GopalKrishna Advanced Rural Research Foundation
GARRF Virtual Engineering & Electronics Laboratory
KALAM ZERO RESEARCH FUNDING LAB
EXPERIMENT 18
RLC CIRCUIT
RESONANCE
FREQUENCY RESPONSE
KALAM ZERO RESEARCH FUNDING LAB
A resistor, inductor and capacitor can behave as a remarkable frequency-selective system. At one particular frequency, the inductive and capacitive effects cancel each other.
This phenomenon is called electrical resonance. It is one of the fundamental concepts behind tuning circuits, filters, communication systems, oscillators and many sensing systems.
For a series RLC circuit:
where:
At resonance:
Therefore the resonant frequency is:
At resonance, the reactive components cancel and the impedance of the ideal series RLC circuit becomes approximately equal to R. Therefore the current reaches its maximum value.
Below resonance: Capacitive behaviour dominates.
At resonance: XL = XC. The circuit is purely resistive in the ideal case.
Above resonance: Inductive behaviour dominates.
This means that the resonant frequency depends primarily on inductance and capacitance, while resistance controls the sharpness of the resonance.
The current reaches a maximum close to the resonant frequency. Increasing resistance makes the resonance curve broader and less pronounced.
| Trial | R | L | C | f | Z | Current | Phase |
|---|
Design an RLC circuit whose resonant frequency is approximately 1 kHz.
Experiment with different combinations of L and C and discover how many different component combinations can produce the same resonant frequency.
For:
The resonant frequency is:
At resonance:
For R = 100 Ω and input voltage of 5 V:
For a series RLC circuit, the approximate bandwidth is:
The quality factor is:
A higher Q generally produces a sharper and more selective resonance. A lower Q produces a broader response.
Why might an engineer want a very high-Q circuit in one application but a low-Q circuit in another?
Resonance is particularly important in piezoelectric systems. PZT materials and structures can exhibit strong frequency-dependent responses.
A Structural Health Monitoring system may excite a structure at different frequencies and observe its response. Changes in resonant frequency, amplitude or damping can provide information about changes in the physical system.
Therefore, the simple RLC resonance experiment provides students with a conceptual foundation for understanding much more advanced frequency-response measurements.
Use CHITTI to explore resonance, RLC circuits, filters, oscillators, frequency response, sensors and robotics applications.
“What exactly happens at resonance?”
“Why does current become maximum in a series RLC circuit?”
“How can I design a 1 kHz resonant circuit?”
“What is Q-factor?”
“How is resonance used in PZT sensors?”
In physical RLC experiments, resonant circuits can produce relatively large currents or voltages across reactive components. Use components with suitable voltage, current and power ratings.
Never connect an experimental RLC circuit directly to mains voltage. Use an appropriate isolated laboratory signal source.
The series RLC circuit demonstrates one of the most important concepts in electrical and electronic engineering: resonance.
At resonance, inductive and capacitive reactances cancel. The circuit then has minimum impedance and maximum current for a series RLC configuration.
Students have also seen how resistance influences bandwidth and selectivity through the Q-factor.
A simple RLC circuit can become a powerful frequency-selective engineering system.