Electronic Measurements, Instrumentation and Practical Applications
What this domain tests
Meter construction and range extension, the CRO and its measurements, bridges and transducers, and correct use of instruments in practical circuits. Shunt and multiplier numericals are standard.
Core rules
- Moving-coil (PMMC) instruments respond to average current, work on DC only, and have a uniform scale because deflection is proportional to current. Moving-iron instruments work on AC and DC but have a non-uniform scale.
- Ammeter range extension uses a low-resistance shunt in parallel: Rsh = IgRg/(I − Ig). Voltmeter range extension uses a high-resistance multiplier in series: Rs = V/Ig − Rg.
- Wattmeter: current coil in series with the load, potential coil in parallel; reading equals VI cos φ.
- CRO: electron gun, deflection plates and fluorescent screen; the time base sweeps horizontally. Deflection sensitivity is quoted in V/cm; the trace height gives peak-to-peak value. Lissajous figures give the frequency ratio from the horizontal and vertical tangencies.
- Transducers: active ones generate an emf (thermocouple, piezoelectric, photovoltaic), passive ones change a parameter (thermistor, strain gauge, LVDT). Strain gauge gauge factor = ΔR/R ÷ ΔL/L.
- Errors are systematic (instrument bias, calibration) or random; a low-resistance voltmeter loads a high-resistance circuit and reads low.
Worked example
A galvanometer with Rg = 50 Ω and full-scale Ig = 1 mA is to become a 10 mA ammeter. Rsh = (0.001 × 50)/(0.010 − 0.001) = 0.05/0.009 = 5.56 Ω. The same meter becomes a 10 V voltmeter with Rs = 10/0.001 − 50 = 9950 Ω. Note the shunt sits in parallel and the multiplier in series.
Traps that cost marks
- Placing the multiplier in parallel or the shunt in series.
- Reading a CRO amplitude as rms when the graticule gives peak-to-peak.
- Expecting a moving-coil meter to read AC correctly without a rectifier.
- Ignoring voltmeter loading when measuring across a high resistance.