DIAC-TRjzjdjsjdjdhehshdIAC-AC REGULATORS.pdf

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1. 4EL01 : Power Electronics
BY : PROF. A.A.DAIYA
1
BY: PROF A.A.DAIYA 10/3/2025

2.  Introduction
DIAC ( Bidirectional Diode Thyristor)
TRIAC ( Bidirectional Triode Thyristor)
Single Phase AC regulators ( With TRIAC AND
DIAC)
 LASCR
Power MOSFETS
IGBT ( Insulated Gate Bipolar Transistor)

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BY: PROF A.A.DAIYA 10/3/2025

3. The proper triggering of a thyristor requires that the source
of the trigger-signal should supply adequate gate current
and voltage, without exceeding the thyristor gate ratings

The basic requirements for the successful firing of a thyristor :

The current supplied to the gate should:
(i)be of adequate amplitude and sufficiently short rise time.
(ii) be of adequate duration.
(iii) occur at a time when the main circuit conditions are
favourable to conduction.
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BY: PROF A.A.DAIYA 10/3/2025

4. 4
 Power semiconductor device is the most important in
Power electronics system. The evolution of power
electronics has followed the evolution of devices.

The progress in power electronics today has been
possible primarily due to advances in power electronics
devices.

As the evolution of new devices continued, the voltage
and current ratings and electrical characteristics of the
devices have improved dramatically.






BY: PROF A.A.DAIYA 10/3/2025

5. 5
A Diac is a two termincal, bidirectional avalanche diode
which can be switched from the off-state to the on-state for
either polarity of applied voltage.

The schematic construction, and circuit symbol of the Diac
are shown in Figure
BY: PROF A.A.DAIYA 10/3/2025
Notice that two terminals
are labeled as T1 and T2 ,
instead of anode and
cathode

The term DIAC is obtained
from
Diode that can work on a.c.

6. 6
Conduction occurs in the
Diac when the breakover
voltage is reached in either
polarity across the two
terminals.

When T1 is positive with
respect to T2, and if voltage
V12 exceeds VBO1 , then the
structure PNPN conducts.

BY: PROF A.A.DAIYA 10/3/2025
Similarly, when terminal T2 is positive with respect to T1
and if voltage V21 exceeds breakover voltage VBO2 , the
structure PNPN conducts.
At voltages less than the breakover voltage, a very small
amount of current called the leakage current flows through
the device.

7. 7
The device remains
practically in non-conducting
mode shown by region OA in
the characteristics. It is called
the blocking state.

At point ‘A’, when the voltage
level reaches the breakover
voltage, the device starts
conducting. During its
conduction, the device
exhibits negative resistance
characteristics.

BY: PROF A.A.DAIYA 10/3/2025
This portion of the characteristic shown by AB in
characteristics is known as the conduction state.

8. 8
The characteristic obtained
in the third quadrant will be
a replica of that obtained in
the first quadrant. This is
because the doping level is
same at the two junctions of
the device.

Once the device starts
conducting, the current
flowing through it is very
high which has to be limited
by some external.

The value of breakover
voltage for a commonly used
Diac type ST2 is 30 volts.

BY: PROF A.A.DAIYA 10/3/2025

9. 9


Diac is mainly used as a trigger device for Triacs which
require either positive or negative gate pulses to turn ON.

In fact, matched Diac-Triac pairs are available in the
market for various types of control circuits.


BY: PROF A.A.DAIYA 10/3/2025

10. 10


We have seen that the conventional thyristor, or SCR, has a
reverse-blocking characteristic that prevents current flow in
the cathode-to-anode direction.

 However, there are many applications, particularly in
a.c.circuits, where bidirectional conduction is required.

 Two thyristors may be connected in inverse parallel, but at
moderate power levels the two antiparallel thyristors can be
integrated into a single device structure. This device is
commonly known as TRIAC.
BY: PROF A.A.DAIYA 10/3/2025

11. 11
BY: PROF A.A.DAIYA 10/3/2025
As the Triac can conduct in both the directions, the terms
anode and cathode are not applicable to Triac. Its three
terminals are usually designated as main terminals, MT1,
MT2 and gate G, as in a thyristor.

12. 12
 The terminal MT1 is the reference point for measurement of
voltages and currents.

The first quadrant is the region wherein MT2 is positive with
respect to MT1 and vice-versa for the third quadrant.

 A gate current of specified amplitude of either polarity will
trigger the Triac into conduction in either quadrant


BY: PROF A.A.DAIYA 10/3/2025

13. 13

When the gate current is positive
with respect to MT1, gate current
flows mainly from the gate lead to the
terminal MT1 through the P2 –- N2
junction, as shown in Fig.

The main structure which ultimately
turns ON through regenerative
action is P1 N1 P2 N2.


BY: PROF A.A.DAIYA 10/3/2025
1.MT2 positive, positive gate current (Mode 1)

14. 14

When the terminal MT2 is positive
and gate terminal is negative with
respect to terminal MT1, gate
current flows through P2 –N3
Junction. and this gate current IG
forward biases the gate junction
P2 –N3 of the auxiliary P1 N1 P2 N3
structure.

A current is thus established in layer
P2 from left to right which forward
biased P2 N2 junction and finally the
main structure
P1 N1 P2 N2 begins to conduct.

This mode is less sensitive and
therefore, more gate current is
required.




BY: PROF A.A.DAIYA 10/3/2025
2. MT2 positive, negative gate current (Mode 2)

15. 15

The main structure that leads to
turn-on is P2 N1 P1 N4 with N2
acting as a remote gate as shown
in Fig.

The main structure which
ultimately turns ON through
regenerative action is P2 N1 P1 N4 .

As the Triac is turned ON by
remote gate N2, the device is less
sensitive in the third-quadrant
with positive gate current



BY: PROF A.A.DAIYA 10/3/2025
3. MT2 négative, positive gate current (Mode 3)

16. 16

In this mode of operation, N3 acts
as a gate. The external gate
current IG forward biases P2 N3
junction

The device is more sensitive in this
mode.




BY: PROF A.A.DAIYA 10/3/2025
4. MT2 negative, negative gate current (Mode 4)

17. 17

From the above four operating modes of Triac, it becomes clear
that the sensitivity of the Triac is greatest in the first quadrant
when turned ON with positive gate current and also in the third
quadrant when turned ON with negative gate current.

The sensitivity of the Triac is slightly lower in the first-quadrant
when turned ON with negative gate current. Further, the Triac is
much less sensitive in the third-quadrant with the positive gate
current.

Thus, the Triac is rarely operated in the first-quadrant with
negative gate current and in the third quadrant with positive gate
current.

The Triac finds widespread use in consumer and light industrial
appliances operating from 50 or 60 Hz a.c. supplies at moderate
power levels.



BY: PROF A.A.DAIYA 10/3/2025

18. 18
A Triac is equivalent to a pair of antiparallel connected
SCRs. we shall discuss the advantages and disadvantages of
the Triac over an SCR.

Advantages:

Triacs can be triggered with positive or negative polarity
voltages.
total space required is more for SCRS.
Simplifies the circuit.

Disadvantages:

Triacs have low dv/dt rating compared to SCRs.
SCRs are available in larger rating compared to Triacs.
Since a Triac can be triggered in either direction, a trigger
circuit with Triac needs careful consideration.
Reliability of Triacs is less than that of SCRs.




BY: PROF A.A.DAIYA 10/3/2025

19. 19
By connecting a reverse parallel pair of thyristors or Triac
between a.c. supply and load, the voltage applied to the load
can be controlled. This type of power controller is known as an
a.c. voltage controller or a.c. regulators.



BY: PROF A.A.DAIYA 10/3/2025

20. 20

Therefore, a.c. voltage regulators converts fixed mains voltage
directly to variable alternating voltage without a change in the
frequency.

Thyristor and Triac a.c. regulators are used in most of the
applications because of high efficiency, compact size and
flexibility in control

The important applications where a.c. voltage controllers are
widely used are: speed control of induction motors, domestic
and industrial heating, light controls etc.


BY: PROF A.A.DAIYA 10/3/2025

21. 21
The versatility of the Triac
and the simplicity of its use
make it ideal for ac regulator.

During the positive half-cycle
(when P is positive), the Triac
requires a positive gate signal
for turning it ON. This is
provided by the capacitor C
when its voltage is above the
breakdown voltage of the
Diac. The capacitor
discharges through the Triac
gate.

When the Triac triggers, the
voltage drop across PQ will
be zero and the capacitor
voltage will be reset to zero.



BY: PROF A.A.DAIYA 10/3/2025

22. 22
A similar operation takes
place in the negative half-
cycle, and a negative gate
pulse will be applied when
the Diac breaks down in
the reverse direction.


The firing angle can be
controlled by varying the
resistance R and hence the
firing angle can be
controlled




BY: PROF A.A.DAIYA 10/3/2025

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BY: PROF A.A.DAIYA 10/3/2025

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BY: PROF A.A.DAIYA 10/3/2025

25. 25
A single-phase a.c. regulator feeds power to a resistive load of
4 Ω from 230 V, 50 Hz source. Determine rms output ac
voltage for α = 30 degree.



BY: PROF A.A.DAIYA 10/3/2025

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BY: PROF A.A.DAIYA 10/3/2025