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Basic Voltmeter Use
12 questions By Tony R. Kuphaldt
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Question 10 of 12
Suppose I were about to measure an unknown voltage with a manual-range voltmeter. This particular voltmeter has several different voltage measurement ranges to choose from:
- • 500 volts
- • 250 volts
- • 100 volts
- • 50 volts
- • 25 volts
- • 10 volts
- • 5 volts
What range would be best to begin with, when first measuring this unknown voltage with the meter? Explain your answer.
Reveal answerBegin by setting the voltmeter to its highest range: 500 volts. Then, see if the movement needle registers anything with the meter leads connected to the circuit. Decide to change the meter’s range based on this first indication.
Notes:I always like to have my students begin their test equipment familiarity by using old-fashioned analog multimeters. Only after they have learned to be proficient with an inexpensive meter do I allow them to use anything better (digital, auto-ranging) in their work. This forces students to appreciate what a “fancy” meter does for them, as well as teach them basic principles of instrument ranging and measurement precision.
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Question 11 of 12
Suppose I needed to test for the presence of DC voltage between all wires connected to this terminal block. What is the fewest number of individual measurements I would have to perform with with a voltmeter in order to test for voltage between all possible wire pair combinations (remember that voltage is always measured between two points!)?
Reveal answer105 voltage measurements are necessary (minimum) to test for voltage between every possible combination of two wires out of 15 wires. Mathematically, the number of two-wire test combinations is specified by the equation:
n ∑ i=1 (i − 1) Where,
n = The number of wires
Challenge question: there is a way to mathematically reduce the expression shown above so that it only contains the variable n, and not i or the “summation” symbol (∑).
Notes:This is an interesting mathematical exercise, to determine the total number of 2-wire combinations resulting from 15 wires. If your students have difficulty determining this number, suggest they try to figure out the total number of 2-wire combinations with a smaller quantity of wires, say four instead of fifteen.
Incidentally, this is a powerful problem-solving technique: simplify the problem into one with smaller quantities, until the solution becomes intuitively obvious, then determine the precise steps needed to arrive at that obvious solution. After that, apply those same steps to the original problem.
The challenge question is actually pre-calculus or calculus level.
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Question 12 of 12
Voltmeters must be connected in parallel with the voltage to be measured:
In order to practically function, an voltmeter must have some finite amount of internal resistance. It is usually a very large amount, but less than infinite. It should be apparent to you that the presence of this resistance will have some effect on the circuit voltage, when compared to the amount of voltage in the circuit without any meter connected, due to the fact that real voltage sources tend to “sag” when subjected to additional loads:
Explain why it is usually safe to ignore the internal resistance of an voltmeter, though, when it is in a circuit. A common term used in electrical engineering to describe this intentional oversight is swamping. In this particular circuit an engineer would say, “The resistance of the light bulb swamps the internal resistance of the voltmeter.”
Reveal answerWhen one quantity “swamps” another, we mean that its effect is huge compared to the effect of the other, so much so that we may safely ignore it in our calculations and still arrive at a reasonably accurate result.
Notes:I have found that the concept of “swamping” is extremely useful when making estimations. To be able to ignore the values of some components allows one to simplify a great many circuits, enabling easier calculations to be performed.
