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.

Figure 1: Thyristor Symbol and 
P-N Junctions

Thyristors have
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.

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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.

Relaxation Oscillator
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. UJT
is said to be a transistor but it has a different characteristics, properties
and operation compared to conventional BJT or FET because it is only used as a switch
unlike to some transistors such as BJT and FET, it also allows the input signal
to be amplified. Waveform generators, thyristor gate control, timers and of oscillators
are some of its application. 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 relaxation oscillator.

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 and its intrinsic standoff

3: PUT Relaxation Oscillator


4: Waveform across the capacitor in a PUT Relaxation Oscillator

Figure 3 shows the PUT Relaxation Oscillator.
(intrinsic standoff ratio) and
VP (Peak Voltage) are  all
dependent with Resistor 1 and Resistor 2.  The resistor connected in the cathode terminal
of the transistor limits the cathode current flowing in PUT.  When VBB (Supply Voltage) is supplied,
the capacitor starts doing its function to charge. Given the condition when the
voltage across the capacitor is greater than the given VP, PUT conducts
into its negative resistance and creates a low resistance path from the
terminal of the transistor which makes the capacitor discharges. Once the
voltage across the capacitor is less than VV (Valley Point Voltage),
the PUT comes back to its initial. Again, the capacitor starts to charge with
the help of the resistor and the cycle is repeated. A sawtooth waveform is the
output when a series of the cycle is applied which is shown in Figure 4.


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