Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?
The correct answer is B: Ensure that the circuit is not powered. When measuring in-circuit resistance with an ohmmeter, you must ensure that the circuit is not powered (turned off and disconnected from power sources). Ohmmeters apply their own voltage to measure resistance, and external voltage from a powered circuit can damage the meter, give incorrect readings, or create dangerous conditions. For amateur radio operators, this is a critical safety and accuracy requirement. Always disconnect power and discharge capacitors before measuring resistance in circuits.
Exam Tip
In-circuit resistance = circuit must be unpowered. Think 'R'esistance measurement = 'R'emove power. Ohmmeters apply their own voltage - external voltage can damage the meter or give wrong readings. Always disconnect power first.
Memory Aid
"In-circuit resistance = circuit unpowered. Think 'R'esistance = 'R'emove power. Ohmmeters apply their own voltage - external voltage damages the meter. Always disconnect power before measuring resistance."
Real-World Application
You want to measure the resistance of a resistor in your power supply circuit. First, you disconnect the power supply from AC power and wait for capacitors to discharge. Only then do you connect the ohmmeter. If you tried to measure with power applied, the external voltage could damage your meter and give completely wrong readings.
Key Concepts
Why Other Options Are Wrong
Option A: Incorrect. Ensuring applied voltages are correct doesn't help - you shouldn't have any applied voltage when measuring resistance. The circuit must be unpowered.
Option C: Incorrect. Grounding the circuit doesn't address the main issue - the circuit must be unpowered. Grounding alone isn't sufficient.
Option D: Incorrect. Operating frequency is irrelevant for resistance measurement - resistance is a DC measurement. The circuit must be unpowered, not operating at any frequency.
题目解析
The correct answer is B: Ensure that the circuit is not powered. When measuring in-circuit resistance with an ohmmeter, you must ensure that the circuit is not powered (turned off and disconnected from power sources). Ohmmeters apply their own voltage to measure resistance, and external voltage from a powered circuit can damage the meter, give incorrect readings, or create dangerous conditions. For amateur radio operators, this is a critical safety and accuracy requirement. Always disconnect power and discharge capacitors before measuring resistance in circuits.
考试技巧
In-circuit resistance = circuit must be unpowered. Think 'R'esistance measurement = 'R'emove power. Ohmmeters apply their own voltage - external voltage can damage the meter or give wrong readings. Always disconnect power first.
记忆口诀
In-circuit resistance = circuit unpowered. Think 'R'esistance = 'R'emove power. Ohmmeters apply their own voltage - external voltage damages the meter. Always disconnect power before measuring resistance.
实际应用示例
You want to measure the resistance of a resistor in your power supply circuit. First, you disconnect the power supply from AC power and wait for capacitors to discharge. Only then do you connect the ohmmeter. If you tried to measure with power applied, the external voltage could damage your meter and give completely wrong readings.
错误选项分析
Option A: Incorrect. Ensuring applied voltages are correct doesn't help - you shouldn't have any applied voltage when measuring resistance. The circuit must be unpowered. Option C: Incorrect. Grounding the circuit doesn't address the main issue - the circuit must be unpowered. Grounding alone isn't sufficient. Option D: Incorrect. Operating frequency is irrelevant for resistance measurement - resistance is a DC measurement. The circuit must be unpowered, not operating at any frequency.
知识点
Ohmmeter safety, In-circuit resistance measurement, Circuit power, Measurement safety
Verified Content
Question from official FCC Technician Class question pool. Explanation reviewed by licensed amateur radio operators.