CST Electrical Systems & Mechanical Components 2 — Questions and Answers
Question 1: What is the primary function of a shunt resistor in a weighing system's electrical circuit?
- To amplify the load cell signal
- To provide a parallel current path for measurement (Correct answer)
- To filter AC noise from the signal
- To regulate supply voltage to the load cell
Correct answer: To provide a parallel current path for measurement
A shunt resistor provides a parallel current path that allows measurement of current without interrupting the main circuit.
Question 2: Which condition would cause a scale's analog-to-digital converter (ADC) to produce clipping errors?
- Input signal within the ADC's rated range
- Input signal exceeding the ADC's reference voltage (Correct answer)
- Insufficient grounding of the chassis
- Excessive cable length between load cell and indicator
Correct answer: Input signal exceeding the ADC's reference voltage
Clipping occurs when the input signal voltage exceeds the ADC's reference voltage, causing the converter to max out at its upper limit.
Question 3: A 4-wire load cell connection differs from a 6-wire connection in that it:
- Includes separate sense lines to compensate for cable resistance
- Uses higher excitation voltage for better resolution
- Lacks remote sense lines to correct for lead wire resistance drops (Correct answer)
- Provides redundant signal paths for fault tolerance
Correct answer: Lacks remote sense lines to correct for lead wire resistance drops
A 4-wire connection cannot compensate for voltage drops across lead wires, whereas a 6-wire connection uses remote sense lines to detect and correct this error.
Question 4: When testing insulation resistance on a scale's wiring harness, a reading below which value typically indicates a failure requiring repair?
- 1 MΩ (Correct answer)
- 10 kΩ
- 100 Ω
- 500 kΩ
Correct answer: 1 MΩ
Insulation resistance below 1 MΩ indicates degraded insulation that can allow leakage currents to corrupt the measurement signal.
Question 5: What mechanical component absorbs shock loads and prevents overloading of the load cell in a platform scale?
- Check rod
- Overload stop (Correct answer)
- Flexure plate
- Summing board
Correct answer: Overload stop
Overload stops mechanically limit the deflection of the weighing platform, protecting the load cell from forces exceeding its rated capacity.
Question 6: In a strain-gauge load cell, the Wheatstone bridge becomes unbalanced when:
- Excitation voltage is removed
- All four gauges experience equal strain
- Applied load causes differential resistance changes in the gauges (Correct answer)
- The ambient temperature drops below freezing
Correct answer: Applied load causes differential resistance changes in the gauges
Load application causes tension and compression gauges to change resistance in opposite directions, creating a differential voltage across the bridge output.
Question 7: Which grounding method is recommended to prevent ground loops in a multi-load-cell scale system?
- Connect all load cell shields to individual earth grounds
- Use a single-point star ground topology (Correct answer)
- Float all signal commons from chassis ground
- Connect chassis ground to the AC power neutral
Correct answer: Use a single-point star ground topology
Star grounding connects all shields and signal commons to one central point, eliminating the potential differences that cause ground loop currents.
CST Electrical Systems & Mechanical Components 2
What is the primary function of a shunt resistor in a weighing system's electrical circuit?