ISCET Test Equipment Operation & Troubleshooting 4 — Questions and Answers
Question 1: A technician uses an ohmmeter to test a capacitor in-circuit and reads near-zero ohms continuously. This indicates:
- The capacitor is open
- The capacitor is shorted (Correct answer)
- The capacitor is within tolerance
- There is a parallel resistive load affecting the reading
Correct answer: The capacitor is shorted
A continuous near-zero ohms reading on a capacitor indicates an internal short circuit rather than the expected brief charging deflection.
Question 2: When using a wattmeter to measure power in a circuit, the wattmeter reads zero even though voltage and current are present. This MOST likely indicates:
- The load is purely reactive (no resistive component) (Correct answer)
- The wattmeter range is set too high
- The current coil is open
- The voltage is below the wattmeter's threshold
Correct answer: The load is purely reactive (no resistive component)
A purely reactive load (purely inductive or capacitive) results in a 90° phase angle between voltage and current, making true power (and thus wattmeter reading) zero.
Question 3: Which test equipment is BEST suited to characterize the frequency response of a bandpass filter?
- Oscilloscope with DC coupling
- Spectrum analyzer in zero-span mode
- Swept frequency network analyzer or tracking generator with spectrum analyzer (Correct answer)
- Digital multimeter in AC voltage mode
Correct answer: Swept frequency network analyzer or tracking generator with spectrum analyzer
A network analyzer or a spectrum analyzer with a tracking generator sweeps a range of frequencies and plots the filter's amplitude response across the band.
Question 4: A current probe for an oscilloscope works on which principle?
- It measures the voltage drop across an internal shunt resistor
- It uses the Hall effect or transformer coupling to sense the magnetic field around a conductor (Correct answer)
- It injects a test current and measures back-EMF
- It measures the electric field capacitively coupled from the wire
Correct answer: It uses the Hall effect or transformer coupling to sense the magnetic field around a conductor
Current probes sense the magnetic field produced by current flow using either a Hall-effect sensor (for DC/AC) or a transformer (AC only), converting it to a voltage displayed by the oscilloscope.
Question 5: A DMM displays 'OL' (overload) when measuring resistance. The technician should FIRST:
- Confirm power is removed from the circuit before measuring (Correct answer)
- Select a lower resistance range
- Replace the DMM fuse
- Use the diode test mode instead
Correct answer: Confirm power is removed from the circuit before measuring
An 'OL' on resistance can occur because voltage in the circuit is driving the meter; always verify power is off and the component is isolated before measuring resistance.
Question 6: What is the purpose of the 'holdoff' control on an oscilloscope?
- It sets the minimum signal amplitude required to trigger
- It delays the start of the next sweep to stabilize complex or repetitive waveforms (Correct answer)
- It adjusts the probe compensation frequency
- It selects the trigger source channel
Correct answer: It delays the start of the next sweep to stabilize complex or repetitive waveforms
Holdoff sets the time after a trigger event during which the oscilloscope will not accept a new trigger, allowing complex waveforms to be displayed stably.
Question 7: When testing a transformer with an ohmmeter, a reading of infinity between the primary and secondary windings indicates:
- A shorted winding
- Normal isolation between windings (Correct answer)
- An open primary winding
- The transformer core is saturated
Correct answer: Normal isolation between windings
Transformers are designed to be electrically isolated between primary and secondary, so infinity (open circuit) between them is normal and expected.
ISCET Test Equipment Operation & Troubleshooting 4
A technician uses an ohmmeter to test a capacitor in-circuit and reads near-zero ohms continuously.
This indicates: