The zero crossing of the thyristor voltage from a negative to a positive value is known as a natural ignition point. Different from a diode, a thyristor is able to remain in the off-state beyond the natural ignition point as long as it is not triggered by a gate current pulse. The thyristor is initially blocked in the forward direction. After passing the natural ignition point, the thyristor can be triggered into the on-state, and a load current starts flowing. 

The basic idea is that with a variable delay of the firing pulse the average load voltage can be controlled without losses. Under the assumption that the holding current level of the thyristor is exceeded, the load current keeps flowing after the removal of the firing pulse, and only stops flowing just before the zero-crossing of the current through the thyristor is reached. The thyristor cannot be directly switched off. In order to switch off the thyristor, external networks have to force the thyristor current to cross zero.

• Change resistance or inductance.
• Set a different triggering angle by changing the signal 'ref'. If 'ref' crosses the triangular signal 'sig', the firing pulse triggers the thyristor into the on-state.
• Drag the time slider from left to right. After passing the trigger point, the trigger pulse is animated. After triggering, the thyristor is in the on-state and the load current keeps flowing.