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Kerala Plus Two Electronics (DHSE) study materials
Kerala Plus Two Electronics (DHSE) study resources

Kerala Plus Two Electronics (DHSE): Electronic Instruments, Measurements and Troubleshooting

Kerala Plus Two Electronics (DHSE) · Work through your exam, one topic at a time.

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Electronic Instruments, Measurements and Troubleshooting

Electronic Instruments, Measurements and Troubleshooting tests CRO measurements, multimeter use, bridge balance, measurement error and systematic fault-finding. Numerical questions often use oscilloscope settings or bridge balance conditions.

Core concepts

  • CRO: the CRT produces an electron beam; the vertical amplifier drives vertical deflection, the time base drives horizontal deflection, and triggering stabilises the display. Voltage is divisions × volts/division × probe factor; frequency is 1/T, where T = divisions × time/division.
  • Lissajous figures compare frequencies; a straight line or ellipse indicates related frequencies and phase.
  • Meters: an ammeter is connected in series and has low resistance; a voltmeter is connected in parallel and has high resistance. A DMM’s high input impedance reduces loading error. Accuracy, precision, resolution and sensitivity are different specifications.
  • Bridges: Wheatstone bridge measures resistance; Maxwell bridge measures inductance; Schering bridge measures capacitance. Balance is indicated by a null detector, not a maximum reading.
  • Troubleshooting: visual inspection, half-splitting, signal tracing, DC rail checks, continuity with power off, and safe probing. Compare expected and measured values at test points.

Worked example

A CRO displays a sine wave 4 divisions peak-to-peak with 2 V/div and a 10× probe. Actual Vpp = 4 × 2 × 10 = 80 V. One cycle occupies 5 divisions at 1 ms/div, so T = 5 ms and f = 1/0.005 = 200 Hz. For a sine wave, Vrms = Vpp/(2√2) = 80/2.828 ≈ 28.3 V.

Traps that cost marks

  • Dividing by the probe factor instead of multiplying by it.
  • Reporting peak-to-peak voltage as rms voltage without converting.
  • Using time/div × divisions as frequency instead of taking the reciprocal.
  • Treating bridge balance as a maximum meter deflection rather than a null.
  • Connecting an ammeter across a supply or a voltmeter in series with a load.
  • Measuring resistance or continuity while the circuit is powered.