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The five layers, n1,p1,n2,p2,n3
can
be combined into a single
Triacs and
Diacs
structure to form a new device.
A power
with four
one direction
When
T1device
is positive
with layers
respectconducts
to T2 byinvoltage
greateronly.
than
Bidirectional
device may
be obtained
byon.
connecting two of
VBO
 (p2,n2,p1,n1)
thyristor
will be
these
back-to-back.
If
Reverse
polarity (p1,n2,p2,n3) will be on.
A five-layer
diac is a device without
used to agate
trigger
canother
be designed
semiconductor
for various
power
breakdown
switches.
Current
voltagesand
andvoltage
currentratings
ratings.are
A diac
determined
is a five-layer
by the
gateless
type
of device.
device.
A five-layer device with gate is called triac.( four stages)
Unijunction Transistor (UJT)
Complementary unijunction transistor (CUJT)
Programmable unijunction transistor (PUT)
The UJT has one pn junction and is used mainly as a triggering
device in thyristor circuits and can also be used in oscillator
circuits. The symbol is similar to a JFET. Note the angle of the
emitter. The other terminals are called base 1 and base 2. The
characteristics are quite different than any other transistor.
The resistive equivalent circuit of a UJT shown makes it easier to
understand its operation. The emitter current controls the value of rB1
inversely. The total resistance or interbase resistance (rBB) equals the
sum of rB1 and rB2. The standoff ratio () is the ratio rB1/ rBB.
UJT Operation:
Unijunction transistor can trigger larger thyristors with a pulse at
base B1.With the emitter disconnected, the total resistance RBB, a
datasheet item, is the sum of RB1 and RB2 . RBBO ranges from 412kΩ for different device types. The intrinsic standoff ratio η is the
ratio of RB1 to RBBO. It varies from 0.4 to 0.8 for different devices.
As VE increases, current IE increases up IP at the peak point.
Beyond the peak point, current increases as voltage decreases in the
negative resistance region. The voltage reaches a minimum at the
valley point. The resistance of RB1, the saturation resistance is
lowest at the valley point.
IP and IV, are datasheet parameters; For a 2n2647, IP and IV are 2µA
and 4mA, respectively. VP is the voltage drop across RB1 plus a
0.7V diode drop; VV is estimated to be approximately 10% of VBB.
Peak votage of UJT Vp
Vp=ηVbb +Vd
Complementary unijunction Transistor (CUJT)
Like standard UJT except that the currents and voltages
applied to it are of opposite polarity. Ideal for stable oscillators,
timers, frequency dividers.
Programmable unijunction Transistor (CUJT)
Although it has the same name as a UJT the programmable
unijunction transistor’s structure is not the same. It is actually
more similar to an SCR.
Programmable unijunction Transistor (CUJT)
The PUT can be “programmed” to turn on at a certain voltage by
an external voltage divider. This yields a curve similar to a UJT.
Programmable unijunction Transistor (PUT)
External PUT resistors R1 and R2 replace unijunction transistor
internal resistors RB1 and RB2, respectively. These resistors allow
the calculation of the intrinsic standoff ratio η.
Programmable unijunction Transistor (PUT)
VR is voltage divider (R1 and R2 can be specified)
Vc capacitor voltage
When Vc > VR the PUT will conduct
Programmable unijunction Transistor (PUT)
unijunction Transistor trigger circuits
LASCR (Light Activated SCR)
End of Lecture