Milli Ohm/Micro Ohm Short Finder
Some times you find your self with a short in a circuit. This device is great for pin pointing the short.
A few years ago I ran across this application note AN106 on the Maxim Semiconductor site. http://pdfserv.maxim-ic.com/en/an/AN106.pdf Local Copy.
I had the parts to build it setting around for a while. It went together quickly, and my initial tests show it to work very well.
To test it I took 2 pieces of solder about 3 feet long and used thumb tacks to hold them about 1/16 of and inch apart on the work bench. I could short them out by just placing a small piece of metal across the two pieces of solder. You could see the resistance change very easily as you got closer to the short. For another test I had a shorted coax jumper that I had set aside a while back. It is about 2 feet long. By putting the current source on the end, and taking my measurement from the far end I easily determined which end the short was on. Took the bad end off and sure enough there was a small piece of braid that was touching the center conductor.
I did not have a 1% .1 Ohm resistor for R4, so my 1 Amp range is only at about 900mA, Next part order I will order a 1%. This does not effect its use as a short finder, but I might as well set it up accurately.
A few notes on the build. I did not have a 2.4M resistor (R1) and substituted a 2 resistors in parallel to get my 2.4M
R2 was replaced with a 10 turn 50K trip pot that I had on hand in series with a 68K resistor. I did this to make the unit easy to calibrate.
The case is something I had laying around.
I had the parts to build it setting around for a while. It went together quickly, and my initial tests show it to work very well.
To test it I took 2 pieces of solder about 3 feet long and used thumb tacks to hold them about 1/16 of and inch apart on the work bench. I could short them out by just placing a small piece of metal across the two pieces of solder. You could see the resistance change very easily as you got closer to the short. For another test I had a shorted coax jumper that I had set aside a while back. It is about 2 feet long. By putting the current source on the end, and taking my measurement from the far end I easily determined which end the short was on. Took the bad end off and sure enough there was a small piece of braid that was touching the center conductor.
I did not have a 1% .1 Ohm resistor for R4, so my 1 Amp range is only at about 900mA, Next part order I will order a 1%. This does not effect its use as a short finder, but I might as well set it up accurately.
A few notes on the build. I did not have a 2.4M resistor (R1) and substituted a 2 resistors in parallel to get my 2.4M
R2 was replaced with a 10 turn 50K trip pot that I had on hand in series with a 68K resistor. I did this to make the unit easy to calibrate.
The case is something I had laying around.
I call this the Johnny 5 finder after the robot in the movie Short Circuit.
I also found this article Resistive Dummy Load Draws Constant Current From 1.2 To 50 V Also from maxim that uses much of the same circuit. This looks very interesting also, and a DC load is on my list of equipment to build.
1-15-2010 I found this much simpler to build unit based on a voltage regulator which measures 0.001 up to 1.999 Ohms. I have not built it, but thought it was a very interesting design.
Here is a link to the article, and a local copy also.
73 de KB9JJA
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