Deep Dive: T7D11
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.
Why Other Answers 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.
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 Example
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.
Source & Coverage
Question Pool: 2022-2026 Question Pool
Subelement: T7D
Reference: 2022-2026 Question Pool · T7 - Practical circuits
Key Concepts
Verified Content
Question from the official FCC Technician Class pool. Explanation reviewed by licensed amateur radio operators and mapped to the T7D topic.