Rocker arm
A relaxation oscillator is an oscillator circuit , more precisely a relaxation oscillator , in which an energy storage device such as a capacitor is continuously charged and, in combination with a component with a falling characteristic , such as a glow lamp or unijunction transistor , is discharged, thereby generating a relaxation oscillation (sawtooth oscillation). Additional circuit components such as analog comparators can be used to derive additional waveforms such as square waves.
history
The first tilting oscillators go back to work from the early 1920s. A glow lamp is connected in parallel to a capacitor C and fed by a DC voltage source via a resistor R.
The function results from the negative differential resistance of the glow lamp between A and B : First of all, the circuit is in the basic state and the capacitor is discharged. It is slowly charged via the resistor until the voltage reaches the level at point A of the characteristic. At this point the glow discharge begins, the glow lamp lights up, and the current through the glow lamp jumps to a value that corresponds to point C. After a very short time the capacitor voltage decreases to the value B . The resistance is so high that the continuous current for point B is not sufficient. The glow discharge in the lamp goes out and the lamp has a high resistance (jump to point D ). The capacitor is then charged again via the resistor until the voltage in point A is reached .
This cycle runs periodically and causes the glow lamp to flash. Depending on the type of glow lamp and the dimensioning of the components, different flashing frequencies can be implemented. The period t is determined with the component values R and C as follows:
with the ignition voltage at the point A and the hold voltage of the glow lamp in the point B . The supply voltage must be higher than the ignition voltage.
Electronic switch
Instead of a glow lamp, every component with a negative characteristic curve, that is the falling characteristic curve section between points B and A in the glow lamp characteristic curve , such as relay tubes , diacs or unijunction transistors in connection with an energy store and suitable dimensioning, can be used as a tilting oscillator in electronic circuits . Relay tubes and the later developed unijunction transistors were replaced by integrated circuits such as the inverting Schmitt trigger . A falling characteristic can also be generated by positive feedback .
First, let the capacitor C be uncharged. Then the output voltage of the Schmitt trigger is + 5 V. The capacitor is charged via the resistor R and when the upper switching threshold of the trigger (approx. 3 V) is exceeded, the output voltage drops to 0 V and the capacitor is discharged via the resistor. After a short time, the value falls below the lower switching threshold, the output jumps back to + 5 V and the period starts again. An almost triangular voltage curve is created at the capacitor, a square voltage curve can be picked up at the output of the Schmitt trigger.
Further electronic oscillating oscillator circuits are based on the widespread IC NE555 .
A direct relative according to the functional principle is the multivibrator , which can be interpreted as two interconnected tilting oscillators.
Individual evidence
- ↑ Stephen Oswald Pearson, Horatio Saint George Anson: Demonstration of Some Electrical Properties of Neon-filled Lamps , Proceedings of the Physical Society of London , Volume 34, No. 1, December 1921, Pages 175 - 176 doi : 10.1088 / 1478-7814 / 34 / 1/435
- ↑ Patent GB201374 : Improvements in advertising and display apparatus. Applied June 5, 1922 , published August 2, 1923 , Applicants: Frederick Walter Baynes, Stephen Oswald Pearson.
- ↑ LED flashes on mains voltage