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http://www.element14.com/community/people/jw0752/blog/2015/05/11/shop-tips--plate-glass
Mind The Wires:
An experiment, that I was conducting a couple weeks ago, was drawing roughly 2 amps. The experiment was to verify that the low "on" DS resistance of a P Ch MOSFET would make a good candidate for polarity protection for a circuit. As the voltmeter that I was using showed the different voltage drops in the circuit something did not add up. I was missing about 3 volts. One of the standard laws of electronics is that the sum of all the voltage drops in a circuit must equal zero. The mystery was not that hard to solve and a quick check showed that each of my power supply hookup wires had a 1.5 volt drop across it.
It occurred to me that these hookup wires were being used on a daily basis, decisions were being made based on circuit measurements, but almost nothing was really known about the important resistance characteristics of the wires. Ideally they were suppose to have zero ohms resistance and it wasn't until my encounter with the missing voltage that I realized this was certainly not the case.
To begin the process of getting better acquainted with the hookup wires they were all laid out on the bench and inspected. Since nearly 55 years have passed since my adventure in electronics began there were a few wires that needed to be discarded. Of the rest, each was inspected and the connectors tightened or re-soldered as necessary. My goal in this project was to measure the resistance of each hookup wire and to tag it so that it would be easy to match sets of similar resistance and to choose appropriate wires for projects that demand different current levels from the power supply.
To make the resistance measurements of the wires I used the simple test setup below:
A circuit was set up with the wire to be tested in series with an ammeter and a load. I like to use an automobile tail / brake light for a load as it will draw one to two amps at 12 volts. The heat energy dissipation can be easily dealt with by placing the bulb on a glass plate. Also, unlike a resistor, the bulb gives a visual indication that it is working properly and warns that it is probably too hot to touch. The next step is to turn the power supply up until the ammeter reads one amp on the ammeter. Finally a voltmeter is placed across the wire that is being tested. The voltage reading of the voltmeter with be a one to one correspondence to the resistance in ohms of the wire under test. Of course the reading of resistance will be only as accurate as the quality that the ammeter and voltmeter permit. This process was repeated for each of the wires in my collection and their individual resistances were recorded.
Now that I had the information for each wire a means of storing it and relating it to the proper wire was needed. It was decided that some plastic strips would be cut from plastic containers and attached to the test wires. Here is how the plastic tags turned out:
The next step was to put the tags onto the wires. It was important to put the smooth shiny side of the plastic tag to the outside of the wire as labels were going to be made and stuck to the plastic tags. If the plastic had a rough surface the labels might not stick as well. Labels were printed up with the measured resistances from before and also some identification. The labels were then applied to the smooth side of each tag. Some of the wires had small ends and easily slipped through the holes in the tags but in a couple cases it was necessary to remove the banana plug or the alligator clip to get the tag on the wire.
With easy access to the resistance data on my power supply hookup wires there should be no excuse for missing voltage in future circuits.
John
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