EETC Circuit Design & Troubleshooting 5 — Questions and Answers
Question 1: What is the function of a bypass capacitor placed close to an IC's power supply pin?
- To block DC power from entering the IC
- To filter high-frequency noise on the supply rail and provide instantaneous current to the IC (Correct answer)
- To increase the operating voltage of the IC
- To set the operating frequency of the IC
Correct answer: To filter high-frequency noise on the supply rail and provide instantaneous current to the IC
Bypass capacitors (typically 0.1µF ceramic) suppress high-frequency noise on the supply line and act as a local charge reservoir for fast current transients.
Question 2: A technician measures the voltage across each component in a series circuit: R1=5V, R2=7V, R3=4V. What should the supply voltage measure?
- 7V
- 9V
- 16V (Correct answer)
- 20V
Correct answer: 16V
By Kirchhoff's Voltage Law, the supply voltage equals the sum of all individual voltage drops: 5 + 7 + 4 = 16V.
Question 3: Which oscilloscope measurement is most useful for diagnosing a phase shift between input and output signals in an amplifier?
- DC offset measurement
- Lissajous figure (X-Y mode) (Correct answer)
- Frequency spectrum in FFT mode
- Rise time measurement
Correct answer: Lissajous figure (X-Y mode)
An oscilloscope in X-Y mode displays a Lissajous figure whose shape (circle, ellipse, or line) directly indicates the phase relationship between two signals.
Question 4: What is the primary cause of skin effect in AC conductors, and how does it affect circuit design at high frequencies?
- Current concentrates at the conductor's center, increasing resistance
- Current concentrates near the conductor's surface, increasing effective resistance (Correct answer)
- Current distributes uniformly, decreasing inductance
- Voltage drops across the conductor's outer layer, increasing capacitance
Correct answer: Current concentrates near the conductor's surface, increasing effective resistance
At high frequencies, eddy currents force AC current to flow near the conductor surface, reducing effective cross-sectional area and increasing resistance.
Question 5: In a transistor biasing circuit, if the base-emitter junction measures 0V instead of the typical 0.6–0.7V, what has most likely occurred?
- The transistor is saturated
- The transistor's B-E junction is open (failed) (Correct answer)
- The collector load resistor is too large
- The power supply voltage is too high
Correct answer: The transistor's B-E junction is open (failed)
A functional silicon BJT base-emitter junction drops approximately 0.6–0.7V; 0V across this junction indicates the junction is open (no current path exists).
Question 6: When designing a circuit for operation in an environment with significant vibration, which capacitor type should generally be avoided?
- Tantalum electrolytic capacitors
- Ceramic disc capacitors
- Large electrolytic capacitors with leads (Correct answer)
- Film capacitors
Correct answer: Large electrolytic capacitors with leads
Large through-hole electrolytic capacitors with leads are susceptible to mechanical failure from vibration fatigue at the lead-body junction.
Question 7: A technician is troubleshooting a switching power supply that produces output voltage but with excessive high-frequency noise. What is the most likely component to inspect first?
- The input fuse
- The switching transistor gate driver
- The output filter inductor and capacitors (Correct answer)
- The primary transformer winding
Correct answer: The output filter inductor and capacitors
Excessive high-frequency noise on the output of a switching supply typically indicates a failed or degraded output filter capacitor or a saturated output inductor.
EETC Circuit Design & Troubleshooting 5
What is the function of a bypass capacitor placed close to an IC's power supply pin?