A 'SIDAC', which stands for 'Silicon Diode for Alternating Current', refers to a two-terminal silicon voltage-triggered switching device that can conduct current in two directions. This property makes it a bidirectional thyristor diode.  It is almost identical in operation to the diac, which is another bilateral thyristor diode.

Just like the diac, a SIDAC starts conducting current only when the voltage across it exceeds a certain threshold value.  This value is known as the SIDAC's 'breakover voltage'.  And just like the diac, the SIDAC continues conducting current even if the voltage across it falls below its breakover voltage, as long as the current passing through it exceeds a certain limit known as its 'holding current'.

Once the current flowing through a SIDAC falls below the holding current, it stops conducting.  The voltage across it should exceed the breakover voltage for it to start conducting again.

SIDAC's are distinguished from diacs by the fact that they are designed to handle greater power than the latter, i.e., they generally exhibit higher breakover voltages and can carry larger amounts of current than diacs.

Figure 1.  Photos of SIDAC's in different packages (left) and the symbol for a SIDAC (right)

See Also:   Diac;  Active Components;  More Articles