The Digital Multimeter (DMM) is the first instrument one buys in this trade. I started out with a 20000 ohms per volt analog meter that was only so accurate. As you get more enthused, better and better test equipment starts to find its way onto your bench. So, right now, I have about 10 DMMs that sport the nifty 4 wire ohms connection. This feature lets you measure low resistance and not have to worry about the resistance of your probe leads.
When you read the manuals for the meters, they talk about 4 wire measurements, but have little info on how to do it practically. I have only seen a few vendors who make 4 wire alligator clips and they are usually priced in the less than $100 range. Fluke has a trick 4 in 2 connection for their latest DMMs, but I didn't get one for my 8845A yet. I used to take standard probes and solder extra "sense" wires to the tips. Then I plug those into the sense inputs on the meter. This works well, but I always wanted some gold-plated, two halves connected by a rubber band type 4 wire alligator clips.
Recently, I ran into an issue using a TDR to measure a solder down differential probe cable. We were getting reproducibility issues. I thought of other methodologies to electrically measure the length of a cable. The wonderful HP (AKA Agilent) Time Interval Counter came to mind. That is the HP 5370B. This speedy machine measures 20 pS time intervals in a single shot. I thought I might be able to resolve under 1 mm cable length variations. One end of the cables has coaxial RF connections, the other a tiny circuit board with two vias at the very end. The TDR was used I think because we couldn't disturb the PCB or the UUT would be ruined.
To solve the connection issue, I reverted to a simple clothespin -- the kind where you squeeze it, the jaws open, and you clamp your clothes to a rope.
I put a medium file in the jaw opening and filed two flat surfaces. Then I installed copper foil on each half. For the cable experiment, I also glued an SMA connector to the clothespin. I was able to measure about 8 nS for electron transit time from one end to the other. We ended up sticking with the TDR test method. But the little clothespin looked like a nifty, cheap, 4 wire alligator. Just add a wire to each halve of the jaw, and the connection is made through the component lead. Cost is very low.
This entry was submitted by Steve Lindberg and edited by Rob Spiegel.
Steve Lindberg has loved electronics since he received his first Weller soldering gun when he was 12. He has 35 years of experience in test, debug, and design.
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