe there is one class of topologies, however, that detects all 23 miswirings, an example of which is illustrated below. Please try to poke holes in it. I may be deluding myself.

Anyone actually wanting to build one of these testers should be mindful that a miswired cable applies reverse voltage to the LEDs and their reverse breakdown voltage must be respected by keeping the open-circuit voltage low or using protection diodes. BTW, there are 40,319 ways to screw up an 8-wire cable  :o

And yes, for those of you who don't find this sort of thing amusing (and sort of elegant), buy one of the dozens of commercially available brute-force cable testers.

--Frank


Andrew Broughton:
The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?

Nathan Riddle:

--- Quote from: Andrew Broughton on July 07, 2019, 08:39:39 PM ---The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?

--- End quote ---

A SR flip-flop would work.

frank kayser:

--- Quote from: Andrew Broughton on July 07, 2019, 08:39:39 PM ---The most important feature of the cheapie Behringer I have is the intermittency tester. I wouldn't buy/build a tester that doesn't have that function. Any idea how to incorporate that feature into your design?

--- End quote ---


I'll second the value of intermittency tester function.  Caught more crab cables with that function.

Steve M Smith:
Here seems like a good place to post my version of an XLR/Phantom Power tester if anyone is interested...




Steve.

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