A the gate. On the other hand, when

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.

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

Figure 3: PUT Relaxation Oscillator


Figure 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 saw tooth waveform is the
output when a series of the cycle is applied which is shown in Figure 4.






Figure 5: Resistance Triggering Circuit of SCR

Figure 5 shows the schematic
representation of Resistance Triggering Circuit of SCR. In this kind of circuitry,
it comprises one variable resistor, one fixed resistor, load resistor and of course,
SCR itself or the Silicon Control Rectifier. Each of these electronic
components has their own function in triggering of SCR. Resistor 1 (R1)
allows the limitation of current by using the gate terminal of the SCR. On the
other hand, Resistor 2 (R2) which is a variable resistor is present
in the circuitry because it has to achieve control over the triggering angle of
the SCR. As you can see in the diagram, there is a presence of a diode to
ensure that there will be no negative voltages to the gate of the SCR. Lastly,
Resistor (R) is treated as a stabilizing resistor to stabilize the whole


or Triode Alternating Current is a three-terminal switch (AC) that can conduct and
operate in both directions whether the applied gate signal is either positive
or negative. Figure 6 below shows the symbol of a TRIAC.

Figure 6: Symbol of a TRIAC

As the figure
displays the symbol of a TRIAC, it composes of three terminals namely MT1, MT2
and G. MT1 denotes the Anode 1, MT2 denotes Anode 2 and G stands for the Gate.
The operation of a TRIAC depends on different conditions. If we will apply a
greater gate voltage than the break-over voltage, the TRIAC can be turned on making
the voltage high. On the other hand, if the break-over voltage is greater than
the applied voltage, gate triggering method is applied to turn the TRIAC ON.

Opto-Isolator or also
called as Opto-Coupler is an electronic component that connects two electrical
circuits which are separated by means of a light sensitive optical interface.
This kind of electronic component comprises both an infrared LED and a photo
detector. Wavelength response tailored to be as identical as possible to permit
the highest measure of coupling possible.

7: Sample image of an Opto-Isolator


Figure 8 below shows
the different types of opto-isolator.

8: Types of Opto-Isolator

When we are
talking about DC Circuits, the photo-transistor and photo-darlington devices
are used while in AC Circuits, photo-SCR and photo-TRIAC allows the circuit to
be controlled. There are a lot of parameter needed to reconsider when
opto-coupler is used. One of those is the CTR or the Current Transfer Ratio. CTR
determines its efficiency. It is maximized


by closely matching spectrally the LED and the
phototransistor (which usually operate in the infra-red range). The
optocoupling efficiency of an optocoupler may be conveniently specified by the
output-to-input current transfer ratio (CTR) i.e., the ratio
of the output current Ic (measured at the collector
terminal of the phototransistor), to the input current IF flowing into the LED. 

Input-to-Output Isolation Voltage (Viso). This is the maximum potential difference (dc) that
can be allowed to exist between the input and output terminals. Typical values
range from 500 V to 4 kV. 

Maximum Collector-Emitter Voltage, VCE (max). This is the maximum allowable dc
voltage that can be applied across the output transistor. Typical values may
vary from 20 to 80 volts.  

Bandwidth. This is the typical maximum signal
frequency (in kHz) that can be use­fully passed through the optocoupler when
the device is operated in its normal mode. Typical values vary from 20 to 500
kHz, depending on the type of device construction. 

Response Time. Divided into rise time tr and
fall time t*. For a phototransistor output stages, tr andtr are
usually around 2 to 5 us. 

A simple isolating optocoupler uses
a single phototransistor output stage and is usually housed in a six-pin
package, with the base terminal of the phototransistor externally available. In
nor­mal use the base is left open circuit, and under such a condition the
optocoupler has a minimum CTR value of 20 % and a useful bandwidth of 300 kHz.