1.2 Thyristor 1 2016 - EEL 2003 Electrical Electronics...

EEL 2003 Electrical / Electronics Department SO1/2
5/15/2016 1 Nasser
Thyristor
In many ways the Silicon Controlled Rectifier, or the Thyristor as it is more commonly
known, is similar to the transistor. It is a multi-layer semiconductor device, hence the
“silicon” part of its name. It requires a gate signal to turn it “ON”, the “controlled” part of
the name and once “ON” it behaves like a rectifying diode, the “rectifier” part of the name
Figure 1. In fact the circuit symbol for the thyristor suggests that this device acts like a
controlled rectifying diode.
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type
material. It acts exclusively as a bistable switch, conducting when the gate receives a current
trigger, and continuing to conduct while the voltage across the device is not reversed (forward-
biased).
Figure 1: Thyristor Symbol
1- Thyristor representation
However, unlike the diode which is a two layer ( P-N ) semiconductor device, or the transistor
which is a three layer ( P-N-P, or N-P-N ) device, the Thyristor is a four layer ( P-N-P-N )
semiconductor device that contains three PN junctions in series, and is represented by the symbol
as shown in figure 2.
Figure 2: Thyristor representation

EEL 2003 Electrical / Electronics Department SO1/2
5/15/2016 2 Nasser
Like the diode, the Thyristor is a unidirectional device, that is it will only conduct current in one
direction only, but unlike a diode, the thyristor can be made to operate as either an open-circuit
switch or as a rectifying diode depending upon how the thyristors gate is triggered. In other
words, thyristors can operate only in the switching mode and cannot be used for amplification.
The silicon controlled rectifier SCR , is one of several power semiconductor devices along with
Triacs (Triode AC’s), Diacs (Diode AC’s) and UJT’s (Unijunction Transistor) that are all
capable of acting like very fast solid state AC switches for controlling large AC voltages and
currents. So for the Electronics student this makes these very handy solid state devices for
controlling AC motors, lamps and for phase control.
The thyristor is a three- terminal device labelled: “Anode”, “Cathode” and “Gate” and consisting
of three PN junctions which can be switched “ON” and “OFF” at an extremely fa st rate, or it can
be switched “ON” for variable lengths of time during half cycles to deliver a selected amount of
power to a load. The operation of the thyristor can be best explained by assuming it to be made
up of two transistors connected back-to-back as a pair of complementary regenerative switches
as shown in figure 3.
2- Thyristor I-V Characteristics Curves
Figure Thyristor Characteristics
Figure 3: Thyristor Characteristics

EEL 2003 Electrical / Electronics Department SO1/2
5/15/2016 3 Nasser
The important points on this characteristic are:
Latching Current I
L
This is the minimum anode current required to maintain the thyristor in the on-state
immediately after a thyristor has been turned on and the gate signal has been removed.