UJT as relaxation oscillator:

The UJT relaxation oscillator is shown in fig. it consist of a UJT and a capacitor C which is charged through a variable resistance R when VBB is switched ON. R1 and R2 are the external resistances such that R1 << R2, R1 is less than 50. The inclusion of these resistances provides spike waveforms. Here the negative resistance region of the UJT is used. The discharge of the capacitor through UJT develops a saw tooth output as shown in fig. when the circuit is switched on, the capacitor c starts charging through R. during charging period, the voltage across capacitor increases in an exponential relation till it reaches the peak point voltage VB. At this instant of time UJT switches to its slow resistances conducting mode. So, the capacitor small due to small valued of R1 so C discharges abruptly. As the capacitor voltage condenser stars charging again and the next cycle begins.