DIAC as PAC
DIAC as PAC:
Fig: Phase angle controller circuit for a single-phase ac regulator using DIAC
Fig: Waveforms (a) Input voltage (b) voltage across capacitor
- A Diac is equivalent to two diodes connected back to back. Also, it is a bidirectional device, in contrast to the diode, which is a unidirectional device, having reverse blocking characteristic, preventing the flow of current from Cathode to Anode.
- So, when it (diac) is in conduction mode, current flows in both directions (forward and reverse). This switching device is called as DIAC (Diode AC switch).
- The two terminals of the diac are designated as T1 and T2. These are similar to the terminals, A (Anode) and K (Cathode), of the diode.
- The diac conducts, when the break-over voltage is reached in either polarity across its two terminals.
- WhenT1 is positive with respect to T2, and if at that time if the voltage,V12 exceeds VBO1 (break-over voltage), the diac conducts in positive direction from to T1 to T2.
- Similarly, when T2 is positive with respect to T1, and if at that time if the voltage,V21 exceeds VBO2 (break-over voltage), the diac conducts in negative direction from T2 to T1 .
- So, a diac can conduct in both directions (positive and negative), whereas a diode conducts only in positive direction from Anode (A) to Cathode (K), if, at that time, the voltage, VAK exceeds VBO (break-over voltage).
- A diode does not conduct in the negative direction, if the VAK voltage, is negative.
- A diode turns off (non-conducting mode), if the current through it, falls below holding current. Similarly, a diac turns off (non-conducting mode), if the magnitude of the current, irrespective of its direction, falls below holding current.
- If the V-I characteristic of diode is known, the V-I characteristic of diac, on the lines of the triac can be developed.
- The diac is symmetrical, unlike the triac. So, the diac can be connected in opposite direction, with T1 in place of T2 , and vice versa, i.e.,T2 ,T1 in place of T1 . But the operation here is described with the connection.
- The triac is not symmetrical, though it conducts in both directions like diac.
- Two reasons are: the presence of third terminal, Gate (G), and the gate signal to be fed between G & MT1 (reference) for triggering.
- The snubber part (Rs & Cs ) is used for the protection of the triac – the power switching device. The remaining part, including the diac used for triggering of the triac, is the controller for the triac.