GopalKrishna Advanced Rural Research Foundation

GARRF Virtual Engineering & Electronics Laboratory

KALAM ZERO RESEARCH FUNDING LAB

EXPERIMENT 17 RL LOW-PASS FILTER INDUCTOR FREQUENCY RESPONSE

🔬 Experiment 17 — RL Low-Pass Filter & Frequency Response

In the previous experiments, we studied RC filters. We now introduce the inductor and investigate how an RL circuit responds to different frequencies.

An RL low-pass filter uses the frequency-dependent impedance of an inductor to reduce higher-frequency components while allowing lower-frequency components to appear at the output.

🎯 Experiment Objectives
  • Understand the behaviour of an inductor in an AC circuit.
  • Understand inductive reactance.
  • Study an RL low-pass filter.
  • Calculate cutoff frequency.
  • Observe frequency-dependent output voltage.
  • Study the relationship between R, L and cutoff frequency.
  • Understand applications in electronics and signal conditioning.
👨‍🎓 Think Before You Start
  • Why does an inductor resist changes in current?
  • What happens to inductive reactance as frequency increases?
  • Why does the circuit behave differently at low and high frequencies?
  • What happens when the inductance is increased?

🔄 1. Capacitor vs Inductor

Capacitor Inductor
Opposes changes in voltage Opposes changes in current
Reactance decreases as frequency increases Reactance increases as frequency increases
XC = 1/(2πfC) XL = 2πfL
Stores energy in electric field Stores energy in magnetic field
💡 Key Idea

The frequency dependence of the inductor is the fundamental reason that an RL circuit can act as a filter.

📚 2. Theory

Inductive Reactance

XL = 2πfL

As frequency increases, the inductive reactance increases. Therefore, the inductor increasingly opposes AC current at high frequencies.

RL Low-Pass Filter

For the virtual experiment, the output is measured across the resistor.

|H(f)| = R / √(R² + (2πfL)²)

Cutoff Frequency

fc = R / (2πL)

At the cutoff frequency the output magnitude is approximately 70.7% of the input voltage, corresponding to approximately −3 dB.

🧩 3. Virtual Experiment Controls

🟢 RL low-pass filter ready.

🔌 4. Virtual RL Low-Pass Circuit

📊 5. Live Results

Resistance 1.0 kΩ
Inductance 10.0 mH
Cutoff Frequency 15.92 Hz
Input Frequency 100 Hz
Inductive Reactance 6.28 Ω
Output Amplitude 0.157 V
Gain 15.76%
Attenuation −16.05 dB

📈 6. Input & Output Waveforms

The output is measured across the resistor. As frequency increases, the high-frequency output becomes increasingly attenuated.

📊 7. RL Frequency Response

🔬 8. Engineering Analysis

📝 9. Student Observation Table

Trial R L fc Input f XL Output Gain

🎓 10. Experiment Procedure

  1. Set R = 1 kΩ.
  2. Set L = 10 mH.
  3. Calculate the cutoff frequency.
  4. Set the input frequency below the cutoff frequency.
  5. Observe the output voltage.
  6. Increase the frequency gradually.
  7. Observe the reduction in output amplitude.
  8. Set the frequency equal to the cutoff frequency.
  9. Verify the approximately −3 dB response.
  10. Increase the frequency further.
  11. Observe the strong attenuation of high-frequency signals.
  12. Record at least five observations.
⭐ Engineering Challenge

Design an RL low-pass filter having a cutoff frequency of approximately 100 Hz.

Try changing both R and L. Find multiple combinations that produce approximately the same cutoff frequency.

📐 11. Sample Calculation

For:

R = 1 kΩ
L = 10 mH

The cutoff frequency is:

fc = 1000 / (2π × 0.01)
fc ≈ 15.92 Hz

The inductive reactance at 100 Hz is:

# XL 2π × 100 × 0.01 ≈ 6.28 Ω

The output is therefore significantly attenuated because the operating frequency is substantially above the cutoff frequency.

🌍 12. Real-World Applications

🧠 13. Connection to Sensors, Robotics & SHM

Inductors are fundamental electromagnetic components. Their frequency-dependent behaviour is particularly important when electrical systems interact with motors, coils, transformers and electromagnetic interference.

🔬 Research Connection

In advanced sensing and Structural Health Monitoring systems, electrical signals can contain unwanted frequency components. Understanding analogue filtering helps students understand how measurement systems isolate useful information before digital processing or AI analysis.

❓ 14. Student Quiz

Q1. Inductive reactance is:
Q2. In this RL low-pass filter, the output is measured across:
Q3. The cutoff frequency is:
Q4. As frequency increases, inductive reactance:
Q5. The RL low-pass filter mainly attenuates:

🤖 CHITTI

GARRF Robotics & Engineering AI Mentor

Students can use CHITTI to explore inductors, RL circuits, frequency response, filtering, robotics and electromagnetic systems.

💡 Try asking:

“What is inductive reactance?”

“Why does an inductor oppose high-frequency current?”

“How do I design a 100 Hz RL filter?”

“What is the difference between RC and RL filters?”

“Where are inductors used in robotics?”

⚠️ 15. Physical Laboratory Safety

Important:

When reproducing this experiment physically, inductors can store magnetic energy and may generate voltage transients when current changes. Use properly rated components and laboratory equipment.

🎓 16. Experiment Conclusion

The RL low-pass filter demonstrates how the frequency-dependent behaviour of an inductor can be used to control the frequency content of an electrical signal.
XL = 2πfL

fc = R/(2πL)
As frequency increases, inductive reactance increases and the output across the resistor decreases. The experiment introduces students to a second major family of analogue filters and establishes an important foundation for electronics, robotics, instrumentation, EMI control and advanced sensor systems. A resistor and an inductor may look simple — but together they become a frequency-selective engineering system.