The DIAC, or Diode for Alternating Current, is a bidirectional trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. When this occurs, the resistance of the diode abruptly decreases, leading to a sharp decrease in the voltage drop across the diode and, usually, a sharp increase in current flow through the diode. The diode remains "in conduction" until the current flow through it drops below a value characteristic for the device, called the holding current. Below this value, the diode switches back to its high-resistance (non-conducting) state. When used in AC applications this automatically happens when the current reverses polarity.
The behavior is typically the same for both directions of current flow. Most DIACs have a breakdown voltage around 30 V. In this way, their behavior is somewhat similar to (but much more precisely controlled and taking place at lower voltages than) a neon lamp.
DIACs are a form of thyristor but without a gate electrode. They are typically used for triggering both thyristors and TRIACs - a bidirectional member of the thyristor family. Because of this common usage, many TRIACs contain a built-in DIAC in series with the TRIAC's "gate" terminal. |
