Thyristor is a type of diode that allows current to flow if and only if a
control voltage is applied to its gate terminal. This kind of diode has three
electrodes namely anode, cathode and gate. The symbol of thyristor is shown in
Figure 1: Thyristor Symbol and
different working principle depending on its classification. Generally, the thyristor is switched off and no current flows between the
anode and the cathode when there is no current flowing into the gate. On the
other hand, when there is a flow of current into the gate, it effectively flows
into the base of the n-p-n transistor, which makes the thyristor operates.
Figure 2: The circuit and its V-I Characteristics
Figure 2 shows the
representation of the circuit (a) used to obtain the V-I Characteristics (b).
Some of the significant points on this characteristic talks about the Holding
Current, Latching Current, Reverse Current, and Forward Break-Over Voltage.
Latching Current (IL) is the amount of the anode current required to
constantly maintain the operation of a thyristor immediately after turning it
on. On the other hand, Holding Current (IH) is the current required
to maintain a thyristor into its on-state. In order for us to turn off a thyristor,
the forward anode current must be less than compared to its IH in a particular
period of time. If it is not maintained properly, the thyristor will not return
to its state of blocking when the voltage across anode-to-cathode increases
again. In other words, if there is no IG applied externally, there
is a chance or possibility to return to its conducting state. Reverse Current
(IR) will only be present and conduct through a device if and only
if it is in a reverse-biased condition. Most
of the time, current flows once the circuit is in a forward-biased condition.
However, there are instances that there is a presence of a reverse current that
conducts in a reverse-biased condition.
Once the thyristor is
turned on by a gate signal and its anode current is greater than the holding
current, the device continues to conduct due to positive feedback even if the
gate signal is removed. This is because the thyristor is a latching device and
it has been latched to the on state.
can be constructed through UJT. UJT or Unijunction Transistor is a break-over
type transistor. It consists of 3 terminals namely Base 1, Base 2 and Emitter. Since
UJT is said to be a transistor, it has a different characteristics and
properties compared to conventional BJT or FET because it is only used as an ON
and OFF switch unlike to BJT and FET, it allows the input signal to be
amplified. Some of the known applications of UJT are waveform generators,
thyristor gate control, timers and of course, oscillators. UJT is used in a
relaxation oscillator because if you’re going to see its characteristics, it
has a negative resistance region which can be easily used and employed in
As technology is keep
on improving and developing, PUT has been invented. PUT stands for Programmable
Unijunction Transistor. From the word itself, its structure and operation is
the same as UJT. It is said to be programmable because it can be adjusted to a
desired VP through external resistance. Also, its parameters