CST Electrical Systems & Mechanical Components 3 — Questions and Answers
Question 1: What is the purpose of a junction (summing) box in a multi-load-cell platform scale?
- To convert the analog signal to digital before the indicator
- To parallel-connect load cells and trim outputs for equal contribution (Correct answer)
- To provide individual power supplies to each load cell
- To isolate each load cell from AC electrical interference
Correct answer: To parallel-connect load cells and trim outputs for equal contribution
A summing box connects multiple load cells in parallel and uses trim resistors to balance each cell's contribution to the total signal.
Question 2: A technician measures zero balance on a new load cell and finds it is -0.3 mV/V. This indicates:
- The load cell is defective and must be replaced
- A normal condition within typical ±1% rated output tolerance (Correct answer)
- The excitation polarity is reversed
- The load cell is operating beyond its temperature range
Correct answer: A normal condition within typical ±1% rated output tolerance
Zero balance offsets of ±1% of rated output are standard and are corrected by the indicator's zero calibration function.
Question 3: Which mechanical symptom most likely indicates worn or damaged check rods on a vehicle scale?
- Erratic readings that change with wind or vibration
- Consistent positive offset at all weights
- Lateral movement of the deck when a vehicle drives on at an angle (Correct answer)
- Sudden drop in load cell output voltage
Correct answer: Lateral movement of the deck when a vehicle drives on at an angle
Check rods (or stay rods) prevent lateral and longitudinal deck movement; wear allows the deck to shift sideways under off-axis loading.
Question 4: When using an ohmmeter to check a load cell, a technician measures 0 Ω between the signal+ and signal− terminals. This most likely indicates:
- A properly balanced Wheatstone bridge
- A short circuit within the load cell bridge (Correct answer)
- Normal resistance for a 350 Ω load cell
- An open circuit in the excitation wiring
Correct answer: A short circuit within the load cell bridge
Signal terminals should show the bridge output resistance (typically 350 Ω or higher); 0 Ω indicates a short across the output arms of the bridge.
Question 5: Electrostatic discharge (ESD) protection in a weighing instrument is most effectively achieved by:
- Using longer cable runs to dissipate charge
- Properly bonding the instrument chassis to earth ground (Correct answer)
- Installing a 1 kΩ resistor in series with the excitation lines
- Increasing the excitation voltage to overcome static interference
Correct answer: Properly bonding the instrument chassis to earth ground
Bonding the chassis to earth ground provides a low-impedance path for static charges to dissipate safely before reaching sensitive electronics.
Question 6: A flexure-type load cell is preferred over a bending beam type in applications requiring:
- Low-cost installations with ample headroom
- High side-load rejection and minimal off-center sensitivity (Correct answer)
- Maximum capacity in the smallest physical footprint
- Direct contact with corrosive liquids
Correct answer: High side-load rejection and minimal off-center sensitivity
Flexure load cells use guided flexure elements that inherently reject side loads and horizontal forces, making them suitable for precision platform scales.
Question 7: What effect does a broken solder joint on one arm of a Wheatstone bridge have on scale output?
- Zero output regardless of applied load
- Full-scale output with no load applied
- Reduced but non-zero sensitivity and a large zero offset (Correct answer)
- No effect because the bridge has redundant paths
Correct answer: Reduced but non-zero sensitivity and a large zero offset
An open arm in the bridge creates an unbalanced circuit that still produces some output, but with degraded sensitivity and a large offset error.
CST Electrical Systems & Mechanical Components 3
What is the purpose of a junction (summing) box in a multi-load-cell platform scale?