Physical structure, schematic symbol and characteristic - SCR, Diac, Triac and UJT. Application of these devices in electronics.

1. INTRODUCTION TO OTHER
ELECTRONIC DEVICES
CHAPTER 6
BY
PN. RUHIYAH NAZIHAH ZAHKAI
ELECTRICAL ENGINEERING DEPARTMENT
POLYTECHNIC SULTAN IDRIS SHAH

2. SILICON CONTROLLED RECTIFIER
(SCR)
 An SCR consist of four layers of alternating P
and N type semiconductor materials.
 As the terminology indicates, the SCR is a
rectifier constructed of silicon material with a
third terminal for control purposes.
 Silicon was chosen because of its high
temperature and power capabilities.

3. PHYSICAL STRUCTURE &
SCHEMATIC DIAGRAM

4. SCR MODES OF OPERATION
 In the normal "off" state, the device restricts
current to the leakage current.
 When the gate-to-cathode voltage exceeds a
certain threshold, the device turns "on" and
conducts current.
 The device will remain in the "on" state even
after gate current is removed so long as current
through the device remains above the holding
current.

5. CONT’D
 Once current falls below the holding current for
an appropriate period of time, the device will
switch "off".
 If the gate is pulsed and the current through the
device is below the holding current, the device
will remain in the "off" state.

6. SCR APPLICATION
 Mainly used in devices where the control of high
power, possibly coupled with high voltage, is
demanded.
 Suitable for use in medium to high-voltage AC
power control applications, such as lamp
dimming, regulators and motor control.
 SCRs and similar devices are used for
rectification of high power AC in high-voltage
direct current power transmission.

9. DIAC
 The DIAC, or 'diode for alternating current', is a
diode that conducts current only after its break-
over voltage has been reached momentarily.
 The diac is basically a two-terminal parallel-
inverse combination of semiconductor layers that
permits triggering in either direction.

10. DIAC SCHEMATIC SYMBOL
A1
A2

11. CONT’D
 DIACs are also called symmetrical trigger
diodes due to the symmetry of their
characteristic curve.
 Because DIACs are bidirectional devices, their
terminals are not labeled as anode and cathode
but as A1 and A2 or MT1 ("Main Terminal") and
MT2.

12. DIAC APPLICATION
 The diacs, because of their symmetrical bidirectional
switching characteristics, are widely used as
triggering devices in triac phase control circuits
employed for lamp dimmer, heat control, universal
motor speed control etc.

16. TRIAC
 TRIAC, from Triode for Alternating Current,
is a genericized tradename for an electronic
component which can conduct current in either
direction when it is triggered (turned on).
 The triac is fundamentally a diac with a gate
terminal for controlling the turn-on conditions of
the bilateral device in either direction.
 For either direction the gate current can control
the action of the device in a manner very similar
to that demonstrated for an SCR.

17. TRIAC SCHEMATIC SYMBOL

18. CONT’D
 It can be triggered by either a positive or a
negative voltage being applied to its gate electrode.
 Once triggered, the device continues to conduct until
the current through it drops below a certain threshold
value, the holding current, such as at the end of a
half-cycle of alternating current(AC) mains power.
 This makes the TRIAC a very convenient switch for
AC circuits, allowing the control of very large power
flows with milliampere-scale control currents.

19. TRIAC APPLICATION
 Low power TRIACs are used in many
applications such as light dimmers, speed
controls for electric fans and other electric
motors.
 In the modern computerized control circuits of
many household small and major appliances.

20. UJT (UNIJUNCTION TRANSISTOR)
 A unijunction transistor (UJT) is
an electronic semiconductor device that has only
one junction.

21.  A slab of lightly doped (increased resistance
characteristic) n-type silicon material has two
base contacts attached to both ends of one
surface and an aluminum rod alloyed to the
opposite surface.

22.  The p-n junction of the device is formed at the
boundary of the aluminum rod and the n-type
silicon slab.
 The single p-n junction accounts for the
terminology unijunction.

23. UJT APPLICATION
 The most important applications of UJTs or
PUTs is to trigger thyristors (SCR, TRIAC, etc.).
 DC voltage can be used to control a UJT circuit
such that the "on-period" increases with an
increase in the DC control voltage. This
application is important for large AC current
control.
 UJTs can also be used to measure magnetic flux.