Aveneu Park, Starling, Australia

Experiment Compare and Contrast the calculated, simulated

Experiment
No. 1

THYRISTORS

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Course:

Section:

Group
No.:

Date
Performed:

Group
Members:

Date
Submitted:

 

Instructor:

 

 

 

       
I.           
Objective(s):

The activity aims
to give some of the basic knowledge including its principle on how thyristors
work and operate through the utilization of different thyristor
circuits.   

     
II.           
Intended
Learning Outcomes (ILOs):

 The students shall be able to:
1)      
Know how thyristors
differ from each other depending on its operation.
2)      
Analyze the output
characteristic curves.
3)      
Compare and
Contrast the calculated, simulated and actual values and determine the
factors that affect the discrepancy.
4)      
Understand some of
the equivalent circuits of thyristors depending on its availability.
 

    III.           
Discussion:

 
A
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 ratio.

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 circuit.
 
TRIAC
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.

Figure 7: Sample image of an Opto-Isolator
 
Figure
8 below shows the different types of opto-isolator.

Figure 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. 
 
 
 

    IV.           
Equipment:

 
·        
De Laguna Module  
·        
Connecting Wires
·        
Digital Multimeter
 

     
V.           
Procedure:

 
Computation Formulas:
 
 
ACTIVITY 1: UJT RELAXATION OSCILLATOR
   
                         
ACTIVITY 2: PUT EQUIVALENT CIRCUIT OF UJT IN
RELAXATION OSCILLATOR
 
ACTIVITY 3: R 
AND RC NETWORK CIRCUIT TRIGGERING SCR
 
ACTIVITY 4: EVALUATING THE CHARACTERISTICS OF A TRIAC
 
ACTIVITY 5: EVALUATING THE CHARACTERISTICS OF AN
OPTOCOUPLER
 

    VI.           
Data
and Results

 
ACTIVITY 1: UJT RELAXATION OSCILLATOR
                            
ACTIVITY 2: PUT EQUIVALENT CIRCUIT OF UJT IN
RELAXATION OSCILLATOR
 
ACTIVITY 3: UJT FIRING SCR
 
ACTIVITY 4: EVALUATING THE CHARACTERISTICS OF A TRIAC
 
ACTIVITY 5: EVALUATING THE CHARACTERISTICS OF AN
OPTOCOUPLER
 
 

  VII.           
Observation:

 
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VIII.           
Interpretation:

 
_____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
 
 
 
 

 
    IX.           
Assessment
Task:

 
1)      
What
is the difference between SCR and a Thyristor?
______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
 
2)      
?
__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
 
3)      
?
_________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
 
 
 

     
X.           
Assessment
(Rubric for Laboratory Performance):

 

CRITERIA

BEGINNER
1

ACCEPTABLE
2

PROFICIENT
3

SCORE

I. Laboratory Skills

 

 

 

Manipulative Skills

Members do not demonstrate needed skills. 

Members occasionally demonstrate needed skills.

Members always demonstrate needed skills.

 

Experimental Set-up

Members are unable to set up the materials.

Members are able to set up the materials with supervision.

Members are able to set up the materials with minimum supervision.

 

Process Skills

Members do not demonstrate targeted process skills.

Members occasionally demonstrate targeted process skills.

Members always demonstrate targeted process skills.

 

Safety Precautions

Members do not follow safety precautions.

Members follow safety precautions most of the time.

Members follow safety precautions at all times.

 

II. Work Habits  

 

 

 

Time Management /
Conduct of
Experiment

Members do not finish on time with incomplete data.

Members finish on time with incomplete data.

Members finish ahead of time with complete data and time to revised
data.

 

Cooperative and Teamwork

Members do not know their tasks and have no defined
responsibilities. Group conflicts have to be settled by the instructor.

Members have defined responsibilities most of the time. Group
conflicts are cooperatively managed most of the time. 

Members are on tasks and have defined responsibilities at all times.
Group conflicts are cooperatively managed at all times.

 

Neatness and Orderliness

Messy workplace during and after the experiment.

Clean and orderly workplace with occasional mess during and after
the experiment. 

Clean and orderly workplace at all times during and after the
experiment.

 

Ability to do independent work

Members require supervision by the instructor.

Members require occasional supervision by the instructor.

Members do not need to be supervised by the instructor.

 

Other comments / Observations:

 

TOTAL SCORE

 

RATING = (Total Score / 20) x 100%

 

 

 

 
ANSWER
KEYS

Data
and Results

 
ACTIVITY 1: UJT RELAXATION OSCILLATOR
                            
ACTIVITY 2: PUT EQUIVALENT CIRCUIT OF UJT IN
RELAXATION OSCILLATOR
 
ACTIVITY 3: UJT FIRING SCR
 
ACTIVITY 4: EVALUATING THE CHARACTERISTICS OF A TRIAC
 
ACTIVITY 5: EVALUATING THE CHARACTERISTICS OF AN
OPTOCOUPLER
 
 

Observation

 
 

Interpretation

 
 

Assessment
Task

 
 

 

 

x

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