Electronic Instruments and Measurement
8. Electronic Instruments and Measurement
This domain tests meters, the cathode-ray oscilloscope, and the errors that appear when an instrument loads a circuit.
Meters
- A PMMC movement deflects in proportion to current and reads the average value; it responds only to DC.
- Ammeter: in series, must have very low resistance; range extended by a shunt.
- Voltmeter: in parallel, must have very high resistance; range extended by a series multiplier.
- Rectifier-type meters measure AC; a digital multimeter uses an ADC and displays digits directly.
- Sensitivity is quoted in ohms per volt: a 20 kΩ/V meter on the 10 V range has 200 kΩ input resistance.
CRO
- Blocks: CRT, vertical amplifier, time-base (horizontal) oscillator, trigger circuit, power supply.
- The electron beam is deflected by electrostatic plates; the fluorescent screen shows the trace.
- Measurements: V_peak-to-peak = vertical deflection × volts/div; period = horizontal divisions × time/div; f = 1/T.
- Lissajous figures compare two frequencies.
Worked example
A 1 mA, 100 Ω PMMC movement is to read 10 V full scale. Multiplier R = V/I_fs − R_m = 10/0.001 − 100 = 10 000 − 100 = 9 900 Ω. The voltmeter resistance is then 10 kΩ, which noticeably loads a high-resistance circuit.
Traps
- Connecting an ammeter across a supply (it behaves as a short and may be destroyed).
- Reading a CRO display as rms when it shows peak-to-peak.
- Ignoring the loading effect of a low-resistance voltmeter.
- Forgetting that a PMMC meter reads zero on AC unless a rectifier is added.