Static V- I Characteristics

A thyristor  works in  three basic  modes

a.Reverse  blocking mode

b.Forwards  blocking  mode and

c.Forward  conducting  mode

In reverse  blocking  mode cathode is made  positive  with respect to anode  and gate  circuit  remains  open as shown  in figure  in this  mode junctions  J₁ and J₃ are seen  to be reverse  biased and junction J₂ is forward  biased. A very  small  current  flows.

                             Figure(a) reverse blocking  mode (b)forwards blocking  mode  (c)forward  conducting mode

An avalanche breakdown  occurs  at junction J₁ and J₃. When  reverse voltage  is increased and  reverse  current  increase  rapidly  as shown in figur.

In forward  blocking  mode anode is positive  with respect to cathode and gate is  opened. Since  the junctions J₁ and J₃ are forward biased whereas junction J₂ is reverse biased. Hence  a small  leakage current  flow  called forward  leakage  current. When  forward  voltage  is increased junction J₂ will have  avalanche  breakdown  at a  voltage named forward  break over voltage  v_Bo.  

In  forward  conduction mode  anode  is made  positive with  respect to  cathode  and gate is  also made  positive with respect  to cathode. As positive  gate  voltage  is applied   between gate and  cathode at a particular  forward  anode  voltage junctions  J₂ comes in  forwards bias  from reverse bias  condition and a  forwards anode  current flow  in the  circuit  with very small  voltage drop  across  the thyristor.