Rf Parts

Which End Is Up?

Stalking the shorted connector.

by Evert Fruitman W7RXV

The boss handed me a 4-foot coaxial cable and told me to replace the shorted end, Since he wouldn't settle for two 2-fixM cables, one gt>od and one bad, I would have to i igure out which end had the short.

For a few minutes, we talked about using an RC bridge to find a break in a cable, and joked about replacing both ends of the cable* which would shorten it too much. Finally, we came up with the following idea.

Smart Thinking

Supply a fairly heavy, current from a constant current sourcc to the cable and measure the voltage drop at the end. The key here is that the the shorted end will have a lower voltage drop than the other end. The difference would be in millivolts hut easily readable wiih a digital multimeter. Figure I shows the setup.

Apply the current to one end of the cable and connect a digital voltmeter directly to the cable end. Write down the reading; say, 70150 m\\ Then, without changing its setting, connect the current source and the meter to the other end nl the cable and write down that reading, possibly 20-50 mV The low reading indicates the end u ith the least resistance. Unless there is a pin in the cable, or the short is in the middle of the cable, the end showing ihe lowest voltage is the one w ith the problem in the fining.

Sometimes, just applying a he^\ current will burn the short out, especially if it was caused by just one strand of the fine wire used in many coaxial cables. Burning out a strand or two, however, may leave a carbon path on

"The key here is that the the shorted end will have a lower voltage drop than the other end/'

the cable. Even if the short clears up after you hit the cable w ith a couple of amps, redo the end for safety, depending on how you plan to use the cable. Especially take precautions for high frequency, high power applications- For law frequency, low power applications, it may not make much difference.

The Curreni-Lirniting Power Source

Tying a 10-20 volt 2 amp power supply across a shorted cable could damage the power supply, the cable, or both Finding a suitable current source is easier than it sounds, A good laboratory power supply, and many home-brew supplies, have a current-limit adjustment on the front panel. Set it for 1 or 2 amps and proceed as above. If your power supply wilt deliver 1 2 amps, but lacks a current-limiting circuit, add an external ltmiter. A large resistor or even a car headlight will do the job. Car headlights draw about 3 amps each on low beam and about 5 amps each on high beam at 13 volts. They make handy, high power resistors. Figure 2 shows the setup using a load (here, a resistor) to limit current.

While l4 to 4 amp should furnish enough voltage drop to see on some voltmeters, the higher values will be easier to see, and they will more accurately indicate which end of the cable has the short.

Using this method, in minutes we had one cable the correct length and in service, and the shorted fitting in the waste basket. The boss was so pleased that he almost bought us lunch.

Perhaps the next time someone hands you a shorted cable, you wili be able to tell which end is up.BB

Figure I Basic setup to test cable^ Power supply with built-in current-limiting Figure 2. Setup with trie ma! iitrrent-limitinii resistor. For a I amp !2 volt set for f -2 amps\ supply, use WO 25 W resistor or cor headhghL

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