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Theory & working of
• Thyristor is a family of semiconductor devices, i.e, SCR,
Triac, P.U.T, RCT, GTO etc. The oldest member of this
thyristor family is SCR (Silicon Controlled Rectifier). Due
to the vast use, the word thyristor has become
synonymous with it and hence, the term thyristor is
used for SCR.
• Compared to transistors, thyristors have lower on-state
conduction losses, and higher power handling capability.
On the other hand, transistors generally have superior
switching performances in terms of fasting switching
speed and lower switching losses.
• Consider the following circuit and and observe the
switch, Thyristor is just like this switch, before the gate
signal is applied. When a triggering signal is applied, the
switch turns on and current starts to flow.
• An SCR is represented by the following symbol, it has
Anode and Cathode terminals like diode with an
additional terminal known as Gate. Gate is the control
terminal and triggering signals are applied at it. On the
right the V-I caracteristics of a thyristor is shown.
• Clarifying that SCR conducts in 1st quadrant only, when a
gate signal is applied an SCR is made sufficiently forward
biased to cross holding current limit, it starts to conduct.
• Once in conduction state, it continues to conduct even if
the gate signal is removed. Special technics are
employed for turning it off, known as commutation.
• Let us now learn the working of Thyristor, by considering
its internal structure. Thyristor is actually a 4 layered P-N
junction device, with 2 P and 2 N portions. Without the
application of any voltage, it has 3 diffusion regions.
• now, if we apply positive at Anode with respet to
Cathode, the junctions J-1 and J-3 become forward
biased while making junction J-2, reverse biased. It will
remain in this state until a positive signal is applied at
the Gate terminal.
• So, when a positive signal is applied at Gate, the junction J-2
turns to forward biased state, and current starts to flow. On
removal of Gate signal, the current continues to flow as
charge is drifted from Anode to Cathode.
• If we observe the internal structure of thyristor closely, it will
be revealed that it is actually made up of a PNP and NPN
transistor, such that the collector of 1st is connected to the
base of 2nd. Gate is connected to the base of NPN transistor.
• On application of signal, the NPN transistor conducts,
sending a signal to the base of PNP transistor which in
turns conducts and send another signal to the base of
NPN transistor. Hence, the process continues.
• Rectification is the conversion of AC to DC. Here is the
model of uncontrolled full-wave rectification. When
node ‘A’ is positive with respect to node ‘B’, the diode
D1 and D3 conduct.
• The direction of current in load is downwards. During
the negative half cycle of AC, the node ‘A’ becomes
negative with respect to node ‘B’. The diodes D2 and D4
now conduct. The direction of current is again
• Hence, in both cycles, the direction of current in load remains
same. Controlled rectification is the basic principle of DC drives
for which thyristors are used.
• In the bridge configuration, a same pulse is applied on two
thyristors per half cycle of AC. The control angle of pulses
determines the amount of power transferred. Firing angle is
monitored using a separate scheme like PWM.
• Accordig to the following diagram, when node ‘A’ is positive with
respect to node ‘B’, the thyristors T1 and T3 will have forward
biased condition but they will not conduct to any Gate pulse. So,
when T1 and T3 are fired together, current flows through the
• Similary, in the negative half cycle of AC, the thyristors T2 and T4
are fired at the same time which results in conduction. The
resultant output is unidirectional but with fluctuations. You can
clearly observe the dependence of output voltage on the firing
pulses of thyristors.
• This is the basic simulation of a DC drive in which closed-
loop speed control with inner current loop and fiels
weaking has benn shown :
• Let us analyse this diagram. The DC motor is coupled to
a techo-generator whose output is fed to a filter to
reduce harmonics in the current and convert this
current to actual equivalent speed.
• This speed is compared to desired speed.the difference
is then passed through speed controller and current
limiter which convert that difference to equivalent
current and then generate a value of reference current,
• On the other hand, the actual current, that is being fed
to the motor, is measured, filtered; to remove any
harmonics; and then compared to the generated value
of reference current.
• Now this difference is passed through a current
controller which specifies the fired angle according to
the difference and dens a signal to the firing circuit.
• From here, pulses are applied to the controlled rectifiers
at calculated intervals.
• In this way, we control the amount of power applied to
the armature of motor and hence control its speed.
Below the base speed, the motor speed in controlled via
inner current loop. While, above the base speed, the
same is controlled using the field weakening.
• For the field control, first, the e.m.f generated by motor,
is calculated. Now this value is compared with the
reference e.m.f that should be generated by the motor
according to the specified speed. The difference is then
fed to field controller, actually a current controller, which
specifies the firing angle for field rectifiers. Hence, the
power supplied to field is controlled.
• Thank you.