Bidirectional trigger diode (DIAC)

The bidirectional trigger diode, also known as a two-terminal AC device (DIAC), came out at the same time as the bidirectional thyristor. Because of its simple structure and low price, it is often used to trigger bidirectional thyristors to form overvoltage protection circuits, timers, etc. The structure, symbol and equivalent circuit of the bidirectional thyristor are shown in Figure 1. It is a three-layer structure, symmetrical two-terminal semiconductor device, which can be equivalent to an NPN transistor with an open base and symmetrical emitter and collector. The forward and reverse volt-ampere characteristics are completely symmetrical, as shown in Figure 2. When the voltage V across the device is less than the forward transition voltage V (BO), it is in a high resistance state; when V>V (BO) it enters the negative resistance region. Similarly, when V exceeds the reverse transition voltage V (BR), the tube can also enter the negative resistance region. The symmetry of the turning voltage is represented by ⊿V(B), ⊿V(B) = V(BO)-V(BR), and generally requires ⊿V(B) <2V. The withstand voltage value (V(BO)) of the bidirectional trigger diode is roughly divided into three levels: 20~60V, 100~150V, and 200~250V. Figure 3 is an overvoltage protection circuit composed of a bidirectional trigger diode and a bidirectional thyristor. When the transient voltage exceeds the V(BO) of DIAC, DIAC turns on and triggers the triac to also turn on, protecting the subsequent load from overvoltage damage. The following describes how to use a megohmmeter and multimeter to check the bidirectional trigger diode. (1) Set the multimeter to R×1k (or R×10). Because the V (BO) value of DIAC is above 20V, the measured forward and reverse resistance values ​​are both infinite. (2) Connect the circuit as shown in Figure 4. The breakdown voltage is provided by a megohmmeter, and the forward transition voltage V (BO) of the DIAC is measured with a DC voltmeter. Then replace the electrodes of DIAC and measure the reverse turning voltage V (BR). Finally, check the symmetry of the transition voltage. Example one: Select the ZC25-3 megger and set the 500 multimeter to the 50VDC range. The trigger diode under test is a DB3 type, its shape is similar to the detection diode, and the tube shell is sky blue. The main parameters are: V (BO) = 35V (typical value), peak pulse current Ipk = 5mA. First, use the R×1k gear to measure the forward and reverse resistances, both of which are infinite. Then measure it twice according to the method shown in Figure 5.9.19: V (BO) = 28.5V, V (BR) = 28.0V. From this, ⊿V(B)=0.5V＜2V is calculated. The forward and reverse transition voltages of this tube are slightly lower than the typical values, but the symmetry of the transition voltage is very good. Example 2: Measure two glass-encapsulated bidirectional trigger diodes with a voltage of 30V (the specific model is not marked), and the measurement data are organized into Table 1. It shows that the performance of the two tested tubes is good. Note: Because the bidirectional trigger diode is symmetrical, any one of the two measured values ​​can be defined as V(BO). Now take the larger value as V(BO) and the smaller value as V(BR).