Relaxation oscillator

A relaxation oscillator is an oscillator circuit, specifically a relaxation oscillator in which an energy storage device such as loading a capacitor continuously and unloaded in combination with a component with a negative slope, such as a neon lamp or Unijunctiontransistor and thereby tilting vibration ( sawtooth wave ) is generated. By additional circuitry such as analog comparators more waveforms such as rectangular wave can be derived.

History

First relaxation oscillator go back to work from the early 1920s. In this case, a glow lamp is connected in parallel with a capacitor C and supplied with a resistance R from a DC voltage source.

The function is derived from the negative differential resistance of the glow lamp between A and B: First, the circuit in the ground state and the capacitor is discharged. It is slowly charged through the resistor until the voltage reaches the level at the point A of the characteristic curve. At this point, the glow discharge begins, the glow discharge lamp is lit, and the current through the glow discharge lamp jumps to a value which corresponds to point C. After a very short time, the capacitor voltage is lowered to the value B. The resistance is such dimensioned that the continuous current for the point B is not sufficient. The glow discharge in the lamp goes out and the lamp becomes high impedance (jump to point D). Subsequently the capacitor is charged again via the resistor up to the reaching of the voltage at point A.

This cycle runs periodically and lets the glow lamp flash. Depending on the type neon lamp and the dimensioning of the components you to realize different flashing frequencies. The period t is determined as with the components values ​​R and C:

With the ignition voltage at the point A and the holding voltage of the neon lamp at point B. The supply voltage must be higher than the ignition voltage.

An electronic circuit

Instead of a neon lamp may be used in electronic circuits of each component with a negative characteristic curve, that is, the falling characteristic curve portion between points B and A in the Glimmlampenkennlinie as relay tubes, diac or unijunction transistor, in conjunction with an energy storage and suitable dimensioning, be used as a relaxation oscillator. Relay tubes and later developed unijunction transistor were replaced by integrated circuits such as the inverting Schmitt trigger.

First, the capacitor C is uncharged. Then, the output voltage of the Schmitt trigger 5 V. The capacitor is charged via the resistance R and if the upper switching threshold of the trigger has been exceeded (for example, 3 V ), tilts the output voltage to 0 V, and the capacitor is discharged via the resistor. After a short time the lower switching threshold is exceeded, the output jumps back to 5 V, and the period starts over. At the condenser, an almost triangular voltage waveform at the output of the Schmitt trigger, a rectangular voltage waveform to be removed.

Further electronic Kippschwingerschaltungen based on the widely used IC NE555.

A direct relative according to the functional principle of the multivibrator, which can be interpreted as two interconnected relaxation oscillator.