Answer / kris paring

The thyristor is a solid-state semiconductor device with
four layers of alternating N and P-type material. They act
as bistable switches, conducting when their gate receives a
current pulse, and continue to conduct for as long as they
are forward biased (that is, as long as the voltage across
the device has not reversed).

Function

The thyristor is a four-layer, three terminal semiconducting
device, with each layer consisting of alternately N-type or
P-type material, for example P-N-P-N. The main terminals,
labeled anode and cathode, are across the full four layers,
and the control terminal, called the gate, is attached to
p-type material near to the cathode. (A variant called an
SCS—Silicon Controlled Switch—brings all four layers out to
terminals.) The operation of a thyristor can be understood
in terms of a pair of tightly coupled bipolar junction
transistors, arranged to cause the self-latching action:

Thyristor.svg

Thyristors have three states:

1. Reverse blocking mode — Voltage is applied in the
direction that would be blocked by a diode
2. Forward blocking mode — Voltage is applied in the
direction that would cause a diode to conduct, but the
thyristor has not yet been triggered into conduction
3. Forward conducting mode — The thyristor has been
triggered into conduction and will remain conducting until
the forward current drops below a threshold value known as
the "holding current"

[edit] Function of the gate terminal

The thyristor has three p-n junctions (serially named J1,
J2, J3 from the anode).
Layer diagram of thyristor.

When the anode is at a positive potential VAK with respect
to the cathode with no voltage applied at the gate,
junctions J1 and J3 are forward biased, while junction J2 is
reverse biased. As J2 is reverse biased, no conduction takes
place (Off state). Now if VAK is increased beyond the
breakdown voltage VBO of the thyristor, avalanche breakdown
of J2 takes place and the thyristor starts conducting (On
state).

If a positive potential VG is applied at the gate terminal
with respect to the cathode, the breakdown of the junction
J2 occurs at a lower value of VAK. By selecting an
appropriate value of VG, the thyristor can be switched into
the on state suddenly.

It should be noted that once avalanche breakdown has
occurred, the thyristor continues to conduct, irrespective
of the gate voltage, until both: (a) the potential VG is
removed and (b) the current through the device
(anode−cathode) is less than the holding current specified
by the manufacturer. Hence VG can be a voltage pulse, such
as the voltage output from a UJT relaxation oscillator.

These gate pulses are characterized in terms of gate trigger
voltage (VGT) and gate trigger current (IGT). Gate trigger
current varies inversely with gate pulse width in such a way
that it is evident that there is a minimum gate charge
required to trigger the thyristor.
[edit] Switching characteristics

In a conventional thyristor, once it has been switched on by
the gate terminal, the device remains latched in the
on-state (i.e. does not need a continuous supply of gate
current to conduct), providing the anode current has
exceeded the latching current (IL). As long as the anode
remains positively biased, it cannot be switched off until
the anode current falls below the holding current (IH).
V - I characteristics.

A thyristor can be switched off if the external circuit
causes the anode to become negatively biased. In some
applications this is done by switching a second thyristor to
discharge a capacitor into the cathode of the first
thyristor. This method is called forced commutation.

After a thyristor has been switched off by forced
commutation, a finite time delay must have elapsed before
the anode can again be positively biased and retain the
thyristor in the off-state. This minimum delay is called the
circuit commutated turn off time (tQ). Attempting to
positively bias the anode within this time causes the
thyristor to be self-triggered by the remaining charge
carriers (holes and electrons) that have not yet recombined.

For applications with frequencies higher than the domestic
AC mains supply (e.g. 50 Hz or 60 Hz), thyristors with lower
values of tQ are required. Such fast thyristors are made by
diffusing into the silicon heavy metals ions such as gold or
platinum which act as charge combination centres.
Alternatively, fast thyristors may be made by neutron
irradiation of the silicon.
[edit] History

The Silicon Controlled Rectifier (SCR) or Thyristor proposed
by William Shockley in 1950 and championed by Moll and
others at Bell Labs was developed in 1956 by power engineers
at General Electric (G.E.) led by Gordon Hall and
commercialized by G.E.'s Frank W. "Bill" Gutzwiller.
A bank of six, 2000 A Thyristors (white pucks).
[edit] Applications
Load voltage regulated by thyristor phase control.
Red trace: load voltage
Blue trace: trigger signal.

Thyristors are mainly used where high currents and voltages
are involved, and are often used to control alternating
currents, where the change of polarity of the current causes
the device to switch off automatically; referred to as Zero
Cross operation. The device can be said to operate
synchronously as, once the device is open, it conducts
current in phase with the voltage applied over its cathode
to anode junction with no further gate modulation being
required to replicate; the device is biased fully on. This
is not to be confused with symmetrical operation, as the
output is unidirectional, flowing only from cathode to
anode, and so is asymmetrical in nature.

Thyristors can be used as the control elements for phase
angle triggered controllers, also known as phase fired
controllers.

Thyristors can also be found in power supplies for digital
circuits, where they can be used as a sort of "circuit
breaker" or "crowbar" to prevent a failure in the power
supply from damaging downstream components. The thyristor is
used in conjunction with a zener diode attached to its gate,
and when the output voltage of the supply rises above the
zener voltage, the thyristor conducts, shorting the power
supply output to ground (and in general blowing an upstream
fuse).

The first large scale application of thyristors, with
associated triggering diac, in consumer products related to
stabilized power supplies within color television receivers
in the early 1970s. The stabilized high voltage DC supply
for the receiver was obtained by moving the switching point
of the thyristor device up and down the falling slope of the
positive going half of the AC supply input (if the rising
slope was used the output voltage would always rise towards
the peak input voltage when the device was triggered and
thus defeat the aim of regulation). The precise switching
point was determined by the load on the output DC supply as
well fluctuations on the input AC supply. They proved to be
unpopular with the AC grid power supplier companies because
the simultaneous switching of many television receivers, all
at approximately the same time, introduced asymmetry into
the supply waveform and, as a consequence injected DC back
into the grid with a tendency towards saturation of
transformer cores and overheating. Thyristors were largely
phased out in this kind of application by the end of the decade.

Thyristors have been used for decades as lighting dimmers in
television, motion pictures, and theater, where they
replaced inferior technologies such as autotransformers and
rheostats. They have also been used in photography as a
critical part of flashes (strobes).
[edit] Snubber circuits

Because thyristors can be triggered on by a high rate of
rise of off-state voltage, in many applications this is
prevented by connecting a resistor-capacitor (RC) snubber
circuit between the anode and cathode terminals in order to
limit the dV/dt (i.e., rate of change of voltage versus time).
[edit] HVDC electricity transmission
Two of three thyristor valve stacks used for long distance
transmission of power from Manitoba Hydro dams

Since modern thyristors can switch power on the scale of
megawatts, thyristor valves have become the heart of
high-voltage direct current (HVDC) conversion either to or
from alternating current. In the realm of this and other
very high power applications, both electronically switched
(ETT) and light switched (LTT) thyristors[4] are still the
primary choice. The valves are arranged in stacks usually
suspended from the ceiling of a transmission building called
a valve hall. Thyristors are arranged into a Graetz bridge
circuit and to avoid harmonics are connected in series to
form a 12 pulse converter. Each thyristor is cooled with
deionized water, and the entire arrangement becomes one of
multiple identical modules forming a layer in a multilayer
valve stack called a quadruple valve. Three such stacks are
typically hung from the ceiling of the valve building of a
long distance transmission facility.[5][6]
[edit] Comparisons to other devices

The functional drawback of a thyristor is that, like a
diode, it only conducts in one direction. A similar
self-latching 5-layer device, called a TRIAC, is able to
work in both directions. This added capability, though, also
can become a shortfall. Because the TRIAC can conduct in
both directions, reactive loads can cause it to fail to turn
off during the zero-voltage instants of the ac power cycle.
Because of this, use of TRIACs with (for example)
heavily-inductive motor loads usually requires the use of a
"snubber" circuit around the TRIAC to assure that it will
turn off with each half-cycle of mains power. Inverse
parallel SCRs can also be used in place of the triac;
because each SCR in the pair has an entire half-cycle of
reverse polarity applied to it, the SCRs, unlike TRIACs, are
sure to turn off. The "price" to be paid for this
arrangement, however, is the added complexity of two
separate but essentially identical gating circuits.

An earlier gas filled tube device called a Thyratron
provided a similar electronic switching capability, where a
small control voltage could switch a large current. It is
from a combination of "thyratron" and "transistor" that the
term "thyristor" is derived.

Although thyristors are heavily used in megawatt scale
rectification of AC to DC, in low and medium power (from few
tens of watts to few tens of kilowatts) they have almost
been replaced by other devices with superior switching
characteristics like MOSFETs or IGBTs. One major problem
associated with SCRs is that they are not fully controllable
switches. The GTO (Gate Turn-off Thyristor) and IGCT are two
related devices which address this problem. In
high-frequency applications, thyristors are poor candidates
due to large switching times arising from bipolar
conduction. MOSFETs, on the other hand, have much faster
switching capability because of their unipolar conduction
(only majority carriers carry the current).
[edit] Failure modes

As well as the usual failure modes due to exceeding voltage,
current or power ratings, thyristors have their own
particular modes of failure, including:

* Turn on di/dt — in which the rate of rise of on-state
current after triggering is higher than can be supported by
the spreading speed of the active conduction area (SCRs &
triacs).
* Forced commutation — in which the transient peak
reverse recovery current causes such a high voltage drop in
the sub-cathode region that it exceeds the reverse breakdown
voltage of the gate cathode diode junction (SCRs only).
* Switch on dv/dt — the thyristor can be spuriously
fired without trigger from the gate if the rate of rise of
voltage anode to cathode is too great

[edit] Silicon carbide thyristors

In recent years, some manufacturers[7] have developed
thyristors using Silicon carbide (SiC) as the semiconductor
material. These have applications in high temperature
environments, being capable of operating at temperatures up
to 350 °C.
[edit] Types of thyristor

* SCR — Silicon Controlled Rectifier
* ASCR — Asymmetrical SCR
* RCT — Reverse Conducting Thyristor
* LASCR — Light Activated SCR, or LTT — Light triggered
thyristor
* DIAC & SIDAC — Both forms of trigger devices
* BOD — Breakover Diode — A gateless thyristor triggered
by avalanche current, used in protection applications
* TRIAC — Triode for Alternating Current — A
bidirectional switching device containing two thyristor
structures
* GTO — Gate Turn-Off thyristor
* IGCT — Integrated Gate Commutated Thyristor
o MA-GTO — Modified Anode Gate Turn-Off thyristor
o DB-GTO — Distributed Buffer Gate Turn-Off thyristor
* MCT — MOSFET Controlled Thyristor — It contains two
additional FET structures for on/off control.
o BRT — Base Resistance Controlled Thyristor
* SITh — Static Induction Thyristor, or FCTh — Field
Controlled Thyristor containing a gate structure that can
shut down anode current flow.

The GTO is a tri state device. with an 8-function setup. it
also has an equation: v=j-o x n/n o

* LASS — Light Activated Semiconducting Switch

Answer / indira pati

THYRISTOR HAS THREE ELECTRODES i.e. Anode, Cathode and gate...Most common type is Silicon controlled Rectifier...it wil conduct only when gate pulse applied...continues to conduct until the voltage between anode and cathode is reversed or reduced below certain threshold voltage...this type of thyristors used in large amount of power can b switched ON....and controlling the small triggering c or v...

Answer / vilas

Thyrister is a silicon controlled rectifier,it has four
layers, it has a gate for firing circuit, applying a small
current pulse to the gate the scr can be fired,and power is
supplied to the d.c. motor,or load, scr is used for power
switching device, with the help of potentiometer, the gate
voltage is controlled, so as firing of scr can be contorlled
and thus power output (d.c. voltage) is controlled,thus it
is used as speed control device,

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