Diacs Sidacs - A Brief on Discrete Semiconductor Products

Know About Diacs Sidacs : The DIAC, or "diode for alternating current", is a diode that conducts current only after its break over voltage, it has been reached momentarily. When that occurs, the diode enters the region of negative dynamic resistance, leading to a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains "in conduction" until the current through it drops below a value characteristic for the device, called the holding current. Below that value, the diode switches back to its high-resistance (non-conducting) state. That behavior is bidirectional, meaning typically the same for both directions of current.

Most DIACs have a three-layer structure with break over voltage around 30 V. DIACs have no gate electrode, unlike some other thyristors that they are commonly used to trigger, such as TRIACs. Some TRIACs, like Quadratic, contain a built-in DIAC in series with the TRIAC's "gate" terminal for that purpose. DIACs are also called symmetrical trigger diodes due to the symmetry of their characteristic curve. Because DIACs are bidirectional devices, their terminals are not labeled as anode and cathode but as A1 and A2 or MT1 ("Main Terminal") and MT2. The SIDAC is a less common electrically similar device, the difference in naming being determined by the manufacturer. In general, SIDACs have higher break over voltages and current handling.

The SIDAC, or Silicon Diode for Alternating Current, is another member of the thyristor family. Also referred to as a SYDAC (Silicon thyristor for Alternating Current), bi-directional thyristor break over diode, or more simply a bi-directional thyristor diode, it is technically specified as a bilateral voltage triggered switch. Its operation is similar to that of the DIAC, but SIDAC is always a five-layer device with low-voltage drop in latched conducting state, more like a voltage triggered TRIAC without a gate. SIDACs have higher break over voltages and current handling capacities than DIACs, so they can be directly used for switching and not just for triggering of another switching device. The SIDAC remains none conducting until the applied voltage meets or exceeds its rated break over voltage. Once entering this conductive state going through the negative dynamic resistance region, the SIDAC continues to conduct, regardless of voltage, until the applied current falls below its rated holding current. At that point, the SIDAC returns to its initial non-conductive state to begin the cycle once again.

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