27.17:
Applications of RC Circuits
Relaxation oscillators are one of the applications of RC circuits.
In a neon lamp oscillator circuit, while charging, the lamp acts as an open circuit. Once the voltage across the capacitor reaches a threshold value, the lamp acts like a short circuit, and the capacitor discharges, creating a bright flash.
After discharging, the process repeats, and the time interval between the flashes is controlled by tuning the R and C values. For this reason, they are used as timers for several applications.
In intermittent windshield wipers, the variable resistor adjusts the frequency of the sweeps.
On increasing the resistance, the RC time constant increases, resulting in a longer sweeping time.
A pacemaker is used to correct abnormalities in the heart rhythm by sending electrical signals to the heart.
The circuit consists of a battery, a capacitor, a computer-controlled switch that flips to charge and discharge the capacitor, and a sensor that senses physical activity and breathing.
The RC timer circuit controls the time period between the electrical signals sent to the heart based on feedback from the sensor.
27.17:
Applications of RC Circuits
A relaxation oscillator is one of the applications of RC circuits. A neon lamp relaxation oscillator comprises a capacitor, a resistor, a voltage source, and a lamp. The lamp acts like an open circuit, with infinite resistance until the potential difference across the lamp reaches a specific voltage. At that voltage, the lamp acts like a short circuit with zero resistance, and the capacitor discharges through the lamp, thus producing light. Once the capacitor is fully discharged through the neon lamp, it starts to charge again, and the process repeats.
Relaxation oscillators have many practical uses. They can be used effectively as a timer for intermittent windshield wipers, pacemakers, and strobe lights. They control indicator lights, which flash at a frequency determined by the R and C values. Some models of intermittent windshield wipers use a variable resistor to adjust the interval between the sweeps of the wiper. Increasing the resistance increases the RC time constant, which increases the time between the operation of the wipers. Relaxation oscillators are often used in electronic circuits, where the neon lamp is replaced by a transistor or a device known as a tunnel diode.
Another application is in pacemakers. The heart rate is normally controlled by electrical signals, which cause the heart muscles to contract and pump blood. When the heart rhythm is abnormal, pacemakers can be used to correct this abnormality. Pacemakers have sensors that detect body motion and breathing to increase the heart rate during physical activity, thus meeting the increased need for blood and oxygen. An RC timing circuit can control the time between voltage signals to the heart.
Suggested Reading
- Young, H.D and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco,CA: Pearson. Section 20.4; pages 867-868.
- OpenStax. (2019). University Physics Vol. 2. [Web version]. Retrieved from https://openstax.org/details/books/university-physics-volume-2; section 10.5; pages 471–472.