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Relaxation oscillator

This is a schematic of a relaxation oscillator. It consists of resistor R and capacitor (which determine the frequency of the oscillator) then transistors T1 and T2 (which are almost in thyiristor configuration) and RL (which is R load that can be speaker or V out can be taken of it). When we look at transistors configuration we see that the only thing that differs this wiring against thyristor is the fact that collector of pnp transistor is not directly connected to the base of npn transistor but it is connected via capacitor. So small current through R (and the base of T1) turns on both transistors and while the capacitor is not full ie. while there is a charging current through it both transistors are in saturation. When cap is full and there is no charging current both transistors turn off and now the base of T1 is inversely polarized. Now Irl=0 and Vo=0 until capacitor discharges. This oscillator gives short pulses of current through RL followed by longer periods of Vo=0 and Irl=0.

The frequency is calculated like this: f=1.4/(R*C)

This is a schematics of a thyristor in most basic configuration. Turning on sw1 for a moment and then off puts the thyristor in a conductive state and the only way to turn it off now is to cut the current through it by turning sw2 off.

When the power is turned on small current trough R is amplified by T1 and then by T2. This puts Vo closer to Vcc and the capacitor begins to charge which instantly puts both transistors in saturation. Because both transistors are in saturation we can assume that the capacitor is directly connected to Vcc and it is charging fastly.It is as if the capacitor is now in parallel with R. When it stops charging that is when current through it is zero suddenly the base of T1 is inversely polarized that is the voltage on base-emitter of T1 is negative (because the cap is charged that way) and both transistors are now turned off. Capacitor is now "discharging" trough R, RL and Vcc and charging to 0.7 V when both transistors are turned on again.