Experiment #1
Thyristor Familiarization - Part 1
I. Theory.
Thyristors are used to control power in numerous applications including light
dimmers and motor speed controls, solid state relays, some microwave ovens,
photocopiers, traction motors for electric locomotives and electric cars, power
inverters for transmission of electric power over long distances, frequency
converters, other DC-DC or DC-AC or AC-AC inverters, AC-DC regulated power
supplies, and many other applications where efficient power control is required.
A Silicon Controlled Rectifier is one type of thyristor used where the power to
be controlled is unidirectional. SCRs are normally off but may be triggered on by a
low current pulse to an input called the gate. Once triggered on, they remain on until
the current flowing through the main terminals of the device drops below a hold value
which is very close to zero.
SCR is a three-element, solid state device, it has a three terminal namely anode
(A), cathode (K) and gate (G) as shown in figure below.
Page 1 of 14
You can test most SCRs with a standard ohmmeter. The SCR under test
should show:
1. A very high resistance when the ohmmeter probe is connected in the
anode and cathode regardless of the ohmmeter polarity.
2. When the ohmmeter probe is connected such as “+” to gate and “-“to
cathode, should show a very low resistance (Forward biased pn
junction).
3. When the ohmmeter probe is connected such as “-” to gate and “+“to
cathode, should show a very high resistance (Reverse biased pn
junction).
II. Objectives.
a. To identify the terminals of the SCR using ohmmeter.
b. To determine if the SCR is in good condition.
c. To show the turn-ON and turn-OFF methods for an SCR
d. To observe the operation of the SCR in DC circuit.
e. To observe the operation of SCR in AC circuit.
III. Materials.
SCR C106
Ohmmeter (Analog Tester)
Connecting Wires
Breadboard
Oscilloscope Transformer 12Vrms Secondary
Function Generator DC Power Supply
Resistors = 100Ω, 22kΩ, 10kΩ, 1kΩ Potentiometer = 500kΩ
Capacitor = 0.02µF Diode = 1N4001
Page 2 of 14
IV. Procedures.
Part 1: SCR Testing With an Ohmmeter
Part 1.1 - Terminal Identification
1. Mark the terminals of SCR as pin 1, 2, and 3.
C106
123
2. Set the ohmmeter in the midrange scale.
3. Connect the ohmmeter leads as indicated in table 1.1. Record the result.
Indicate whether it is very high (VH) or very low (VL).
4. Analyze the above result. Determine the terminal (anode, cathode, and
gate) that corresponds to the pin number: Record your answer in table
1.2.
Part 1.2 – Testing a good SCR
5. Set the ohmmeter to midrange scale.
6. Refer to figure 1.1. What is the expected reading? (Very high/very low).
Record it in table 1.3.
Negative
Positive
Page 3 of 14
Figure 1.1
Reverse-Biased Open Gate
(Reverse Probe: Forward-Biased Open Gate)
7. Use the ohmmeter to determine the resistance and record in table 1.3.
8. Refer to figure 1.1 again, with the ohmmeter probe reversed. What is the
expected reading? (Very high/very low). Record it in table 1.3.
9. Use the ohmmeter to determine the resistance and record in table 1.3.
10. Refer to figure 1.2. What is the expected reading? (Very high/very low).
Record it in table 1.3.
Negative
Positive
Figure 1.2
Reverse-Biased Shorted Anode-Gate
(Reverse Probe: Forward-Biased Shorted Anode-Gate)
11. Use the ohmmeter to determine the resistance and record in table 1.3.
12. Refer to figure 1.2 again, with the ohmmeter probe reversed. What is the
expected reading? (Very high/very low). Record it in table 1.3.
13. Use the ohmmeter to determine the resistance and record in table 1.3.
14. Still keep the probes in the SCR. Now, short the terminals G and K as
illustrated in figure 1.3. What is the expected ohmic value (very high or
very low)? Record it in table 1.2.
Page 4 of 14
Positive
Negative
Figure 1.3
Forward-Biased Shorted Cathode-Gate
(Reverse Probe: Reverse-Biased Shorted Cathode-Gate)
15. Use the ohmmeter to determine the resistance and record in table 1.3.
16. Refer to figure 1.3 again, with the ohmmeter probe reversed. What is the
expected reading? (Very high/very low). Record it in table 1.3.
17. Use the ohmmeter to determine the resistance (Very high/very low). And
record it in table 1.3.
Part 2 – SCR DC Operation
To turn SCR in the conduction mode, the SCR must have its anode more
positive than its cathode and the gate voltage must made more positive than its
cathode. When SCR turns ON current can flow from anode to cathode therefore
there will be a voltage drop across the load. It follows that, if the gate voltage is
made equal to or more negative than the cathode, SCR stays in the ON condition
because SCR is already triggered.
The SCR can be turned OFF by reducing the anode current below its holding
current. This can be accomplished using anode current interruption or forced
Page 5 of 14
commutation method. In this OFF condition, no current is flowing from anode to
cathode, therefore there will no voltage drop across the load.
1. Construct the circuit shown in figure 2.1.
+V=12V
R1 = 22-kohms RL= 100-ohms
A SCR
SW1
B
SW2 B
R2 = 10-kohms A
Figure 2.1 SCR DC Circuit
2. Refer to table 2.1.
3. Follow the sequence of combination; determine the theoretical
condition of the SCR. Record your answer in table 2.1 (column 5).
4. Set the switch combination as indicated in table 2.1 (column 2 & 3).
5. Verify your answers in #4 by measuring the voltage across the load
(RL). Record it in table 2.1 (column 4).
6. Write the experimental condition of the SCR based the value obtained
from step #5. Record your answer in table 2.1 (column 6).
Part 3: SCR AC Operation
Silicon controlled rectifier has many application especially in ac power control.
These include on-off control of current, half-wave power control, lighting system for
power interruptions, and an over-voltage protection circuit.
Page 6 of 14
The popular terms used to describe how an SCR is operating are: firing delay
angle and conduction angle. Conduction angle is the number of degrees of an ac
cycle during which the SCR is turned on. Firing delay angle is the number of degrees
of an ac cycle that elapses before the SCR is turned on.
Figure shows waveforms for an SCR control circuit for two different firing
delay angles.
At the time the ac cycle starts its positive alternation, the SCR is turned off.
Therefore, it has an instantaneous voltage across its anode to cathode terminals
equal to the supply voltage. Since the SCR is dropping the entire supply voltage, the
voltage across the load is zero during this time. When the SCR is triggered at about
60°, it conducts for approximately 120° and maximum power is delivered to the load,
as shown in Figure (a). The firing delay angle and conduction angle always total
180°. When the ac input goes negative, the SCR turns off and does not conduct
again until the firing delay angle on the next positive half-cycle. Figure (b) shows the
same waveforms for a difficult firing delay angle. In these waveforms, the firing delay
angle is about 135° and the conduction angle is about 45°. Less power is delivered
to the load
1. Construct the circuit shown in figure 3.1.
2. Place the oscilloscope across the load.
3. Set the value of Rpot as indicated in table 3.1. Draw each waveform
obtained
Page 7 of 14
RL = 100Ω
R1 = 1kΩ
Rpot = 500kΩ
12Vrms SCR
60Hz C106
1N4001
Figure 3.1 SCR AC Circuit
4. Calculate the theoretical value of ‘α’ for each R2 setting. Record it in
table 3.1. Also calculate the % error. Show your calculation in a separate
sheet of paper.
5. Replace RL by a DC bulb or DC motor. Observe what happens to these
loads as you adjust Rpot from minimum to maximum (vice versa).
Page 8 of 14
V. Data and Result.
Part 1
Table 1.1
Red Black Remarks
Probe Probe
1 2
2 1
1 3
3 1
2 3
3 2
Table 1.2
Pin Terminal
1
2
3
Table 1.3
Steps Observed Value Expected Value Remarks
(Good/Defective)
Figure 1
Figure 1 (R)
Figure 2
Figure 2 (R)
Figure 3
Figure 3 (R)
Note: R = reversed
Part 2
Page 9 of 14
Table 2.1
SCR SCR
Combination SW1 SW2 VRL (ON or OFF) (ON or OFF)
Page 10 of 14
Theoretical Experimental
1 B B
2 B A
3 A A
4 B A
5 B B
Part 3
Table 3.1
Rpot setting Measured α Calculated α % Error for α
(kΩ)
Note: α = firing delay angle
Waveforms
Rpot = _______kΩ Rpot = _______kΩ
Page 11 of 14
Rpot = _______kΩ Rpot = _______kΩ
Rpot = _______kΩ Rpot = _______kΩ
Page 12 of 14
VI. Observations and Conclusions.
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
Page 13 of 14
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
__________________________________________________________________
Page 14 of 14
