Each straight line can be characterised by an equation based on 2 transistors.
- The breakover or trigger voltage is the same as any power transistor, Vbe and current gain for trigger current,
- the foldback negative resistance depends on gm in two transistors and resulting saturation voltages plus 1 diode drop and Rs or ESR.
- The follow-on positive resistance depends on the power rating and junction size - the Rs or ESR is proportional to Pd power rating of the device. e.g. Rs=k/Pd where k is approximately = 1
- ( the same applies to all diodes and LED's ) with a wide tolerance
- the reverse characteristics are likely similar to transistor PIV breakdown.
Sure, but since the thyristor has different operating conditions there is no single equation.
Like the JFET equation, in which you study differents equations before and after pich-off, in the case of the thyristor you should study the behavior when it is in blocking state, in on state and what happens when during the turn-on and turn-off process.