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.