basic features and applications of multimeter and CRO

basic features and applications of multimeter and CRO

1. What They Are (two essential lab instruments)

Multimeter = handheld/portable instrument that measures $V$ (AC/DC), $I$ (AC/DC), $R$, and often $C$, frequency, diode, continuity with one device by rotary switch selecting function/range. Modern digital multimeter (DMM) uses ADC and LCD.

CRO (Cathode Ray Oscilloscope) = bench instrument that displays $v(t)$ vs time as waveform on screen (CRT or LCD), allowing to see shape, frequency, amplitude, phase, distortion. It has electron beam deflected by $X$ (time base) and $Y$ (signal) plates.

2. Basic Features

Feature	Multimeter (DMM)	CRO (DSO)
Measured quantities	DCV (200mV–1000V), ACV (RMS), DCA/ACA (µA–10A), Ω (Ω–MΩ), continuity beep, diode, $h_{FE}$, $C$, $f$ on some	Voltage vs time ($v(t)$), frequency via period, phase via Lissajous, rise time, FFT on DSO
Input impedance	10 MΩ on $V$ (high, loads little)	**1 MΩ
Resolution	3½ digit (1999) to 4½ (19999)	8–12 bit ADC, bandwidth (20 MHz–1 GHz)
Display	Digits	Waveform
Use mode	Connect parallel for $V$, series for $I$ (with break), power off for $R$	Connect probe across signal and ground clip

3. Applications (exam list)

Instrument	Typical Uses
Multimeter	Check continuity (beep), measure battery $V$, resistor value, diode forward $V_{γ}$, mains $V_{rms}$, current through load, fault finding (open/short)
CRO	See sine/square waveform, measure $V_m$, $f=1/T$, phase between $v$ and $i$ (two channels), rise time of pulse, check op-amp clipping, ripple in power supply, modulation, Lissajous for frequency ratio

4. How to Use (brief steps)

• Multimeter DCV: selector to DCV 20V, red to +, black to GND, parallel across component.
• Resistance: power off, selector to Ω, across resistor.
• CRO: CH1 probe to signal, ground clip to ground, set Volts/div (e.g., 1V/div) and Time/div (e.g., 1ms/div) to get 2–3 cycles stable via trigger.

5. Worked Checks

Need to measure 230V RMS mains: DMM on ACV 750V ⇒ reads ~230.

Need to see ripple on 12V DC supply: CRO AC coupling, 0.5V/div ⇒ ripple ~0.5V p-p visible.

Need to check diode: DMM diode mode shows ~0.7V forward, OL reverse.

CRO frequency: 2 cycles in 4 divisions at Time/div 1ms ⇒ T=2ms ⇒ f=500Hz.

6. Exam Points

• List features (DMM measures $V,I,R$; CRO displays $v(t)$) — 5 marks, very frequent (almost certain to ask to compare).
• State DMM 10 MΩ input and CRO 1 MΩ probe — 3 marks, frequent.
• Give 3 applications each — 5 marks, very frequent.
• Mention how to connect (parallel for $V$, series for $I$) — 3-5 marks, frequent.