A semiconductor device (thyristor) consisting of two terminals and four layers which, having been properly energized, could conduct and transmit current in any direction, is called diac. Diac is a two-way Breakover triggering device. It is also known as a double diode or bilateral switch or bi-directional trigger (bi-directional diode means a diode that could operate in both directions). This bi-directional switch can be used up to a few hundred volts. Remember that diac is basically a Triac. The only difference is that diac does not have a gate terminal.
Diac consists of two terminals. One of the terminals is called A1, whereas the other is A2. It has five doped regions, as can be seen in figure 6.31 (a). In figure (b) diac’s symbols have also been depicted, which reflect two diodes fixed in inverse directions. Both these four layers of diodes are fitted parallel to each other. Diac has been designed in such a manner that it could trigger triacs, or provide protection against overvoltage. Its rating is reflected through current and voltage.
Figure 6.31 construction and symbol of DIAC
When voltages from any direction or of any polarity are provided parallel to a diac, a diode becomes a forward bias (or becomes ON). While the second diode gets reverse biased or off. Therefore, switching from off to on mode is possible via the provision of voltages of any polarity. In other words, it can be operated as an ON /off switch by providing adequate voltages. If the value of any polarity voltages provided parallel to its terminals is low, it does not conduct (does not work). It works when the voltage value provided to it from any direction is greater than its Breakover voltage (approx. 30 volts). (In other words, diac does not operate until voltages provided to it from any directions reach Breakover voltages). As soon as its voltages provided from any direction exceed breakover voltages (voltages on which diac enters forward or reverse biasing regions), it fires immediately or conducts. Remember that it can break down in any course.
When anode A1 is positive, the current route is P2-N2 P1-N1. Similarly, when anode A2 is positive, current path is P1-N2-P2-N3. In figure 6.32, characteristics of diac have been illustrated through the specific curve. Its shape is just like the English alphabet “Z”. As soon as Diac gets proper value breaking voltages, it starts passing current according to the polarity of voltages received parallel to its terminals. And it remains in conducting mode until the current passing through it gets less than the holding current (i.e. the device turns off when the current decreases from the holding value (IH). Remember that value of anode current, under which a device switches from a forward conduction region to a forward blocking region, is known as holding current. In other words, the current which is required for keeping a four-layer device or switching in on mode, is called holding current?
Figure6.32 diac characteristics curve
As diac consists of bilateral switching characteristics, therefore it is vastly used in a triac control circuit as a triggering device. Furthermore, it is also commonly used for light dimming, speed control of universal motors and heat control, etc.
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