Diacs are often used in conjunction with triacs to provide symmetrical triggering.
Diac Figure 21 Figure 21 (a) shows the schematic symbol of a diac , and Figure 21 (b) shows the bidirectional symbol.
Diac Constructions Figure 22 Shows the diac internal constructions.
Diac Construction Neither terminal is referred to as cathode Contain 2 anodes, anode 1(electrode 1) and anode 2(electrode 2) When anode 1 is positive, the applicable layers are p 1 n 2 p 2 and n 3 When anode 2 is positive, the applicable layers are p 2 n 2 p 1 and n 1
Diac Figure 23 Shows the diac characteristic curve.
Diac Basic Operation From A1 to A2 Q1 & Q2 forward-biased Q3 & Q4 reverse-biased Operate on the upper right portion of the characteristic curve.
Diac Basic Operation From A2 to A1 Q3 & Q4 forward-biased Q1 & Q2 reverse-biased Operate on the lower left portion of the characteristic curve.
Diac Applications Trigger circuit for the Triac Proximity sensor circuit
Triac A triac is a bi-directional thyristor used to control the power in ac circuits. It is like a diac with a gate control or equivalent to two SCRs in parallel. A triac has two leads designated M T1 , and M T2 or A 1 and A 2 . It can conduct current in either direction when it is trigger ON (depending on polarity of voltage across it terminal A 1 and A 2 ). A triac has a gate lead which is used to control its conduction,which can be turned on by a pulse of gate current and does not require the breakover voltage to initiate conduction.
Triac Figure 24 The schematic symbol for a triac is shown in Figure 24 (a). There are two anode terminals, A 2 and A 1 , and a gate lead. The triac is the equivalent of two SCRs connected in parallel, as shown in Figure 24 (b).
Triac Figure 25 A 2 Gate Mode + + 1 + − 2 − + 3 − − 4 Figure 25 shows the four operating modes for a triac. When the anode 2 (A 2 ) terminal is positive, the triac can be turned on with either a positive or negative gate voltage. When the anode 2 (A 2 ) terminal is negative, the triac can be turned on with either a positive or negative gate voltage.
Triac Figure 26 Figure 26 shows a very effective way to provide a wide range of control over load current. R 1 -C 1 and R 2 -C 2 provide the required phase shift necessary for full control of the load current.
Triac Construction Neither terminal is referred to as cathode Contain 2 anodes, anode 1(electrode 1) and anode 2(electrode 2) & 1 gate terminal.
Triac Characteristic Curve
Current in direction depending on the polarity across the terminal. It turn OFF when the current drop sufficient low level. Breakover potential decrease as the gate current increase. Triac Characteristic Curve
Triac Basic Operation Terminal A1 is biased positive with respect to A2
Triac Basic Operation Terminal A2 is biased positive with respect to A1.
Triac Typical Packages and Pin
- Fall '15
- SCR, SCRs, basic operation