In electronics a relaxation oscillator is a nonlinear electronic oscillator circuit that produces a nonsinusoidal repetitive output signal, such as a triangle wave or square wave. The circuit consists of a feedback loop containing a switching device such as a transistor, comparator, relay, op amp, or a negative resistance device like a tunnel diode, that repetitively charges a capacitor or inductor through a resistance until it reaches a threshold level, then discharges it again. The period of the oscillator depends on the time constant of the capacitor or inductor circuit. The active device switches abruptly between charging and discharging modes, and thus produces a discontinuously changing repetitive waveform. This contrasts with the other type of electronic oscillator, the harmonic or linear oscillator, which uses an amplifier with feedback to excite resonant oscillations in a resonator, producing a sine wave.
Relaxation oscillators are used to produce low frequency signals for applications such as blinking lights (turn signals) and electronic beepers and in voltage controlled oscillators (VCOs), inverters and switching power supplies, dual-slope analog to digital converters, and function generators.
The term relaxation oscillator is also applied to dynamical systems in many diverse areas of science that produce nonlinear oscillations and can be analyzed using the same mathematical model as electronic relaxation oscillators. For example, geothermal geysers, networks of firing nerve cells, thermostat controlled heating systems, coupled chemical reactions, the beating human heart, earthquakes, the squeaking of chalk on a blackboard, the cyclic populations of predator and prey animals, and gene activation systems have been modeled as relaxation oscillators. Relaxation oscillations are characterized by two alternating processes on different time scales: a long relaxation period during which the system approaches an equilibrium point, alternating with a short impulsive period in which the equilibrium point shifts. The period of a relaxation oscillator is mainly determined by the relaxation time constant. Relaxation oscillations are a type of limit cycle and are studied in nonlinear control theory.
Electronic relaxation oscillators
The first relaxation oscillator circuit, the astable multivibrator, was invented by Henri Abraham and Eugene Bloch using vacuum tubes during World War I. Balthasar van der Pol first distinguished relaxation oscillations from harmonic oscillations, originated the term "relaxation oscillator", and derived the first mathematical model of a relaxation oscillator, the influential Van der Pol oscillator model, in 1920. Van der Pol borrowed the term relaxation from mechanics; the discharge of the capacitor is analogous to the process of stress relaxation, the gradual disappearance of deformation and return to equilibrium in an inelastic medium. Relaxation oscillators can be divided into two classes Sawtooth, sweep, or flyback oscillator: In this type the energy storage capacitor is charged slowly but discharged rapidly, essentially instantly, by a short circuit through the switching device. Thus there is only one "ramp" in the output waveform which takes up virtually the entire period. The voltage across the capacitor approximates a sawtooth wave, while the current through the switching device is a sequence of short pulses. Astable multivibrator: In this type the capacitor is both charged and discharged slowly through a resistor, so the output waveform consists of two parts, an increasing ramp and a decreasing ramp. The voltage across the capacitor approximates a triangle waveform, while the current through the switching device approximates a square wave. Before the advent of microelectronics, simple relaxation oscillators often used a negative resistance device with hysteresis such as a thyratron tube, neon lamp, or unijunction transistor, however today they are more often built with dedicated integrated circuits such as the 555 timer chip.