Measurements and Instrumentation
What this domain tests
This section tests instrument construction and application: PMMC and moving-iron meters, shunts and multipliers, bridges, CRO operation, digital meters, transducers, and measurement error. Expect practical range-extension and bridge-balance calculations.
Core concepts
- A PMMC meter responds to average current and works only on DC unless a rectifier is added. A moving-iron meter responds to RMS and can measure AC or DC.
- A shunt extends current range: Rsh = ImRm/(I - Im). A series multiplier extends voltage range: Rm = (V/Im) - Rmeter.
- Wheatstone bridge measures resistance; Maxwell bridge measures inductance; Wien bridge measures frequency or capacitance. Balance occurs when the detector current is zero.
- A CRO displays voltage versus time. The timebase setting gives period; vertical scale gives amplitude. Probes may have 10× attenuation.
- Accuracy is closeness to true value; precision is repeatability. Loading effect occurs when an instrument draws current from the circuit.
Worked example
A PMMC meter has full-scale current 1 mA and resistance 100 ohm. To extend its range to 10 mA, the shunt carries 9 mA at full scale. Rsh = ImRm/(I - Im) = (1 mA × 100 ohm)/9 mA = 11.11 ohm.
Traps
Do not use a PMMC meter directly on AC without a rectifier. Remember that a 10× probe divides the displayed voltage by 10 before the scope input. Bridge balance is a null condition, not a maximum reading. Avoid confusing accuracy with precision.
Key points
- PMMC: average/Dc; moving iron: RMS.
- Shunt extends current range.
- Multiplier extends voltage range.
- Wheatstone bridge measures resistance.
- CRO timebase gives period; vertical scale gives voltage.