industrial electronics lab manual

There is a quotation that I have seen, which I

am told was originally stated

by Confucius. I present it here as a

very practical statement on how the human

mind approaches the learning process:

I hear, and I forget.

I see, and I remember.

I do, and I understand.

One of the best ways to understand something

is to get your hands on it and actually

experiment with it. In electronics, this means

putting small circuits together, powering them

up, and seeing firsthand what they do.

SAFETY WARNING

Before using this laboratory, read, understand and follow the Safety Precautions mentioned

inside this manual.

This is an educational laboratory where high-voltage terminals and large current-carrying

components and circuits are exposed for ease of measurements. Therefore, regardless of the

voltage and current levels, these should be treated as high voltages and high currents, and the

safety precautions mentioned in the manual must be followed.

SAFETY PRECAUTIONS

1. Why is safety important?

Attention and adherence to safety considerations is even more important in this laboratory than is

required in any other laboratories. Power electronic circuits can involve voltages of several

hundred volts and currents of several tens of amperes. By comparison the voltages in many

teaching laboratories rarely exceed 20V and the currents hardly ever exceed a few hundred

milliamp.

In order to minimize the potential hazards, we will use DC power supplies that never exceed

voltages above 40-50V and will have maximum current ratings of 5A or less. However in spite

of this precaution, power electronics circuits on which the student will work may involve

substantially larger voltages (up to hundreds of volts) due to the presence of large inductances in

the circuits and the rapid switching on and off of amperes of current in the inductances. For

example a boost converter can have an output voltage that can theoretically go to infinite values

if it is operating without load. Moreover the currents in portions of some converter circuits may

be many times larger than the currents supplied by the DC supplies powering the converter

circuits. A simple buck converter is an example of a power electronics circuit in which the output

current may be much larger than the input DC supply current.

2. Potential problems presented by Power Electronic circuits

Electrical shock may take a life.

Exploding components (especially electrolytic capacitors) and arcing circuits can cause

blindness and severe burns.

Burning components and arcing can lead to fire.

3. Safety precautions to minimize these hazards

3.1 General Precautions

Be calm and relaxed, while working in Lab.

When working with voltages over 40V or with currents over 10A, there must be at least

two people in the lab at all times.

Keep the work area neat and clean.

No paper lying on table or nearby circuits.

Always wear safety glasses when working with other than signal-level power.

Use rubber door mats to insulate yourself from ground, when working in the Lab.

Be sure about the locations of fire extinguishers and first aid kits in lab.

A switch should be included in each supply circuit so that when opened, these switches

will de-energize the entire setup. Place these switches so that you can reach them quickly

in case of emergency, and without reaching across hot or high voltage components.

3.2 Precautions to be taken when preparing a circuit

Use only isolated power sources (either isolated power supplies or AC power through isolation

power transformers). This helps in using a grounded oscilloscope. This reduces the possibility of

risk of completing a circuit through your body. This also reduces the possibility of destroying the

test equipment.

3.3 Precautions to be taken before powering the circuit

Check for all the connections of the circuit and scope connections before powering the

circuit, to avoid shorting or any ground looping, which may lead to electrical shocks or

damage of equipment.

Check any connections for shorting two different voltage levels.

Check if you have connected load at the output. This is very important in Boost and

Buck-Boost Converters and converters based on them.

Double check your wiring and circuit connections. It is a good idea to use a point-to-point

wiring diagram to review when making these checks.

3.4 Precautions while switching ON the circuit

Apply low voltages or low power to check proper functionality of circuits.

Once functionality is proven, increase voltages or power, stopping at frequent levels to

check for proper functioning of circuit or for any components is hot or for any electrical

noise that can affect the circuit‟s operation.

3.5 Precautions while switching on or shutting down the circuit

Reduce the voltage or power slowly till it comes to zero.

Switch of all the power supplies and remove the power supply connections.

Let the load be connected at the output for some time, so that it helps to discharge

capacitor or inductor if any, completely.

3.6 Precautions while modifying the circuit

Switch on the circuit as per the steps in section 3.5.

Modify the connections as per your requirement.

Again check the circuit as per steps in section 3.3, and switch ON as per steps in section

3.4.

3.7 Other Precautions

No loose wires or metal pieces should be lying on table or near the circuit, to cause shorts

and sparking.

Avoid using long wires, that may get in your way while making adjustments or changing

leads.

Keep high voltage parts and connections out of the way from accidental touching and

from any contacts to test equipment or any parts, connected to other voltage levels.

When working with inductive circuits, reduce voltages or currents to near zero before

switching open the circuits.

BE AWARE of bracelets, rings, metal watch bands, and loose necklace (if you are

wearing any of them), they conduct electricity and can cause burns. Do not wear them

near an energized circuit.

Learn CPR and keep up to date. You can save a life.

When working with energized circuits (while operating switches, adjusting controls,

adjusting test equipment), use only one hand while keeping the rest of your body away

from conducting surfaces.

Experiment No: 1

AIM: To draw the characteristics of SCR

OBJECTIVES: To plot the forward and reverse characteristics of a SCR.

To find the latching current and holding current.

THEORY: The Silicon Controlled Rectifier (SCR) is a three terminal semiconductor switching

device which is probably the most important circuit element after the diode and the transistor.

SCR are used as a controlled switch to perform various functions such as rectification, inversion

and regulation of power flow. The SCR has assumed paramount importance in electronics

because it can be produced inversions to handle currents upto several thousand amperes and

voltages upto more than 1 kV. It is a unidirectional power switch and is being extensively used in

switching DC and AC, rectifying AC to give controlled DC output, converting DC into AC etc.

In a silicon controlled rectifier, load is connected in series with anode. The anode is always kept

at positive potential w.r.t. cathode. The working of SCR can be studied as follows.

Fig 1a shows the SCR circuit with gate open i.e. no voltage applied to the gate. Under this

condition, junction J2 is reverse biased while junctions J1 and J3 are forward biased. Hence the

situations in the junctions J1 and J3 is just as in a npn transistor with base open. Consequently, no

currents flows through the load RL and the SCR is cut off. However, if the applied voltage is

gradually increased, a stage is reached when reverse biased junction J2 breaks down. The SCR

now conducts heavily and is said to be in the ON state. The applied voltage at which SCR

conducts heavily without gate voltage is called breakover voltage.

The SCR can be made to conduct heavily at smaller applied voltage by applying a small positive

potential to the gate as shown in fig 1b. Now junction J3 is forward biased and junction J2 is

reverse biased. The electron from n type material start moving across junction J3 towards left

where as holes from p type toward the right. Consequently, the electrons from junction J3 are

attracted across junction J2 and gate current starts flowing. As soon as the gate current flows,

anode current increases. The increased anode current in turn makes more electrons available at

junction J2. This process continues and in an extremely small time junction J2 breaks down and

the SCR starts conducting heavily. Once SCR starts conducting, the gate loses all controls. Even

if gate voltage is removed, the anode current does not decrease at all. The only way to stop

conduction (i.e. bring SCR in off condition) is to reduce the applied voltage to zero.

APPARATUS: SCR characteristics trainer (AT-304), patch cords

PROCEDURE:

Forward Characteristics:

A. With open gate:

1. Connect the circuit as shown by dotted lines (in fig 2) through patch cords.

2. Keep gate power supply control knob (i.e. set gate current) to minimum position

so that gate current becomes zero.

3. Select milliammeter range to 1.2 mA and voltmeter range to 30 V.

4. Switch ON the instrument using ON/OFF toggle switch provided on the front

panel.

5. Increase anode-cathode power supply VAK in small steps and note down

corresponding anode current IA. As IA is small SCR is in OFF state.

NOTE: Breakover voltage of SCR with open gate will take place at higher voltages

(say 100V), maximum permissible forward voltage. It is undesirable to apply this

voltage as SCR is never used with open gate.

B. When gate is positive w.r.t. cathode:

6. Connect the circuit as shown in fig 2

7. Repeat step (2) to (4) as given in case of open gate circuit.

8. Select milliammeter range to 30 mA.

9. Increase gate current Ig in small steps, at a particular value of Ig, SCR will turn

ON resulting sudden increase in anode current IA with decrease in anode-cathode

voltage VAK.

10. Change the range of voltmeter to 1.2 V after triggering of SCR. Record all

possible value of IA (say between 10 mA to 30 mA) and corresponding VAK (may

be in the range of 0.8 V to 1 V).

11. Also note down the gate current Ig required for triggering the SCR at a given VAK.

12. Repeat the experiment for different anode-cathode voltages (VAK).

13. Plot a graph between VAK and IA by taking VAK along x-axis and IA along y-axis.

To record holding current IH

14. When the SCR turns ON, decrease IA by decreasing anode-cathode (VAK) power

supply in small steps. At certain value of VAK, IA drops suddenly towards zero.

This value of anode current (IA) is the holding current (IH). Below IH SCR will

remain in OFF state. On the other hand above IH SCR remains in ON state.

Reverse characteristics:

15. Connect the circuit as shown by dotted lines (in fig 3) through patch cords.

16. Repeat all the steps as in case of forward characteristics procedure and plot a

graph between VAK and IA as shown in fig 4.

In reverse characteristics the SCR will never be turn ON at the application of gate current

Ig because it is harmful to operate the SCR in reverse direction. It may damage the SCR

because SCR is a unidirectional device. So it is required in case of SCR that it should not

operate in the Avalanche breakdown region.

OBSERVATIONS:

Gate current Ig ( ) =____ (open gate) Gate current Ig () =____ (gate is +ve)

Anode voltage Va (V) Anode Current Ia () Anode voltage Va (V) Anode Current Ia ()

For Gate current Ig (mA) =________the forward break over voltage is _______ (V).

Latching Current (mA) = ________

Holding Current (mA) = ________

PRECAUTIONS:

1. The supply voltage between the anode and the cathode should never exceed the peak

inverse voltage of the device.

2. The value of gate current (IG) must always be well below that of IG (max).

3. In case a.c. signal is applied as gate current, it should be ensured that the cathode is never

positive with respect to the gate.

4. The SCR gate should be supplied with positive polarity.

5. The SCR terminals should be checked before connecting it in the circuit.

6. When work has been completed, disconnect the wiring and leave the equipment tidy.

7. Great care should be taken in handling meters and apparatus.

8. Always close a switch confidently, never in a hesitating manner. If there is any circuit

fault, the protective gear will deal with it and dangerous arcing at the switch contacts will

be avoided.

Experiment No: 2

AIM: To draw the characteristics of DIAC.

OBJECTIVES: To plot the characteristics of a DIAC.

To find the breakover voltages.

THEORY:

DIAC is a two terminal, three layer semiconductor device. It is a bi-directional diode i.e. it can

be made to conduct in either direction. It has no gate terminal. Fig 1a shows the basic structure of

a DIAC and Fig 1b shows the symbol of a DIAC. The two leads are connected to p-regions of

silicon separated by an n-region.

The structure of the diac is somewhat like a transistor with the following basic differences:

i) There is no terminal attached to the base layer.

ii) The doping connections are identical (unlike a Bipolar Transistor) to give the device

symmetrical properties.

Circuit diagram of the diac is shown in fig 2a and V-I characteristics are shown in fig 2b. When a

positive or negative voltage is applied across the terminals of a diac, only a small leakage current

IBO will flow through the device. As the applied voltage increased, the leakage current will

continue to flow until the voltage reaches the breakover voltage VBO. At this point, avalanche

breakdown of the reverse biased junction occurs and the device exhibits negative resistance i.e.

current through the device increases with the decreasing values of applied voltage. The voltage

across the device then drops to „breakback‟ voltage VW.

Fig 2a

Fig 2b shows the V-I characteristics of a diac, for applied positive voltage less than +VBO and

negative voltage less than -VBO, a small leakage current (±IBO) flows through the device. Under

such conditions, the diac blocks the flow of current and effectively behaves as an open circuit.

The voltage +VBO and –VBO are the breakdown voltages and usually have a range of 30 to 50

Volts. When positive or negative applied voltage is equal to or greater than the breakdown

voltage, diac begins to conduct and the voltage drop across it becomes a few volts. Diacs are

used primarily for triggering of triacs in adjustable phase control of AC mains power. Some of

the circuit applications of diac are light dimming, heat control and universal motor speed control.

APPARATUS: DIAC characteristic trainer (AT), patch cords

PROCEDURE:

1. Connect the circuit as shown in fig 2a.

2. Switch ON the instrument using ON-OFF toggle switch provided on front panel.

3. Increase the voltage in steps with the voltage control potentiometer and note down the

corresponding increase in current.

4. At a particular voltage, when applied voltage approaches the breakover voltage VBO as

shown in fig 2b the device exhibits negative resistance i.e. current through the device

increases with the decreasing values of applied voltage.

5. Draw a graph between voltage and current, by taking voltage across x-axis and current

across y-axis.

OBSERVATIONS:

Sr. No.

Forward Biased Reverse Biased

Diac Voltage V (V) Diac Current I () Diac Voltage V (V) Diac Current I ()

MT1 is positive w.r.t. MT2 the break over voltage is ______ (V).

MT2 is positive w.r.t. MT1 the break over voltage is ______ (V).

RESULT:

In the V-I characteristics of DIAC the forward break over voltages, when MT1 is positive w.r.t.

MT2 ______ (V) and when MT2 is positive w.r.t. MT1 is ______ (V).

PRECAUTIONS:





3. The DIAC terminals should be checked before connecting it in the circuit.





be avoided.

7. All circuits must be regards “alive” at all time. Before touching any circuit, distribution

terminals or exposed copper, ensure yourself that it is “dead”. When dealing with high

voltage i.e. bigger than 50V, it is essential to prevent your body from touching the

circuits.

8. Apply low voltages or low power to check proper functionality of circuits.

9. Once functionality is proven, increase voltages or power, stopping at frequent levels to



10. No loose wires or metal pieces should be lying on table or near the circuit, to cause shorts

and sparking. Avoid using long wires, that may get in your way while making

adjustments or changing leads.

11. Keep high voltage parts and connections out of the way from accidental touching and


12. When working with inductive circuits, reduce voltages or currents to near zero before


Experiment No: 3

AIM: To draw the characteristics of TRIAC.

OBJECTIVES: To plot the characteristics of a TRIAC.

To find the latching current and holding current.

THEORY:

The TRIAC corresponds to a pair of antiparallel SCRs as regards its operation. With single gate

signal, the TRIAC triggered to conduct symmetrically in both the directions. Due to these

characteristics, the TRIAC is very useful in controlling AC power as in AC motor control, heat

control of a furnace, lamp dimmers, etc. The circuit symbol of a TRIAC is shown in fig. Since

the terms „anode‟ and „cathodes‟ are not applicable, connections are designed by terminals MT1

and MT2. Terminal MT1 is a reference point for measurement of voltage and current at gate

terminal G and main terminal MT2. The first quadrant of control characteristics signifies that

MT2 is positive with respect to MT1. The gate trigger characteristics are similar to that of an

SCR and are not turned ON unless power more than a minimum value specified for a device is

applied to the gate. Since the TRIAC is a bidirectional device and can have its terminals at

various combinations of positive and negative voltages, there are four possible electrodes

potential.

Fig 1 TRIAC Symbol

The V-I characteristics of TRIAC are shown below:-

Fig 2 V-I characteristics of TRIAC

APPARATUS: TRIAC characteristic trainer (AT), patch cords

CIRCUIT DIAGRAM:

Figure 3: Circuit Diagram for Plotting V-I characteristics of TRIAC

PROCEDURE:

1. Connect the circuit as shown in fig.

2. Adjust DC, Dc to minimum value (1.2V) and potentiometer P1 to its maximum value.

3. Adjust DC to 2V. Gradually increase the DC voltage in the gate circuit to the maximum

voltage. Now gradually decrease the P1 value until the TRIAC starts conducting. This is

indicated by the current meter in the DC circuit.

4. Note the TRIAC voltage, TRIAC current and Gate current.

5. For observing the load current (through 100E/5W) at different voltage, the TRIAC has to

be brought back to the non-conducting state (cut-off). This is done by increasing the POT

P1 to maximum value and breaking or disconnecting the anode circuit temporarily.

6. Repeat the steps 3,4,5,6 for the other TRIAC voltages like 4V, 6V, 8V, 10V, 12V, 14V,

15V etc. Tabulates the result and draw the graph between TRIAC voltage and current at

different gate currents. Tabulate the readings.

7. Reverse connects the voltages DC and repeats the steps from 2 to 7. Tabulate the

readings.

OBSERVATIONS:

Table 1

MT1 is positive w.r.t. MT2

Positive Gate current Ig (mA) =____

MT1 is positive w.r.t. MT2

Negative Gate current Ig (mA) =____

Voltage (V) Current (mA) Voltage (V) Current (mA)

Table 2

MT1 is Negative w.r.t. MT2


MT1 is Negative w.r.t. MT2


Voltage (V) Current (mA) Voltage (V) Current (mA)

1. For Positive Gate current Ig (mA) =____ the forward break over voltage when MT1 is

positive w.r.t. MT2 is _______ (V).

2. For Negative Gate current Ig (mA) =____ the forward break over voltage when MT1 is


3. For Positive Gate current Ig (mA) =____ the forward break over voltage when MT2 is


4. For Negative Gate current Ig (mA) =____ the forward break over voltage when MT2 is


PRECAUTIONS:

1. All the components used in the experiment should be thoroughly tested before connecting

them in the circuit.

2. A triac-diac matched pair should be used in the circuit.

3. The proper value of gate signal should be used for triggering the triac.





be avoided.




circuits.





10. Keep high voltage parts and connections out of the way from accidental touch and from

any contacts to test equipment or any parts, connected to other voltage levels.



RESULT:

In the V-I characteristics of TRIAC the characteristics is same in first and third quadrant with

different break over voltages with positive and negative gate currents as shown in observations.

The latching current is ________mA and Holding current is ________mA.

Experiment No: 4

AIM: To draw the characteristics of UJT.

THEORY: The uni-junction transistor is a three terminal semiconductor device with negative

resistance characteristics. It consists of a bar of n –type silicon with a small silicon p- type insert

(emitter) near to one of the ends. Two ohmic contacts at the end of the n-type bar constitute two

terminals, Base 1 and Base 2. The rectifying contact is called emitter. The device shows negative

resistance characteristics between emitter and base 1 terminal. The construction of this device is

indicated in figure 1a. A bar of high resistivity n-type silicon called the base B, has attached to it

at opposite ends two ohmic contacts B1 and B2. An aluminum wire, called the emitter E is

alloyed to the base to form p-n rectifying junction, Uni-Junction transistor (UJT).The standard

symbol for this device is shown in figure 1b. Note that the emitter is inclined and points towards

B, whereas the ohmic contact B1 and b2 are brought as right angles to the line, which represents

the base. As usually employed, a fixed interbase potential Vbb is applied between B1 and B2. The

most important characteristics of UJT is that of the input diode between E and B1, if B2 is open-

circuited so that IB2 = 0, then the input volt-ampere relationship is that of the usual p-n junction

diode. In figure.2 the input current-voltage characteristics are for IB2=0 and also for a fixed

value of interbase voltage Vbb. The latter curve is seen to have the current –controlled negative-

resistance characteristic, which is single, valued in current but may be multivalued voltage.

Figure 1 UJT Symbol

The V-I characteristics of UJT are shown below:-

Figure 2: V-I characteristics of UJT

APPARATUS: UJT Characteristics trainer Kit (SPL-505), 0-50 V DC Voltmeter, 0-25mA DC

Ammeter and connecting wires.

CIRCUIT DIAGRAM:

Figure 3: Circuit Diagram for Plotting V-I characteristics of UJT

PROCEDURE:

1. Connect the circuit as shown in figure 3.

2. Fixed the Dc voltage from Base2 to Base1.

3. Adjust emitter to Base1 DC voltage by varying potentiometer P1 from its minimum value

to maximum value.

4. Note down the emitter current corresponding to emitter current.

5. Find the peak voltage, peak current, valley voltage and valley current.

6. Find the intrinsic standoff ratio with the above data.

7. Plot the graph between emitter voltage and emitter current.

8. Repeat the steps from 2 to 7 with different values of Base 2 to Base1 voltage.

OBSERVATIONS:

VBB (V) =____ VBB (V) =____

Emitter Voltage (V) Current (mA) Emitter Voltage (V) Current (mA)

Peak Voltage (V) = _________

Peak Current (mA) = ________

Valley Voltage (V) = _________

Valley Current (mA) = ________

Intrinsic standoff Ratio η =________

PRECAUTIONS:

1. The three terminals of the UJT, i.e., E, B1, B2 should be checked properly before

connecting the device in the circuit.

2. The two supplies VBB and VEE should be within the safe range of the UJT.





be avoided.




circuits.












RESULT:

In the V-I characteristics of UJT the intrinsic standoff ratio is ______ and different currents and

voltages are given in observations.

Experiment No: 5

AIM: To draw the different waveforms of half wave controlled rectifier.

THEORY: The phase controlled rectifier, a circuit which converts the AC input into

controllable DC output voltage. The basic principle of phase controlled rectifier is to control the

point at which the SCR is allowed to conduct during the each AC cycle. Thus it is possible to

select the time segment of the AC voltage waves which appears at the DC terminals and the

mean output voltage is controlled continuously. When the AC voltage across the SCR is negative

it is turn off.

The working principle of single phase half wave controlled rectifier is as follows:

During the positive half cycle of supply voltage i.e. when the anode of SCR is positive w.r.t.

cathode and the SCR is fired and power is delivered to the load.

During the negative half cycle of supply voltage i.e. when the anode of SCR is negative w.r.t.

cathode and the SCR is reverse biased and no current can flow through the load.

APPARATUS: Half wave controlled rectifier trainer Kit (VPL PET-EC/SM/01), CRO (SM440)

and connecting wires.

CIRCUIT DIAGRAM:

Figure 1: Circuit diagram

PROCEDURE:

1. Connect the circuit according to the circuit diagram.

2. Connect the CRO across the input supply and output load terminals

3. Apply the gate signal at different firing angles α.

4. Observe the voltage across output terminals and different waveforms across different

components.

5. Repeat the above steps for different firing angels.

OBSERVATIONS:

S. No. Firing Angle α Output Voltage (V)

WAVEFORMS:

PRECAUTIONS:








5. The value of gate current (IG) must always be well below that of IG (max).




circuits.




be avoided.












RESULT:

As the firing angle is increased the output voltage decreases and various output voltages are

measured across load with different firing angles as shown in observations.

Experiment No: 6

AIM: To draw the different waveforms of full wave controlled rectifier.

THEORY: The phase controlled rectifier, a circuit which converters the AC input into

controllable DC output voltage. The basic principle of phase controlled rectifier is to control the

point at which the SCR is allowed to conduct during the each Ac cycle. Thus it is possible to

select the time segment of the Ac voltage waves which appears at the DC terminals and the mean

output voltage is controlled continuously. When the Ac voltage across the SCR is negative it is

turn off.

The working principle of single phase full wave controlled rectifier is as follows:

During the positive half cycle of supply voltage i.e. when the anode of SCR1 and SCR2 is

positive w.r.t. cathode and the SCRs are fired and power is delivered to the load.

During the negative half cycle of supply voltage i.e. when the anode of SCR3 and SCR4 is

positive w.r.t. cathode and the SCRs are fired and power is delivered to the load. At the same

time SCR1 and SCR2 are reversed biased and turn off.

The above processes are repeated for each cycle of AC supply.

APPARATUS: Full wave controlled rectifier trainer Kit (VPL PET-EC/SM/01), CRO (SM 440)


CIRCUIT DIAGRAM:

Figure 1: Circuit diagram

PROCEDURE:

1. Connect the circuit according to the circuit diagram.

2. Connect the CRO across the input supply and output load terminals

3. Apply the gate signal at different firing angles α.

4. Observe the voltage across output terminals and different waveforms across different

components.

5. Repeat the above steps for different firing angels.

OBSERVATIONS:

S. No. Firing Angle α Output Voltage (V)

WAVEFORMS:

PRECAUTIONS:

1. The SCRs used in the two arms must be exactly of the same rating.



3. The signal at the gate of two SCRs should be given through two separate diodes instead

of giving them directly.





circuits.




be avoided.





10. No loose wires or metal pieces should be lying on table or near the circuit, to cause

shorts and sparking. Avoid using long wires, that may get in your way while making






RESULT:

As the firing angle is increased the output voltage decreases and various output voltages are

measured across load with different firing angles as shown in observations.

Experiment No: 7

AIM: To study the different waveforms of relaxation oscillator using UJT.

OBJECTIVES: To observe the wave shape generated by UJT Relaxation Oscillator.

Calculation of ON Time period of a Relaxation Oscillator.

THEORY: A Uni-Junction Transistor (UJT) is a three terminal semiconductor switching device.

This device has a unique characteristic that when it is triggered, the emitter current increases

regeneratively until it is limited by emitter power supply. Due to this characteristic, the Uni-

Junction Transistor can be employed in a variety of applications, such as switching, pulse

generator, saw tooth generator etc.

The Uni-Junction Transistor (UJT) is a semiconductor which has a single PN junction, a P-type

emitter and an N-type bar. The N-type bar is symmetrically pressed against a split gold film

deposited on a ceramic base and the bar makes resistive (non rectifying) contacts with the film.

The bottom gold film is called base 1 (B1); the top is called base 2 (B2). The emitter junction is

physically closer to B2 than to B1.

The UJT connected as a relaxation oscillator as shown in fig generates a voltage waveform

across B1 which can be applied as a triggering pulse to an SCR gate to turn on the SCR. When

instrument is switched ON, capacitor C1 starts charging exponentially through R4 to the applied

voltage V1. The voltage across C1 is the voltage VE applied to the emitter of the UJT. When C1

has charged to the peak-point voltage VP of the UJT, the UJT is turned ON, decreasing greatly

the effective resistance RB1 between the emitter and base 1. A sharp pulse of current IE flows

from base 1 into the emitter, discharging C1. When the voltage across C1 has dropped to

approximately 2V, the UJT turns off and the cycle is repeated. The waveforms in fig illustrate

the sawtooth voltage VE generated by the charging of C1 and the output pulse VB1 developed

across R1. VB1 is the pulse which will be applied to the gate of an SCR to trigger the SCR. The

frequency f of the relaxation oscillator depends on the time constant C1R4 and on the

characteristics of the UJT. For values of R1<100, the period of oscillation T is given

approximately by the equation.

𝑇 =1

𝑓= 𝑅4𝐶1𝑙𝑛

1

1 − η(𝑒𝑡𝑎)

The value of η (eta) lies in the range of 0.51 to 0.81.

APPARATUS: UJT relaxation oscillator kit (SPL-505), CRO (SM 440) and connecting wires.

PROCEDURE:

1. Connect channel 1 of dual trace CRO across R3 (between B1 and Ground point) and

channel 2 across C1 (i.e. across Emitter and Ground). Set the mode of CRO to dual trace.

2. Switch ON the instrument using ON/OFF toggle switch provided on the front panel.

3. Observe the output waveform on CRO.

4. Calculate the time period of oscillations by using the formula:

𝑇 =1

𝑓= 𝑅4𝐶1𝑙𝑛

1

1− η(𝑒𝑡𝑎)

5. Vary the value of C1 and R4 and every time plot the output waveform.

OBSERVATIONS:

S. No. C1 R4 T(observed) T(calculated) %age error

PRECAUTIONS:

1. The three terminals of the UJT, i.e., E, B1, B2 should be checked properly before

connecting the device in the circuit.

2. The two supplies VBB and VEE should be within the safe range of the UJT.




circuits.




be avoided.












Experiment No: 8

AIM: To trigger the SCR using relaxation oscillator.

THEORY:

The ability of SCR‟s to switch rapidly from non-conducting to conducting status makes it

possible to control the amount of power applied from ac source to load. Control circuits are used

to vary the speed of motor, the brightness of the light & many other applications when the power

is to be varied. SCR is turned on by trigger pulse at beginning of each ac cycle, and maximum

power flows to load. Under these conditions the trigger is said to be “in phase” with the source

voltage. This is a conduction angle of 180 or a firing angle of 0˚.Fig 1 is a graphic representation

of this situation on the upper graph is shown the triggering signal, which consists of a series of

short pulses. The exact nature of the trigger is not very critical, except there are min & max

voltages and current specified for each type of SCR. The lower graph shows the actions in the

load circuit. Note that the time axis is same for both graphs. The dotted sine wave is source

voltages applied to anode of SCR and shaded area measure of power flowing through SCR to the

load. The firing angle represents the voltage sine wave has gone through before the trigger pulse

occurs in this case. The conduction angle represents actual time the SCR is turned on.

APPARATUS: SCR trigger kit with UJT relaxation oscillator (SPL-501), CRO (SM440) and

connecting wires.

CIRCUIT DIAGRAM:

WAVEFORMS:

PROCEDURE:

1. Connect the required supply and switch ON the unit.

2. Connect the current meter at the appropriate place in the circuit with the indicated

polarity.

3. Remove the link connecting the gate of SCR the output of UJT and observe the pulse

obtained at the output of UJT.

4. Calculate the frequency of the pulses and note in the observation table.

5. Keep the pot P1 in fully anti-clockwise position.

6. Connect the jumper link, connecting the output of UJT to the gate of SCR.

7. Note the current flowing and observe its output on the CRO.

8. Vary the load by varying P1 and note the current.

PRECAUTIONS:

1. The three terminals of SCR should be checked properly before connecting the device in

the circuit.

2. The voltage applied between the anode and cathode of the SCR must be below its peak

inverse voltage.







circuits.




be avoided.






and sparking. Avoid using long wires, which may get in your way while making






Experiment No: 9

AIM: To control light intensity of bulb using TRIAC-DIAC pair.

THEORY:

The major thrust of thyristor technology has been field of industrial process control. The

intensity of illumination of a lighting system can be controlled simply by rheostat method of

control. It is manual method and has the disadvantage of undesired power loss across the resistor.

Solid state methods of illumination control are much more effective and efficient as compared to

the rheostat method of control. The power loss caused in a solid state circuit is almost negligible

and a smooth control of illumination is possible. Illumination controllers can also be made

automatic using solid state devices. Such circuits are widely used for automatic stage lighting.

Solid – state dimmer circuits can be fabricated using SCRs and TRIAC-DIAC. They are also

cheaper than using rheostats or dimmer stats. The intensity of illumination of a source can be

varied over a wide range by using TRIAC. A TRIAC is a bidirectional device which requires a

triggering pulse at its gate for both the half cycles. For this reason, a DIAC is used as a triggering

agent. In fact, DIAC – TRIAC matched pairs are available in the market which can be used in the

circuit. The connection diagram for such a controller has been shown in figure. The R-C

triggering process has been adopted for firing the TRIAC by means of a DIAC. A pot VR1, 100

K ohms has been chosen as the variable resistance and a capacitor C3 of 0.1 micro farads has

been taken to form the RC triggering circuit. In the RC circuit, since VR1 is variable, the rate of

charging of capacitor C3 can be varied by changing the value of VR1. The DIAC will trigger

only when its break over voltage is reached. If VR1 is sufficiently large, the capacitor voltage

Vc3 will not exceed the DIAC break over voltage. In this case the DIAC will not conduct and as

a result the TRIAC will remain in OFF state. As the resistance of VR1 is decreased, the Vc3 will

increase. When the voltage exceeds the value of the break over voltage of DIAC, the DIAC is

triggered. Once the DIAC starts conducting, it wills exhibit a negative resistance characteristic in

both the directions, which will in turn trigger the TRIAC by sending the gate signal. A similar

operation takes place in the negative half cycle and a negative gate pulse will be applied when

the DIAC break down in reverse direction. By changing the value of VR1, the firing angle of the

DIAC can be controlled which in turn will control the firing angle of TRIAC. Less the value of

the firing angle of the TRIAC, more will be the voltage across the lamp and hence more will be

intensity of illumination and vice versa.

APPARATUS: Intensity control kit using TRIAC-DIAC pair (MARS-ME 794), bulb and

connecting wires.

CIRCUIT DIAGRAM:

PROCEDURE:

1. Study the circuit configuration given on the front panel carefully.

2. Mount 40 W bulb in bulb holder mounted on the front panel.

3. Connect the gate to diac by connecting dotted line through patch cord.

4. Switch ON the instrument using ON/OFF toggle switch provided on the front panel.

5. Now vary the value VR1 and observe the corresponding effect on the intensity of light.

PRECAUTIONS:



2. A triac-diac matched pair should be used in the circuit.

3. The proper value of gate signal should be used for triggering the triac.




circuits.



Experiment No: 10

AIM: To control the speed of a universal motor using SCR.

THEORY: In many applications variation in speed of electric drives is essential. For example in

process industries the speed control of motors in different stages are required. The introduction

of Thyristors solves these problems very efficiently. The fast response, reliability and less cost

dominate this scheme very popular today. This set up is designed to study of basic speed control

of small shunt wound dc motor using silicon controlled rectifier. It has following features:

Fully isolated ac supply is used for control and motor unit. The isolator has capacity of 75 VA

which is sufficient for the experiment. Phase control motor speed system based upon silicon

controlled rectifier type number TIC 612 T, has PIV equal to 600 V and current rating 12 Amp

according to data sheet. It requires 4 to 6 mA gate current for rated operation in set up. High

power resistors and plus rated capacitor to ensure long life for the set up. 1/16 HP shunt wound

motor has rated armature voltage 90 V, current .5 max at 2500 RPM. Field supply is 30 V at .5

Amps, shaft dia. 10 mm and rated torque 1 Kg-cm.

APPARATUS: Speed control kit (EC/SM/01 OR EC/SM03), universal motor, CRO(SM 440)


CIRCUIT DIAGRAM:

PROCEDURE:

1. Connect the given line cord with main‟s outlet. Adjust pot VR at its mid way. Switch on

power. The motor will start to run.

2. Connect CRO ground lead with (1) socket and live with (2). Observe voltage waveform

at UJT oscillator. Observe the sharp dip occurs when UJT triggers. The positive voltage

across UJT goes to zero at zero cross of input ac cycle. This leads to synchronize the

firing pulse with line.

3. Connect CRO with gate trigger, socket 3, and its other channel with socket 1:10

attenuated at socket 4.

4. Connect multimeter in DC voltage mode and measure dc voltage across the armature.

5. Adjust speed control VR for low speed of motor and high gradually. Observe the firing

pulse position and armature voltage waveform. Note the decrease in α, cause to increase

the armature voltage.

6. The motor speed N, is nearly proportional to armature voltage Ea, so

a. N = [(Ea-IaRa)/ Kaφ] and Ea = [(Em/П (1+cos α)]

b. Where, Ka is armature constants, φ is field flux, α is firing angle and Em is peak

input voltage.

7. Adjust VR, and measure the firing angle α, from the CRO waveform. Note the dc voltage

across armature. Plot a curve between voltage and firing angle. Since motor speed is

proportional to dc voltage Ea, thus smooth speed control characteristic is shown by the

curve.

8. The voltage waveform across the thyristor can be observed connecting single channel

CRO across the both 1:10 attenuated sockets keeping ground lead with 1:10 attenuated of

socket 4. Note no other channel use this time.

9. The motor current waveform can be observed connecting CRO across socket 1 and 5.

10. To measure α, connect CRO with socket 2 and 1, where 1 is for ground lead. Trigger

CRO with this signal. Adjust CRO time base to obtain single waveform as shown in fig.

connect second channel with socket 3 and measure α as shown in fig

11. There is carbon brush noise appears in waveform.

WAVEORMS:

PRECAUTIONS:



2. Isolation transformer should preferably be used in the supply before the bridge rectifier.

3. The signal at the gate of the SCR should be given through a diode instead of giving it

directly.





circuits.




be avoided.






and sparking.

11. Avoid using long wires, that may get in your way while making adjustments or changing

leads.

Experiment No: 11

AIM: To draw the different waveforms of a chopper circuit.

THEORY:

Speed control of DC motor using Thyristor voltage is needed wherever the DC supply is already

available from AC supply. The chopper circuits are a means of providing variable (average) DC

supply to loads by varying the turn ON/OFF ratio of the switching voltages. As the name

chopper implies a DC voltages is converted into AC voltage by switching through thyristor ON

and OFF in a form shown in fig-1.

Figure 1: Principle of Chopper

This implies that, although the input voltage is constant DC, the average of DC voltage given to

load can be adjusted. There are ways of obtaining the variable mark space ratio or ON and OFF

time (time ratio control, TRC)for voltage control.

ton constant and T (or frequency) adjustable.

T constant and ton adjustable or

ton and T both adjustable, and for all three

Vo = Vton/t

The frequency of switching is made high so that filtering should be minimum and the response is

high compared to that of the power frequency with phase controlled methods. Frequencies

between 500 and 2000HZ are common chosen at higher frequencies the commutating capacitors

do not have sufficient time to charge up to its full value.

APPARATUS: Chopper circuit kit (VPL PET-EC/SM/01), CRO (SM 440) and connecting

wires.

CIRCUIT DIAGRAM:

PROCEDURE:

1. Connect 250V, DC power supply to the appropriate terminals of the Morgan chopper

circuit.

2. Connect isolated firing pulses to the SCR‟s gate and put in minimum frequency position.

3. Connect 40W lamp load and switch ON the system.

4. Observe the waveforms across load

5. Compare the waveforms

WAVEFORMS:

PRECAUTIONS:



2. The signal at the gate of SCR should be given through diodes instead of giving them

directly.





circuits.




be avoided.



and sparking.


leads.





Experiment No: 12

AIM: To study the SMPS Kit.

APPARATUS: SMPS Kit (Nvis-Technologies, NV-7002).

THEORY:

The SMPS NV7002 trainer is a very adaptable product that has been designed to explain a very

remarkable and frequently used Switching based power supply - The SMPS (Switched Mode

Power Supply).

The Trainer is designed to understand each section of SMPS in straight forward way. Various

test points has been provided so that one can observe the inputs and outputs of each block

contained. Being different from a conventional block diagram internal structures of different

blocks are also shown. Switching Transformer and Chopper (the Heart of SMPS) are also

presented to readily understand their operation and pin configuration.

SMPS consists of a rectifier section, filter section, switching section and regulator section. Each

section is explained separately and the internal structure of different blocks is also described.

Switching transformer and chopper controller circuit are the main parts of SMPS. Switching

Transformer works at high frequency, so it is also called as HFT i.e, High Frequency

Transformer and the chopper controller is simply DC to DC controller. It gives constant output

even when the AC mains is varied from 80V to 270V. Students can vary Voltage by using

Variac.

Input rectifier stage: If the SMPS has an AC input, then its first job is to convert the input to

Dc. This is called rectification. The rectifier circuit can be configured as a voltage doubler by the

addition of a switch operated either manually or automatically. This is a feature of large supplies

to permit operation from nominally 120 volt or 240 volt supplies. The rectifier produces an

unregulated DC voltage which is then sent to a large filter capacitor.

Input rectifier

and filter

Inverter

“Chopper”

Output

transformer

Input rectifier

and filter

Chopper

Controller

Block Diagram of Mains operated AC-DC SMPS with output voltage regulation

Inverter stage:

The inverter stage converts DC, whether directly from the input or from the rectifier stage

described above, to AC by running it through a power oscillator, whose output transformer is

very small with few windings at a frequency of tens or hundreds of kilohertz. (KHz). The

frequency is usually chosen to be above 20 kHz, to make it audible to humans. The output

voltage is optically coupled to the input and thus very tightly controlled.

Voltage converter and output rectifier:

If the output is required to be isolated from the input, as is usually the case in Mains power

supplies, the inverted AC is used to drive the primary winding of a high frequency transformer.

This converts the voltage up or down to the required output level on its secondary winding. The

output transformer in the block diagram serves this purpose.

If a DC output is required. The AC output from the transformer is rectified. For output

voltages above ten volts or so, ordinary silicon diodes are used. For lower voltages, Schottky

diodes are commonly used as rectifier elements; then they have the advantages of faster recovery

times than the silicon diodes (allowing low pass operation at higher frequencies) and a lower

voltage drop when conducting. For even lower output voltages, MOSFET transistors may be

used as synchronous rectifiers; compared to Schottky diodes, these have even lower ON state

voltage drops.

The rectified output is then smoothed by a filter consisting of inductors and capacitors.

For higher switching frequencies, components with lower capacitance and inductance are

needed.

Experiment No: 13

AIM: To study the temperature control.

THEORY:

Phase Control Circuits may be used for regulating temperature. Fig shows the connection

diagram for one such diagram.

It is a simple full-wave phase control circuit. By adjusting resistance R1 and R2 we can fix the

reference temperature for the load. Z1 is a Zener diode which gives a fix voltage across it. This

voltage appears across the thermistor R4. When the voltage across thermistor R4 is sufficient to

charge the capacitor C1 to a voltage equal or more than the break over voltage of the diac, the

diac is triggered and sends a trigger pulse to the gate of the triac. The triac starts conducting, thus

connecting the heater element in the circuit. As the temperature increases, the thermistor

resistance decreases and as such, the voltage across the capacitor is reduced. This increases the

firing angle of the triac thus reducing the voltage across the heater element accordingly and

consequently reduction of heat takes place. Gradually a stage comes when the voltage across the

capacitor C1 becomes insufficient to trigger the diac and the triac is automatically switched off.

This results in the disconnection of the heater element from the circuit

APPARATUS: Temperature control kit (EC/SM/02).

PROCEDURE:

1. Connect 220V, 50 Hz AC power supply to the appropriate terminals of the circuit.

2. Adjust the resistances R1 and R2 to fix the reference temperature.

3. Observe the temperature across thermistor.

4. Compare the waveforms across triac.

PRECAUTIONS:



2. For 220V, 50 Hz AC, R1 may be chosen as 47k, 2 W.

3. Capacitor should be of 0.1µF.

4. Rating of Zener diode may be decided by the break over voltage of the diac.




circuits.




be avoided.






and sparking.


leads.



Experiment No: 14

AIM: To study UPS.

THEORY:

An Uninterruptible power supply (UPS), also known as a battery backup, provides

emergency power and, depending on the topology, line regulation as well to connected

equipment by supplying power from a separate source when utility power is not available. It

differs from an auxiliary or emergency power system or standby generator, which does not

provide instant protection from a momentary power interruption. A UPS, however, can be used

to provide uninterrupted power to equipment, typically for 5–15 minutes until an auxiliary power

supply can be turned on, utility power restored, or equipment safely shut down.

While not limited to safeguarding any particular type of equipment, a UPS is typically used to

protect computers, data centers, telecommunication equipment or other electrical equipment

where an unexpected power disruption could cause injuries, fatalities, serious business disruption

or data loss. UPS units range in size from units to back up single computers without monitor

(around 200 VA) to units powering entire data centers, buildings, or even cities (several

megawatts).

Offline / standby UPS:-

The Offline / Standby UPS (SPS) offer only the most basic features, providing surge protection

and battery backup. With this type of UPS, a user's equipment is normally connected directly to

incoming utility power with the same voltage transient clamping devices used in a common

surge protected plug strip connected across the power line. When the incoming utility voltage

falls below a predetermined level the SPS turns on its internal DC-AC inverter circuitry, which is

powered from an internal storage battery. The SPS then mechanically switches the connected

equipment on to its DC-AC inverter output. The switchover time can be as long as 25

milliseconds depending on the amount of time it takes the Standby UPS to detect the lost utility

voltage. Generally speaking, dependent on the size of UPS connected load and the sensitivity of

http://en.wikipedia.org/wiki/Emergency_power_system

http://en.wikipedia.org/wiki/Computer

http://en.wikipedia.org/wiki/Data_center

http://en.wikipedia.org/wiki/Telecommunication

http://en.wikipedia.org/wiki/Volt-ampere

the connected equipment to voltage variation; the UPS will be designed and/or offered

Offline / standby UPS

(specification wise) to cover certain ranges of equipment, i.e. Personal Computer, without any

obvious dip or brownout to that device.

Line-Interactive UPS

http://en.wikipedia.org/wiki/File:Standby_UPS_Diagram.png

http://en.wikipedia.org/wiki/File:Standby_UPS_Diagram.png

http://upload.wikimedia.org/wikipedia/commons/5/56/Line-Interactive_UPS_Diagram.png

The Line-Interactive UPS is similar in operation to a Standby UPS, but with the addition of a

multi-tap variable-voltage autotransformer. This is a special type of electrical transformer that

can add or subtract powered coils of wire, thereby increasing or decreasing the magnetic field

and the output voltage of the transformer.

This type of UPS is able to tolerate continuous under voltage brownouts and overvoltage surges

without consuming the limited reserve battery power. It instead compensates by auto-selecting

different power taps on the autotransformer. Changing the autotransformer tap can cause a very

brief output power disruption, so the UPS may chirp for a moment, as it briefly switches to

battery before changing the selected power tap.

Autotransformers can be engineered to cover a wide range of varying input voltages, but this also

increases the number of taps and the size, weight, complexity, and expense of the UPS. It is

common for the autotransformer to only cover a range from about 90V to 140V for 120V power,

and then switch to battery if the voltage goes much higher or lower than that range.

In low-voltage conditions the UPS will use more current than normal so it may need a higher

current circuit than a normal device. For example to power a 1000 watt device at 120 volts, the

UPS will draw 8.32 amps. If a brownout occurs and the voltage drops to 100 volts, the UPS will

draw 10 amps to compensate. This also works in reverse, so that in an overvoltage condition, the

UPS will need fewer amps of current.

http://en.wikipedia.org/wiki/Autotransformer

http://en.wikipedia.org/wiki/Electrical_transformer

http://en.wikipedia.org/wiki/Brownout_(electricity)

Experiment No: 15

AIM: To study the inverters.

THEORY:

Charging is required in most of the instruments. Charger provides a voltage equal or greater to

that of battery voltage to charge the battery. A charger consists of step-down transformer which

converts high I/P ac voltage to low O/P AC voltage. Using rectifier it is converted into DC

voltage. A filter circuit is used after rectifier to filter the harmonics of DC voltage. Then voltage

regulator is used to control its O/P and then this O/P is directly given to battery for charging. The

battery charger is an ac-dc converter that will supply the battery with a dc voltage so it remains

charged. While the ac line is powered the charge will complete this conversion. If power to the

ac line is lost the charger will remain idle until power is restored, and it will continue charging

the battery.

APPARATUS: Inverter kit (Nivs-Technologies-NV-6001) and CRO (SM 440).

CIRCUIT DIAGRAM:

BLOCK DIAGRAM:

PROCEDURE:

To understand the functioning of inverter Trainer:-

1. Keep the controls of the training board as follows.

2. Power switch should be in OFF position.

3. Inverter switch should be in OFF position.

4. Connect battery (+) ve and (-)ve terminals to the trainer board very carefully to the same

polarity.

5. Note: If battery (+) ve and (-)ve terminals to the trainer board are connected to the

opposite polarity in that case fuse which is provided on the board will damage.

6. Connect an appropriate load to an O/P socket.

7. Press the inverter switch to ON position.

8. Check the load on the O/P socket that whether it is working or not.

To understand the functioning of inverter Trainer in presence of main supply and

understand the charging of battery.

1. Keep the controls of the training board as follows.

2. Power switch should be in OFF position.

3. Inverter switch should be in OFF position.

4. Connect main cord to trainer board.

5. Switch ON the power supply for unit.

6. Set multimeter on AC range.

7. Measure 230V AC directly on O/P socket.

8. When inverter switch is in OFF position and mains power is present then the inverter

circuit is not working due to relay switching and on O/P socket 230V supply is available.

9. Now measure the 230V AC on the I/P of step down transformer at TP1 and TP2.

10. Measure the O/P AC voltage of step down transformer at TP12 and TP13.

11. Set mulimeter on DC range.

12. Now measure the O/P of the battery section on TP16 & TP3. This is DC voltage used for

charging.

13. This DC voltage is converted from low AC voltage by passing them through Rectifier

circuit and then through filter circuit to make it pure DC voltage.

PRECAUTIONS:

1. Use only isolated power sources (either isolated power supplies or AC power

through isolation power transformers). This helps in using a grounded oscilloscope. This

reduces the possibility of risk of completing a circuit through your body. This also

reduces the possibility of destroying the test equipment.

2. Check for all the connections of the circuit and scope connections before powering the

circuit, to avoid shorting or any ground looping that may lead to electrical shocks or

damage of equipment.

3. Check any connections for shorting two different voltage levels.

4. Double check your wiring and circuit connections. It is a good idea to use a point to-point

wiring diagram to review when making these checks.






and sparking.


leads.





WAVEFORMS: