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Basic Ohmmeter Use
10 questions By Tony R. Kuphaldt
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Question 7 of 10
A technician picks up a resistor with the following color bands:
Color code: Org, Wht, Blu, Gld
Having forgotten the resistor color code, and being too lazy to research the color code in a book, the technician decides to simply measure its resistance with an ohmmeter. Holding one lead of the resistor and one test lead of the ohmmeter between the thumb and index finger of the left hand, and the other resistor lead and meter test lead between the thumb and index finger of the right hand (to keep each test lead of the meter in firm contact with the respective leads of the resistor), the technician obtains a resistance measurement of 1.5 MΩ.
What is wrong with the technician’s measurement?
Reveal answerThe resistance measurement is much too low (it should be closer to 39 MΩ) because the technician’s body resistance is in parallel with the resistor.
Notes:This is a very common mistake made by beginning students of electronics: holding resistors improperly when measuring their resistance values. Discuss with your students some alternative methods of maintaining resistor lead contact with the meter probes so that the person’s body does not become part of the resistance being measured.
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Question 8 of 10
What is a megger, and what type of resistance measurements would it typically be used for? Explain how the design of a “megger” differs from that of a normal ohmmeter.
Also, explain how improper use of a “megger” may actually be hazardous, either to the operator of the meter or to the circuit under test.
Reveal answerA “megger” is a high-range ohmmeter, typically used to measure the resistance of wire insulation. They utilize relatively high voltage (upwards of 1000 VDC) to measure resistance.
Follow-up question: what is a hi-pot meter, and what function is it typically used for?
Challenge question: why are special, high-voltage ohmmeters even needed? What’s wrong with using a regular ohmmeter that outputs just a few volts to measure resistance? What does the existence of these specialized ohmmeter types tell us about the nature of electrical resistance?
Notes:“Meggers” are very useful tools, but they can cause damage as well. Your students need to be aware of proper and improper uses of “meggers” before they are entrusted with the use of one.
The challenge question is worth exploring if time permits! In real life, the simplicity of Ohm’s Law is deceptive . . .
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Question 9 of 10
High-precision ohmmeters use a four-wire electrical connection to the resistance under test, rather than a two-wire connection. Explain how the four-wire system works, and why it provides better accuracy than two-wire resistance measurement.
Reveal answerTwo-wire ohmmeters measure not only the resistance under test, but also the added resistance of their own test leads. Four-wire ohmmeters only measure the resistance under test.
Notes:It is one thing to explain how four-wire resistance measurement works, but another to describe how you would set up your own four-wire (“Kelvin”) resistance measurement system using normal two-wire meters. Challenge your students with this problem, to test their comprehension of the concept.
