Basic Electricity

Basic Ohmmeter Use

10 questions By Tony R. Kuphaldt

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Question 1 of 10

Explain how an ohmmeter is able to measure the resistance of a component (in this case, a light bulb) when there is no battery or other source of power connected to it:

Also, identify the reading you would expect the ohmmeter to indicate if the light bulb were burnt out (failed “open”).

Reveal answer

Ohmmeters always contain a battery or some other internal source of electrical power so that the component under test may be supplied with a small amount of current, in order to measure how hard it is for current to go through. Read how to use an ohmmeter.

If the light bulb were burnt open, the ohmmeter would register an extremely large (infinite) amount of resistance.

Notes:
Unlike voltmeters or ammeters, ohmmeters must contain their own power sources. An implication of this fact is that ohmmeters must never be used to measure the resistance of an energized component. Discuss this important caveat with your students, being sure to ask them to explain why connecting an ohmmeter to an energized component might give erroneous measurements (if it doesn’t destroy the meter first!).

In regard to the ohmmeter reading for an open bulb, I have found that many math-weak students have a difficult time grasping the differentiating zero from infinity. They recognize both as being extreme conditions (nothing versus everything), but many make the mistake of regarding “infinity” as identical to zero. Quite to the contrary, “infinity” means bigger than big, and huger than huge. Do not be surprised if one or more of your students harbor this same misconception, and be ready to address it!
Question 2 of 10

What would be wrong about measuring the resistance of a light bulb while it is being powered by a battery?

Reveal answer

The voltage impressed across the light bulb by the battery will interfere with the ohmmeter’s measurement.

Notes:
This principle is very easy to illustrate, simply by building a “live” circuit and trying to measure the resistance of a known component in that circuit, while energized. Different ohmmeters will, of course, respond differently to this abuse, but none will yield the proper value for that component’s resistance.
Question 3 of 10

Suppose we needed to determine the resistance of the light bulb in this circuit, while energized:

Now, we know we cannot simply connect an ohmmeter to an energized circuit, so how is it possible to obtain the resistance measurement we desire?

Reveal answer

Measure voltage across the light bulb, and current through the light bulb, and then use Ohm’s Law to calculate filament resistance from these measurements.

Challenge question: explain how it would be possible to make these measurements without de-energizing the light bulb (not even once!).

Notes:
Quite often, resistance measurements on a component are not practical because the component cannot be “powered down,” or perhaps because the component’s resistance is different when energized versus when de-energized (incandescent light bulbs being an excellent example of this). Knowing how to derive a resistance measurement from voltage and current measurements is a valuable skill.

The “challenge” question, to measure light bulb voltage and current without breaking the circuit, is also very practical. Once students figure out how to do this (no, using an inductive ammeter is not an acceptable answer!), it would be a great classroom activity to actually build such a circuit and have students practice the technique on it.