#24. How do I use a continuity tester? a test light? a volt-ohm meter?
I hate to tell you to read the instructions...but that would be a good place to start. Each one tests for something related but different and will give you different information.
A continuity tester checks to see if two things (two parts, two spots on a circuit) are electrically connected. So, if you touch one probe of the continuity tester to one end of a wire and the other probe to the other end of the wire, the tester should “buzz” (the usual audible output of those units). That tells you the wire is “continuous”. If the tester doesn’t buzz, the wire must have a break in it...no wonder the circuit didn’t work!
So, a continuity tester will tell you if the two things are connected, but it won’t tell you how well they are connected, whether they are connected grounds as opposed to connected power leads or even whether or not there is power there at all. That makes a continuity test great for making sure that two things are NOT connected (such as by a short between them) - no buzz, no connection, no short. It is also a quick way to test whether or not a switch is open/closed, whether a relay is making/breaking, etc. It will tell you connected or not connected, period.
A test light, on the other hand is specifically testing for “hot” power. Usually it will have a clip that gets attached to a ground and a pointed probe that is touched to various spots in the circuit. If the light comes on, that spot is hot. But again, it doesn’t tell you anything else that may be going on or the quality of the circuit or the hot power. In fact, I have seen cases where a test light gave very false information by being an additional “leg” in the circuit, thereby changing the circuit and messing up the test. A pretest diagram might help prevent this type error, too.
So, a test light checks for a hot circuit, which makes it very good for assuring that the power got turned OFF before clipping a wire by mistake, for example. It can also assure you that the power is being turned on/off by a switch or relay. But don’t ask it to tell you the quality of that hot power.
Which brings us to a Volt-Ohm meter, which can tell us voltage and resistance (hence the name!) This allows a much better diagnosis of a circuit because we can get more information. Compare the volt meter feature alone to the continuity tests or test light checks that I described, above. A voltage measurement can better tell the quality of the hot power at any point in a circuit, either compared to ground (zero voltage) as as a voltage drop across a component. But I don’t want to get into details here. This is a case where you must read the directions. They should tell you not only how to use the meter but should also give you some examples to help interpret results. Happy testing!
Steve Watson, Watson's StreetWorks, http://www.watsons-streetworks.com
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