Thyristors are electronic devices made up of semiconductor elements that, through internal feedback, produce a commutation, which are often used to control the electrical power of a circuit.
A triac is a semiconductor device, belonging to the thyristor family. Unlike the thyristor previously explained, the triac is a bi-directional device. That is why they are usually applied to control external currents, to act as a static switch, electronic switch, speed controller in electric motors or light dimmers, among others.
The diac or alternating current diode is a double semiconductor instrument with two connections. Being a bi-directional, self-tripping diode, it directs the current when its alternative trip voltage is exceeded and when the circulating current is greater than the triple value of volts of the device. Diacs are often used to autocomplete the varied rhythm of triac firing.