ISCET Component-Level Troubleshooting 2 — Questions and Answers
Question 1: A technician measures voltage across a diode in forward bias and reads 0 V. What is the most likely fault?
- Diode is shorted (Correct answer)
- Diode is open
- Diode is correctly biased
- Supply voltage is too high
Correct answer: Diode is shorted
A shorted diode drops 0 V because it conducts without resistance, unlike a normal forward-biased diode which drops ~0.6–0.7 V.
Question 2: When testing a capacitor with an ohmmeter, the needle deflects then slowly returns to infinity. This indicates the capacitor is:
- Shorted
- Open
- Leaky
- Functional (Correct answer)
Correct answer: Functional
A good capacitor charges from the ohmmeter's battery, causing a momentary deflection, then returns to infinite resistance as the charge stabilizes.
Question 3: Which instrument is best suited for checking the switching speed of a transistor in a digital circuit?
- Digital multimeter
- Oscilloscope (Correct answer)
- LCR meter
- Frequency counter
Correct answer: Oscilloscope
An oscilloscope displays waveform timing and transition characteristics, making it ideal for measuring transistor switching speed.
Question 4: A NPN transistor has VBE = 0 V and VCE = VCC. The transistor is most likely:
- In saturation
- In cutoff (Correct answer)
- Operating normally in active region
- Shorted collector to emitter
Correct answer: In cutoff
With VBE = 0 V, no base current flows, placing the transistor in cutoff, so VCE equals the full supply voltage.
Question 5: A resistor measures 10% above its marked value. The technician should:
- Replace it immediately
- Accept it if within its tolerance rating (Correct answer)
- Increase the supply voltage to compensate
- Solder a parallel resistor to reduce value
Correct answer: Accept it if within its tolerance rating
If the measured deviation falls within the resistor's specified tolerance (e.g., ±10% or ±20%), the component is acceptable.
Question 6: What does a reading of OL (overload) on a DMM set to resistance indicate when measuring a capacitor?
- The capacitor is shorted
- The capacitor is fully charged and appears open (Correct answer)
- The DMM is malfunctioning
- The capacitor value exceeds the range
Correct answer: The capacitor is fully charged and appears open
A healthy capacitor initially charges from the DMM's test current and then presents an open circuit, causing an OL reading.
Question 7: In a series RC circuit, if the capacitor is shorted, what will the voltage across the resistor measure?
- Zero volts
- Full supply voltage (Correct answer)
- Half supply voltage
- The capacitor's rated voltage
Correct answer: Full supply voltage
A shorted capacitor acts as a wire, so all supply voltage drops across the resistor in series with it.
ISCET Component-Level Troubleshooting 2
A technician measures voltage across a diode in forward bias and reads 0 V.
What is the most likely fault?