Explain and perform the measurement of voltage and current by C.R.O. (Cathode Ray Oscilloscope). (90 Marks)

Introduction

The Cathode Ray Oscilloscope (C.R.O.) is a very important electronic measuring instrument used to observe and analyze electrical signals. It displays voltage signals in graphical form (waveform) with respect to time. This makes it extremely useful for electricians in testing, troubleshooting, and maintenance work.

In this practical, we study how to measure voltage directly and current indirectly using a CRO.

Definition

C.R.O. is an electronic device that converts electrical signals into visual waveforms on a fluorescent screen.

Main Parts of CRO

• Cathode Ray Tube (CRT) – Main display unit
• Electron Gun – Produces electron beam
• Deflection Plates – Control beam movement
• Fluorescent Screen – Displays waveform
• Time Base Generator – Provides time axis
• Amplifiers – Strengthen signals
• Power Supply

Working Principle

The CRO works on the principle of electron beam deflection. When voltage is applied, the electron beam moves vertically. The time base moves it horizontally. This creates a waveform showing voltage versus time.

Measurement of Voltage using CRO

Aim

To measure voltage using a CRO.

Apparatus Required

• CRO
• Function Generator / AC Supply
• Connecting Leads

Procedure

1. Switch ON the CRO and allow warm-up.
2. Connect input signal to vertical input (Y-axis).
3. Adjust focus and intensity.
4. Set volt/div and time/div.
5. Observe waveform.
6. Count vertical divisions.
7. Multiply by volt/div value.

Formula

Voltage = Vertical divisions × Volt per division

Example

If waveform height = 5 divisions and volt/div = 2V

Voltage = 5 × 2 = 10V

Measurement of Current using CRO

Aim

To measure current using CRO.

Principle

CRO cannot measure current directly. It measures voltage across a known resistor. Then current is calculated using Ohm's Law.

Formula

I = V / R

Procedure

1. Connect a known resistor in series.
2. Measure voltage across resistor using CRO.
3. Apply formula I = V / R.

Example

Voltage = 12V, Resistance = 6Ω

Current = 12 / 6 = 2A

Advantages

• Accurate measurement
• Displays waveform clearly
• Useful for AC and DC
• Fast response

Disadvantages

• Costly equipment
• Requires training

Applications

• Electronic circuit testing
• Signal analysis
• TV/Radio repair
• Industrial troubleshooting

Precautions

• Do not apply high voltage directly
• Check grounding properly
• Use proper probes
• Handle carefully

Result

Voltage and current were measured successfully using CRO.

Conclusion

The CRO is a powerful instrument for analyzing electrical signals. It helps electricians understand waveform behavior and measure voltage accurately. Current can be measured indirectly using resistance.

Viva Questions

1. What is CRO?
2. What is CRT?
3. How is voltage measured?
4. Why CRO cannot measure current directly?
5. What is time base?
6. What is volt/div?
7. What is waveform?