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T7D10

PRACTICAL CIRCUITS · Using basic test instruments: voltmeter, ammeter, and ohmmeter; Soldering

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What reading indicates that an ohmmeter is connected across a large, discharged capacitor?

  1. AIncreasing resistance with time
  2. BDecreasing resistance with time
  3. CSteady full-scale reading
  4. DAlternating between open and short circuit
Why An ohmmeter works by pushing a small current from its internal battery through the component and measuring the voltage. A discharged capacitor initially looks like a short, so a lot of current flows and the meter reads near zero ohms. As the capacitor charges up toward the meter's battery voltage, the current tapers off and the indicated resistance climbs toward infinity (open circuit). That rising reading is the classic sign you are looking at a large capacitor.
Watch out The choice saying resistance decreases with time has the charging curve backwards; current starts high and falls, so resistance starts low and rises. A steady full-scale reading would mean an already charged capacitor or a simple open circuit.
Discharged cap starts as a short and ends as an open, so the ohms reading only goes up.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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The rest of T7D

  1. T7D01 Which instrument would you use to measure electric potential?
  2. T7D02 How is a voltmeter connected to a component to measure applied voltage?
  3. T7D03 When configured to measure current, how is a multimeter connected to a component?
  4. T7D04 Which instrument is used to measure electric current?
  5. T7D05 How does an ohmmeter measure the resistance of a circuit or component?
  6. T7D06 Which of the following can damage a multimeter?
  7. T7D07 Which of the following measurements are made using a multimeter?
  8. T7D08 Which of the following types of solder should not be used for radio and electronic applications?
  9. T7D09 What is the characteristic appearance of a cold tin-lead solder joint?
  10. T7D10 What reading indicates that an ohmmeter is connected across a large, discharged capacitor?
  11. T7D11 Which of the following precautions should be taken when measuring in-circuit resistance with an ohmmeter?
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