Experiment: V-I Characteristics of UJT and Its
Application as Relaxation Oscillator
Aim
1.​ To study the V-I characteristics of a Unijunction Transistor (UJT).
2.​ To determine the Peak Point Voltage and Valley Point Voltage of the UJT.
3.​ To study the operation of UJT as a Relaxation Oscillator and observe the
generated waveform.
Required Components
●​ UJT (2N2646)
●​ DC Power Supply (0-30 V)
●​ Resistors (1 kΩ, 4.7 kΩ, 10 kΩ)
●​ Variable Resistor (100 kΩ)
●​ Capacitor (0.01 μF to 1 μF)
●​ Milliammeter
●​ Voltmeter
●​ Breadboard/Trainer Kit
●​ Connecting Wires
●​ Cathode Ray Oscilloscope (CRO)
Theory
A Unijunction Transistor (UJT) is a three-terminal semiconductor device having one
emitter (E) and two bases (B1 and B2). It consists of a lightly doped N-type silicon bar
with a P-type emitter junction. Unlike BJT, the UJT is not used for amplification. It is
mainly used in triggering circuits, timing circuits, pulse generation, and relaxation
oscillators.
The resistance between terminals B1 and B2 is called inter-base resistance (RBB).
When a voltage VBB is applied across B2 and B1, a voltage appears at the emitter
junction. The ratio of internal resistance from emitter to B1 and total inter-base
resistance is known as intrinsic stand-off ratio (η).
Intrinsic Stand-Off Ratio
𝑅
𝐵1
η = 𝑅 +𝑅
𝐵1
𝐵2
The emitter junction remains reverse biased until the emitter voltage reaches the Peak
Point Voltage.
Peak Point Voltage
𝑉𝑃 = η𝑉𝐵𝐵 + 𝑉𝐷
Where,
●​ 𝑉𝑃 = Peak Point Voltage
●​ 𝑉𝐵𝐵 = Inter-base Voltage
●​ 𝑉𝐷 = Diode Drop (≈0.6 V)
When emitter voltage reaches 𝑉𝑃, the emitter junction becomes forward biased. The
emitter current increases rapidly while emitter voltage decreases. This region exhibits
negative resistance characteristic.
After reaching the valley point, the device enters saturation region where emitter current
increases normally with voltage.
Circuit Diagram for V-I Characteristics
​
GND
Operation
1.​ Apply a constant voltage VBB between B2 and B1.
2.​ Increase emitter voltage gradually using the variable resistor.
3.​ Initially emitter current is nearly zero.
4.​ At Peak Point Voltage, emitter current suddenly increases.
5.​ The emitter voltage decreases while current increases, producing a negative
resistance region.
6.​ After valley point, the device enters saturation region.
Thus the V-I characteristic consists of:
●​ Cut-off Region
●​ Negative Resistance Region
●​ Saturation Region
Procedure
Part A: V-I Characteristics
1.​ Connect the circuit as shown.
2.​ Apply a constant inter-base voltage VBB.
3.​ Keep emitter voltage initially zero.
4.​ Increase emitter voltage slowly.
5.​ Note corresponding emitter current readings.
6.​ Continue until the complete characteristic is obtained.
7.​ Plot emitter voltage (VE) versus emitter current (IE).
Part B: UJT Relaxation Oscillator
1.​ Connect the relaxation oscillator circuit.
2.​ Connect CRO across capacitor.
3.​ Switch ON the supply.
4.​ Observe charging and discharging waveforms.
5.​ Measure oscillation frequency.
6.​ Compare measured frequency with theoretical value.
Circuit Diagram of UJT Relaxation Oscillator
+VBB​
GND
Theory of Relaxation Oscillator
In a UJT relaxation oscillator, capacitor C charges through resistor R toward the supply
voltage.
When capacitor voltage reaches Peak Point Voltage:
𝑉𝑃 = η𝑉𝐵𝐵 + 𝑉𝐷
the UJT turns ON.
The capacitor rapidly discharges through emitter and B1.
When capacitor voltage falls to Valley Voltage 𝑉𝑉, the UJT turns OFF.
The capacitor starts charging again.
This repetitive charging and discharging produces sawtooth waveforms and trigger
pulses.
Frequency of Oscillation
𝑓=
1
( )
1
𝑅𝐶𝑙𝑛 1−η
For η = 0.63,
1
𝑓 ≈ 𝑅𝐶
Observation Table
V-I Characteristics
S.No.
Emitter Current IE
(mA)
Emitter Voltage VE (V)
1
0.0
0
2
2.0
0
3
4.0
0
4
6.0
0
5
6.8
0.05
Relaxation Oscillator
Parameter
Value
Supply Voltage VBB
10v
Resistance R
10 kΩ
Capacitance C
0.01 μF
Intrinsic Stand-off Ratio η
0.63
Precautions
1.​ Ensure correct identification of UJT terminals E, B1, and B2.
2.​ Do not exceed the rated supply voltage.
3.​ Make all connections with power OFF.
4.​ Use properly calibrated meters.
5.​ Increase emitter voltage gradually.
6.​ Ensure proper polarity of power supply.
7.​ Avoid loose connections.
Result
1.​ The V-I characteristics of the UJT were studied successfully.
2.​ Peak Point Voltage and Valley Point Voltage were observed.
3.​ The negative resistance region of the UJT was verified.
4.​ UJT was successfully operated as a relaxation oscillator.
5.​ Sawtooth waveform and trigger pulses were obtained, confirming oscillatory
operation.
Experiment: Study of V-I Characteristics of DIAC
Aim
To study the Voltage-Current (V-I) characteristics of a DIAC and determine its breakover
voltage in both forward and reverse directions.
Required Components
●​ DIAC (DB3 or equivalent)
●​ Variable AC/DC Power Supply
●​ Resistor (1 kΩ to 10 kΩ)
●​ Milliammeter
●​ Voltmeter
●​ Breadboard/Trainer Kit
●​ Connecting Wires
Theory
A DIAC (Diode for Alternating Current) is a two-terminal, bidirectional semiconductor
switching device. It conducts current in both directions only after the applied voltage
exceeds a certain value called the Breakover Voltage (VBO).
Unlike ordinary diodes, a DIAC has no gate terminal and no preferred direction of
conduction. It remains in a high-resistance state until the applied voltage reaches the
breakover voltage. Once this voltage is reached, the DIAC switches suddenly to a
low-resistance state and conducts heavily.
The DIAC is widely used for triggering TRIACs in AC power control circuits such as
lamp dimmers, fan regulators, and motor speed controllers.
Construction
A DIAC is a three-layer or five-layer semiconductor device designed to conduct
symmetrically in both directions. Because of its symmetrical construction, its forward
and reverse characteristics are nearly identical.
Working Principle
1.​ When a small voltage is applied, only leakage current flows through the DIAC.
2.​ As the voltage increases and reaches the breakover voltage 𝑉𝐵𝑂, avalanche
breakdown occurs.
3.​ The DIAC suddenly turns ON.
4.​ Current increases rapidly while voltage across the device decreases.
5.​ This region exhibits negative resistance characteristics.
6.​ The DIAC remains ON until the current falls below the holding current.
Circuit Diagram
​
​
GND
For reverse characteristics, reverse the polarity of the supply.
Operation
1.​ Apply voltage across the DIAC through a current-limiting resistor.
2.​ Initially the DIAC remains OFF and only leakage current flows.
3.​ Increase the applied voltage gradually.
4.​ At breakover voltage, the DIAC switches ON suddenly.
5.​ Current increases sharply while voltage across the DIAC decreases.
6.​ Continue increasing voltage and note the corresponding current values.
7.​ Repeat the experiment for reverse polarity.
The forward and reverse characteristics should be nearly symmetrical.
Procedure
1.​ Connect the circuit as shown in the diagram.
2.​ Keep the supply voltage at zero initially.
3.​ Switch ON the power supply.
4.​ Increase the voltage gradually and note voltmeter and ammeter readings.
5.​ Observe the breakover point where current suddenly increases.
6.​ Record several voltage-current readings beyond the breakover region.
7.​ Reverse the polarity of the supply.
8.​ Repeat the measurements for reverse bias.
9.​ Plot voltage (V) on the X-axis and current (I) on the Y-axis.
10.​Determine the breakover voltage from the graph.
Observation Table
Forward Characteristics
S.No.
Voltage (V)
Current (mA)
1
10
0.01
2
20
0.03
3
28
0.08
4
32
8.5
5
35
15.2
Reverse Characteristics
S.No.
Voltage (V)
Current (mA)
1
-10
-0.01
2
-20
-0.03
3
-28
-0.07
4
-32
-8.2
5
-35
-15.0
Characteristics of DIAC
Regions of Operation
1. Non-Conducting Region
●​ Applied voltage is less than breakover voltage.
●​ Only leakage current flows.
●​ DIAC behaves as an open switch.
2. Breakover Region
●​ Applied voltage reaches 𝑉𝐵𝑂.
●​ Avalanche breakdown occurs.
●​ Device switches ON suddenly.
3. Negative Resistance Region
●​ Current increases rapidly.
●​ Voltage decreases while current increases.
●​ Important switching region of DIAC.
4. Conducting Region
●​ DIAC behaves as a closed switch.
●​ Large current flows with small voltage drop.
Precautions
1.​ Use a current-limiting resistor to prevent excessive current.
2.​ Make all circuit connections with power OFF.
3.​ Increase supply voltage gradually.
4.​ Ensure proper meter connections.
5.​ Do not exceed the rated current of the DIAC.
6.​ Record readings carefully near the breakover point.
7.​ Avoid loose connections.
Result
The V-I characteristics of the DIAC were studied successfully. The breakover voltage in
both forward and reverse directions was observed, and the symmetrical bidirectional
switching behavior of the DIAC was verified. The negative resistance region of
operation was also confirmed.
Experiment: Study of V-I Characteristics of SCR
Aim
To study the Voltage-Current (V-I) characteristics of a Silicon Controlled Rectifier (SCR)
and determine its forward breakover voltage, latching current, and holding current.
Required Components
●​ SCR (TYN604, C106, or equivalent)
●​ DC Power Supply (0–30 V)
●​ Gate Supply (0–5 V)
●​ Resistors (220 Ω, 1 kΩ, 10 kΩ)
●​ Rheostat/Variable Resistor
●​ Ammeter (mA range)
●​ Voltmeter
●​ Breadboard/Trainer Kit
●​ Connecting Wires
Theory
A Silicon Controlled Rectifier (SCR) is a four-layer, three-junction semiconductor device
having three terminals: Anode (A), Cathode (K), and Gate (G). It belongs to the thyristor
family and acts as a controlled switch.
The SCR remains in the OFF state when the anode is positive with respect to the
cathode but no gate signal is applied. When a suitable gate current is supplied, the SCR
turns ON and starts conducting heavily from anode to cathode.
Once turned ON, the SCR remains conducting even if the gate signal is removed. It can
be turned OFF only when the anode current falls below a certain minimum value called
the Holding Current.
Modes of Operation
1. Reverse Blocking Mode
When the cathode is positive with respect to the anode, the SCR blocks current except
for a small leakage current.
2. Forward Blocking Mode
When the anode is positive but no gate current is applied, the SCR remains OFF and
blocks current.
3. Forward Conduction Mode
When sufficient gate current is applied, the SCR turns ON and conducts heavily.
Important Parameters
Forward Breakover Voltage (𝑉𝐵𝑂)
The minimum forward voltage at which the SCR turns ON without any gate signal.
Latching Current (𝐼𝐿)
The minimum anode current required to keep the SCR in the ON state immediately after
triggering.
Holding Current (𝐼𝐻)
The minimum anode current below which the SCR turns OFF.
Generally,
𝐼𝐿 > 𝐼𝐻
Circuit Diagram
Operation
1.​ Apply a forward voltage between anode and cathode.
2.​ Initially keep the gate current zero.
3.​ The SCR remains in the forward blocking region.
4.​ Apply a gate pulse through the gate resistor.
5.​ The SCR switches ON and enters the conduction region.
6.​ Once ON, the SCR continues conducting even if the gate signal is removed.
7.​ Reduce the anode current gradually.
8.​ When the current falls below the holding current, the SCR turns OFF.
Procedure
Forward Characteristics
1.​ Connect the circuit as shown.
2.​ Keep the gate current zero initially.
3.​ Apply a forward voltage between anode and cathode.
4.​ Increase the anode voltage gradually.
5.​ Record corresponding anode current readings.
6.​ Apply a small gate current.
7.​ Again increase anode voltage and note the readings.
8.​ Repeat for different values of gate current.
9.​ Plot Anode Current (𝐼𝐴) versus Anode-Cathode Voltage (𝑉𝐴𝐾).
Determination of Holding Current
1.​ Trigger the SCR using gate current.
2.​ Ensure the SCR is conducting.
3.​ Gradually decrease the anode current.
4.​ Note the current at which the SCR turns OFF.
5.​ This value is the Holding Current.
Determination of Latching Current
1.​ Trigger the SCR.
2.​ Increase current gradually.
3.​ Find the minimum current at which the SCR remains ON after removing the gate
signal.
4.​ This value is the Latching Current.
Observation Table
Forward Characteristics
Gate Current IG
(mA)
Anode Voltage
VAK (V)
Anode Current
IA (mA)
1
0
18
0.1
2
2
12
5
3
4
8
15
4
6
4
35
5
8
2
60
S.No.
Holding Current
Trial
Holding Current IH (mA)
1
7
2
8
Average
7.5
Latching Current
Trial
Latching Current IL (mA)
1
11
2
12
Average
11.5
Characteristics of SCR
1. Reverse Blocking Region
●​ SCR behaves like an open switch.
●​ Only leakage current flows.
2. Forward Blocking Region
●​ Anode positive with respect to cathode.
●​ Device remains OFF.
●​ Small leakage current flows.
3. Breakover Region
●​ Forward voltage reaches breakover value.
●​ SCR turns ON suddenly.
4. Conduction Region
●​ Large anode current flows.
●​ Voltage drop across SCR becomes very small (typically 1–2 V).
Effect of Gate Current
●​ Increasing gate current reduces breakover voltage.
●​ SCR turns ON at a lower anode voltage.
●​ Triggering becomes easier.
Precautions
1.​ Verify SCR terminals (Anode, Cathode, Gate) before connections.
2.​ Use proper current-limiting resistors.
3.​ Make all connections with power OFF.
4.​ Increase supply voltage gradually.
5.​ Do not exceed the rated current of the SCR.
6.​ Avoid loose connections.
7.​ Remove the gate signal only after the SCR is triggered.
Result
The V-I characteristics of the SCR were studied successfully. The forward blocking
region, breakover region, and conduction region were observed. The effect of gate
current on triggering was verified, and the latching current (𝐼𝐿) and holding current (𝐼𝐻)
of the SCR were determined experimentally.
​
Experiment: Study of Single Phase AC Voltage
Controller
Aim
To study the operation and performance of a Single Phase AC Voltage Controller using
SCRs/TRIAC and observe the variation of load voltage with firing angle.
Required Components
●​ Single Phase AC Voltage Controller Trainer Kit
●​ SCRs/TRIAC
●​ Triggering Circuit
●​ Resistive Load (Lamp or Rheostat)
●​ CRO (optional)
●​ Voltmeter
●​ Ammeter
●​ Connecting Wires
●​ Single Phase AC Supply (230 V, 50 Hz)
Theory
A Single Phase AC Voltage Controller is a power electronic converter that converts a
fixed AC input voltage into a variable AC output voltage at the same frequency. The
output voltage is controlled by varying the firing angle (α) of SCRs or TRIAC.
The controller works on the principle of phase control. By delaying the firing instant of
the SCR during each half cycle, only a portion of the AC input waveform is applied to
the load. Thus the RMS value of load voltage changes while the supply frequency
remains constant.
Single-phase AC voltage controllers are extensively used in lamp dimmers, electric
heaters, industrial furnaces, fan regulators, and induction motor speed control.
In practical circuits, two SCRs are connected in anti-parallel or a TRIAC is used to
control both positive and negative half cycles.
RMS Output Voltage
For a resistive load:
𝑉𝑜(𝑟𝑚𝑠) = 𝑉𝑠
1
⎡(π −
π⎣
α) +
𝑠𝑖𝑛2α
⎤
2 ⎦
where
●​ 𝑉𝑠 = RMS supply voltage
●​ α = Firing angle
Important Observation
●​ As firing angle increases, output voltage decreases.
●​ As output voltage decreases, load power decreases.
●​ Frequency remains unchanged.
Circuit Diagram
Operation
1.​ During the positive half cycle, SCR1 becomes forward biased.
2.​ SCR1 remains OFF until a gate pulse is applied at firing angle α.
3.​ After triggering, SCR1 conducts for the remaining part of the positive half cycle.
4.​ During the negative half cycle, SCR2 becomes forward biased.
5.​ SCR2 is triggered at the same firing angle and conducts for the remaining
negative half cycle.
6.​ By changing α from 0° to 180°, the conduction period changes.
7.​ Hence the RMS output voltage across the load is controlled.
Special Cases
α = 0°
●​ Full conduction
●​ Maximum output voltage
●​ Maximum load power
α = 90°
●​ Half-cycle conduction delayed
●​ Reduced output voltage
α = 180°
●​ No conduction
●​ Output voltage nearly zero
Procedure
1.​ Connect the circuit as per the circuit diagram.
2.​ Keep the firing angle control at minimum position.
3.​ Switch ON the AC supply.
4.​ Measure the load voltage.
5.​ Increase the firing angle gradually.
6.​ Record the load voltage for different firing angles.
7.​ Observe changes in lamp brightness or load power.
8.​ If CRO is available, observe input and output waveforms.
9.​ Tabulate all readings.
10.​Plot firing angle versus output voltage.
Observation Table
Firing Angle α
(Degree)
S.No.
Output Voltage (V)
1
0°
230
2
30°
215
3
60°
185
4
90°
150
5
120°
95
6
150°
40
Applications
1.​ Light dimmer circuits
2.​ Fan speed regulators
3.​ Electric heating control
4.​ Temperature control systems
5.​ Induction motor speed control
6.​ Industrial power control systems
Precautions
1.​ Ensure proper isolation from AC mains supply.
2.​ Verify SCR/TRIAC connections before switching ON.
3.​ Avoid loose connections.
4.​ Increase firing angle gradually.
5.​ Do not exceed the load rating.
6.​ Use insulated probes while taking measurements.
7.​ Switch OFF the supply before modifying connections.
Result
The operation of a Single Phase AC Voltage Controller was studied successfully. It was
observed that the RMS output voltage and load power can be controlled by varying the
firing angle of the SCRs/TRIAC while maintaining constant supply frequency. The output
voltage decreased as the firing angle increased.
Experiment: Study of V-I Characteristics of TRIAC
Aim
To study the Voltage-Current (V-I) characteristics of a TRIAC and determine its
breakover voltage and conduction characteristics in both directions.
Required Components
●​ TRIAC (BT136/BT139 or equivalent)
●​ Variable AC/DC Power Supply
●​ Resistors (220 Ω, 1 kΩ, 10 kΩ)
●​ Ammeter
●​ Voltmeter
●​ Breadboard/Trainer Kit
●​ Connecting Wires
●​ Gate Triggering Circuit
Theory
A TRIAC (Triode for Alternating Current) is a three-terminal bidirectional thyristor used
for controlling AC power. It can conduct current in both directions and is equivalent to
two SCRs connected in anti-parallel with a common gate terminal.
The terminals of a TRIAC are:
●​ Main Terminal 1 (MT1)
●​ Main Terminal 2 (MT2)
●​ Gate (G)
Unlike an SCR, which conducts only in one direction, a TRIAC conducts during both
positive and negative half cycles of an AC supply. It is widely used in AC voltage
regulators, lamp dimmers, fan speed controllers, and heating control circuits.
Initially, the TRIAC remains in the OFF state and only a small leakage current flows.
When a gate pulse is applied, the device switches ON and starts conducting. Once
triggered, it remains ON even after removal of the gate signal, provided the load current
remains greater than the holding current.
Modes of Operation
A TRIAC can operate in four triggering modes:
Mode I
●​ MT2 positive with respect to MT1
●​ Gate positive with respect to MT1
Mode II
●​ MT2 positive with respect to MT1
●​ Gate negative with respect to MT1
Mode III
●​ MT2 negative with respect to MT1
●​ Gate negative with respect to MT1
Mode IV
●​ MT2 negative with respect to MT1
●​ Gate positive with respect to MT1
Mode I and Mode III are the most sensitive and commonly used modes.
Circuit Diagram
Operation
1.​ Apply voltage between MT2 and MT1.
2.​ Initially the TRIAC remains OFF.
3.​ Apply a gate current through the gate terminal.
4.​ The TRIAC turns ON and conducts current.
5.​ Current increases rapidly while the voltage across the TRIAC decreases.
6.​ The device continues conducting until the current falls below the holding current.
7.​ Reverse the polarity and repeat the experiment.
8.​ Similar characteristics are obtained in the opposite direction.
Procedure
1.​ Connect the circuit as shown in the diagram.
2.​ Keep the gate current zero initially.
3.​ Apply voltage between MT2 and MT1.
4.​ Gradually increase the applied voltage and observe the current.
5.​ Apply a gate pulse through the gate resistor.
6.​ Note the voltage and current at which the TRIAC turns ON.
7.​ Record several readings in the conduction region.
8.​ Reverse the polarity of the supply and repeat the measurements.
9.​ Plot the graph of MT2-MT1 Voltage versus Current.
10.​Observe the symmetry of characteristics in both directions.
Observation Table
Positive Half-Cycle Characteristics
S.No.
MT2-MT1 Voltage (V)
Current (mA)
1
5
0,02
2
10
0,05
3
20
0.10
4
30
12
5
35
25
Negative Half-Cycle Characteristics
S.No.
MT2-MT1 Voltage (V)
Current (mA)
1
-5
-0.02
2
-10
-0.05
3
-20
-0.10
S.No.
MT2-MT1 Voltage (V)
Current (mA)
4
-30
-11
5
-35
-24
Characteristics of TRIAC
1. Blocking Region
●​ TRIAC remains OFF.
●​ Only leakage current flows.
●​ Device behaves as an open switch.
2. Breakover Region
●​ Applied voltage reaches breakover voltage.
●​ TRIAC switches ON suddenly.
3. Conduction Region
●​ Large current flows through the device.
●​ Voltage drop across TRIAC becomes very small (approximately 1–2 V).
4. Bidirectional Operation
●​ Similar characteristics are obtained in both positive and negative directions.
●​ Hence TRIAC is suitable for AC power control.
Applications
1.​ Lamp dimmer circuits
2.​ Fan speed controllers
3.​ AC motor speed control
4.​ Heater temperature control
5.​ Power regulators
6.​ Home appliance control systems
Precautions
1.​ Verify MT1, MT2, and Gate terminals before connections.
2.​ Use a current-limiting resistor in the gate circuit.
3.​ Make all connections with power OFF.
4.​ Increase supply voltage gradually.
5.​ Do not exceed the rated current of the TRIAC.
6.​ Ensure proper insulation while working with AC circuits.
7.​ Avoid loose connections.
Result
The V-I characteristics of the TRIAC were studied successfully. The breakover and
conduction characteristics were observed in both positive and negative directions. The
bidirectional switching behavior of the TRIAC was verified, confirming its suitability for
AC power control applications.