UJT as a  relaxation  oscillator

The UJT  is a highly  efficient  switch  it is  used as trigger  a device for SCR. No sinusoidal oscillators saw tooth generators phase control  and timing circuits. Since  UJT  shows negative resistance characteristics it is mostly used  as  a relaxation oscillator.  Relaxation oscillator. Initially the voltage  across capacitor is zero  hence UJT  is off in this condition a small  reverse leakage  current  as a emitter current will be  flowing. As shown  in capacitor  C will  begin  to charge by VBB through  resistor R.