LAB TASK

Equipment Use:

Generate the signal (Volt) using the function generator and measure the signal using the CRO to verify the results.

RC Circuits analysis:

Now that you have reviewed the RC circuits and time constants, its time to practically observe the things. Perform the following experiment and verify the charging time of the RC circuit.

Square waves are used in digital circuits, which respond to signals that are either ON (also called “high”) or OFF (also called “low”). Most signal generators found in labs will have a square wave function. The photograph shows a CRO display of square waves.

 

Connect the circuit in the following topology

The CRO will not display square waves, but a saw-tooth profile.

The pattern can be explained:

• When the voltage is at +VS, we see the capacitor charging up, and the voltage increasing with the charge. The voltage rises exponentially.
  
• When the voltage is 0, the capacitor voltage decays exponentially.
  

The picture above shows the voltage display of a 500 Hz square wave across a 0.1 mF capacitor in series with a 1000 W resistor. 1 horizontal division (cm) corresponds to a time period of 0.5 milliseconds.  

The RC time constant = 0.1 ´ 10^-6 F ´ 1000 W = 1.0 ´ 10^-4 s = 0.1 ms.

Period, T = 1/f = 1/500 = 0.002 s = 2 ms.

Therefore the time take for 1 half-cycle is 1 ms. If we consider that the time taken for the capacitor to charge fully is about 5 RC, we can see that the capacitor is fully charged by 0.5 ms, which is 1 cm on the CRO screen. The diagram below shows the voltage across the capacitor compared the source square wave:

 We can alter the profile of the saw tooth by changing the RC time constant. Here we see a profile of a 0.5 mF capacitor. The RC time constant is 0.5 ms. The frequency is still 500 Hz.

Here we can see that the capacitor has not quite fully charged up before the reverse half cycle.

The higher the frequency, or the longer the RC time constant, the less the capacitor gets charged up. So the capacitor voltage is less. This next picture shows the voltage display of the 0.5 mF capacitor at a frequency of 1000 Hz.

Notice that the voltage (vertical display) is lower than the voltage in the previous picture. This is because the capacitor has not charged up as much.

This observation is consistent with the previous observations of the reactance of the capacitor reducing as the frequency gets higher. Since V = IXc, if V is reduced, and I remains the same (as it would in a series circuit), the reactance Xc must be reduced.

 Now we will alter the circuit so that the CRO reads the voltage across the resistor:

 

The wave pattern on the CRO is shown below:

 

 
 

The pattern can be explained:

• When a capacitor is being charged up, the current falls exponentially.
  
• When it gets discharged, there is also an exponential fall off in the current. Notice that the current is positive when the capacitor is charged, and negative when it is discharged.
  

 Since V = IR, we can say that the voltage across the resistor represents the current.

When the charge on a capacitor is zero, the current flowing on the plates is a maximum, while the voltage is zero.

So if we increase the frequency, we will see this happening:

Notice that the current does not fall off as much as when the frequency is lower . If we made the frequency very high the current would be very close at its maximum value. In effect the capacitor would have very little “opposition” to the flow of the current. This is consistent with the reactance of the capacitor being very low at high frequencies.

 
 

We can summarise the behavior of RC filter circuits in this table:

Frequency	VR	Vc	Xc
Low	Low	High	High
Break Frequency	Vc = VR	Vc = VR	Xc = R
High	High	Low	Low

 

This experiment has been taken from the following source:

http://www.antonine-education.co.uk/Physics_A2/options/Module_9/Topic_2/square_waveforms.htm

OSCILLOSCOPE

Oscilloscope is used for the several purposes:

• Its primary use is to see the waveform of the signals on the screen.
  
• It can be used to determine the frequency of the signal.
  
• It can be used to compare two signals.
  
• It can be used to see the AC component of the signal separately
  
• It can be used for the measurement of the signal amplitude.
  
• It can be used for the adjustment of the timing of the signals.
  

   

  1 – Channel input
  

  The oscilloscopes available in college lab have two input channels. Channel 1 and 2.
  

  2
  

  –

  Volts/Division
  

  This tab is used to adjust the scaling for the amplitude of the signal. The tab selects the volts/division for the signal display. If the tab is set at 2V/div it means that each vertical division equals 2 V.
  

  3 – Time/Division
  

  This tab adjusts the scaling in the horizontal axis. If the tab is set to 1 ms/ div it means that each division corresponds to 1ms of time unit.
  

  4 – Slide Switch: This switch can be used to visualize the AC and DC components of the signal.
  

  At top is the AC mode. In this mode the signal input is passed through a capacitor in series which blocks the DC shift of the signal and only the AC component is visible.
  

  In the middle is the GND, when the switch is in this position the there will be no signal on screen except for a dead line at the centre (if the centre line is calibrated at the centre otherwise it is shifted accordingly.
  

  At the bottom position, the signal is displayed as such with full DC and AC components.
  

  5 – Vertical Position
  

  This tab adjusts the vertical position of the centre. It must be calibrated such that the centre line (dead line) is at the exact centre if slide switch is set to GND. ADD will add both the signals.
  

  6 –
  

  7 – Horizontal Position
  

  This tab adjusts the horizontal position of the origin. This tab is usually used to shift the signal right or left during measurements.
  

  8 –
  

  9 – Power button
  

  10 – Magnification
  

  11 – Intensity For adjusting the intensity of the display signal
  

  12 – Illumination Used to adjust the illumination of screen (back light).
  

  13 – Focus This will adjust the focus of the display signal. Adjust it for best focus.
  

  14 – Channel Select
  

  CH1 and CH2 will select channel 1 or 2 individually. ALT will display both channels simultaneously. CHOP will display each channel individually at alternate triggering cycles.
  

  15 – Channel invert This button if pushed will invert the signal on the channel 2 of oscilloscope.
  

  16 – Trigger Mode Under normal conditions set this to top most position.
  

   

  17 – Trigger Source For normal operation set this to the internal trigger source/
  

  18 – Trigger Level Set this to middle value for normal trigger postiion
  

  19 – Slope
  

  20 – Calibration Signal
  

  The Probe is placed here to calibrate the Oscilloscope. The signal available here is 1 kHz 1 V peak to peak. The Volts/div and the time/div scales are adjusted so that the signal appears in the corresponding divisions. The rest adjustments are made to make sure the oscilloscope is working and calibrated.
  

  21- GND
  

   

  See the animation at: http://hyperphysics.phy-astr.gsu.edu/Hbase/electronic/scope.html#c1
  

Procedure for calibration of OSCILLOSCOPE

First the calibration probe is inserted in the calibration pin just below the button 10 and the earth part of the probe is inserted in port 21 which is the ground. The default specifications of the output from this probe are 1 KHz and 1 V peak-to-peak. The slide switch (4) is pointed to GND and then the dead line is calibrated such that it is in the centre of the screen by using the Vertical and Horizontal Position Buttons the time/division (button 3)
is
set to 1 ms/division. The volt/division scale (button 2) is selected appropriately.

Power Supply

POWER SUPPLY

The right knobs are for the adjustment of voltage and the left knobs are for the adjustment of the current. The left knobs are often used for restricting the current output of the supply.

The supply can be used as voltage source, as in normal operation, and current source when the current is restricted. It will provide constant current and not voltage as a current source. These lab power supplies are designed for good regulation however the maximum current drive is less and can not exceed a certain amount of current (approx 4 A) after this the circuit breaker disconnects the supply.

KIRCHHOFF’S LAWS

Kirchhoff’s Current Law

At any point in an electrical circuit that does not represent a capacitor plate; the sum of currents flowing towards that point is equal to the sum of currents flowing away from that point.

Adopting the convention that every current flowing towards the point is positive and that every current flowing away is negative (or the other way around), this principle can be stated as picture below.

 

  n is the total number of currents flowing towards or away from the point.

Kirchhoff’s Voltage Law

The directed sum of the electrical potential differences around any closed circuit must be zero.
Similarly to KCL, it can be stated as in picture above.

Here, n is the total number of voltages measured.

Inductors

The inductor is another example of a passive element that stores energy. Inductors do this by storing energy in the form of a magnetic field, or B-field.

An inductor consists of a coil of wire wound around an iron or non-magnetic (air) core. Current passing through the inductor establishes a magnetic flux. The resulting magnetic field can store energy that is transferred back and forth between the electric circuit and the magnetic circuit produced by the inductor. The amount of inductance is measured in Henrys, the amount of magnetic flux produced by each ampere of current.

    V = L*ΔI/ΔT        (electrical definition for inductance)

Capacitors

A capacitor is a device that stores energy in an electric field, by accumulating an internal imbalance of electric charge. It is made of two conductors separated by a dielectric (insulator).

Capacitors will pass AC currents but not DC. Throughout electronic circuits this very important property is taken advantage of to pass ac or RF signals from one stage to another while blocking any DC component from the previous stage.

The capacitance of a capacitor is a ratio of the amount of charge that will be present in the capacitor when a given potential (voltage) exists between its leads. The unit of capacitance is the farad which is equal to one coulomb per volt. This is a very large capacitance for most practical purposes; typical capacitors have values on the order of microfarads or smaller.

Where C is the capacitance in farads, V is the potential in volts, and Q is the charge measured in coulombs.
The basic equation for capacitance  and the current flow through capacitor is:

Capacitor Labeling

            First two numbers forms the 2 digit value (in pF) and the third is the exponent.

K = Maximum working DC voltage

M = Tolerance

Types

Ceramic Disc, Ceramic Dipped, Resin-potted mylar/polyester, Electrolytic, Tantalum

 

Capacitors in series and parallel
Capacitors in parallel ADD together as C1 + C2 + C3 + ….. While

capacitors in series REDUCE by:
1 / (1 / C1 + 1 / C2 + 1 / C3 + …..).

Applications
Low Pass Filters (Remove high frequency noise)
Voltage Regulation
Block voltage spikes

Resistors

            As the name suggests, these elements resist the flow of electric current.

Resistance in series and parallel

Resistors in series add together as R1 + R2 + R3 + ….. While resistors in parallel reduce by 1 / (1 / R1 + 1 / R2 + 1 / R3 + …..).

Resistance and Power

It follows as a fundamental rule in using resistors in electronic circuits that the resistor must be able to comfortably handle the power it will dissipate. A rule of thumb is to use a wattage rating of at least twice the expected dissipation. (P = I * I * R)

Common resistors in use in electronics today come in power ratings of 0.25W, 0.5W, 1W and 5W. Other special types are available to order. Because of precision manufacturing processes it is possible to obtain resistors in the lower wattage ratings which are quite close in tolerance of their designated values. Typical of this type are the .25W range which exhibit a tolerance of plus / minus 2% of the value.

Resistance color chart codes

Technical report writing

Article by Muhammad Uzair Sukhera

De – 27 Electrical Engineering

uzairasif@hotmail.com

 

 

 

Do not make report writing a headache! Make it fun for you. Always document your project while it is in progress. Documenting the project once it is complete is unfortunately a bad practice in many engineers. This late documentation not only makes the report writing a formality but also misses out important technical details that were taken care of during the implementation phase!

 

Documentation is an essential part of all engineering projects. The technical details of each and every module of a project can not be kept in memory! These technical details are critical to system design, functionality and more often to troubleshooting. Thus it is essential to have a record of all the technical details in a well documented manner. Well, I stress “well documented”, why? Imagine the technical details and documentation in a scrambled form in some computer hard disk. When the need arises, lot of time and energy will surely be wasted to look for the right stuff.

 

“Imagine in big industrial unit, a catastrophe is caused just because the project engineer did not document the critical temperatures for the chemical process!”

Report writing has no specific fixed format. It is written according to one’s need and the purpose. However there are certain essentials that technical reports MUST HAVE

1.       Statement of problem

State clearly the task at hand. It includes the purpose of the report.

Example: The given task was to write a report for the lab experiment.

 

2.       Diagram

You are writing a “technical report”, o yes, but it doesn’t mean that it simply contains mere facts and figures! It will make the reader sleep. Make it as much interactive as possible. The engineering processes are often best explained through visual aids than through words!

Example: Diagram and/or flow chart is to give visual description of the task assigned.

 

3.       Strategy

Engineering is all about innovation: Applying the theory to solve the problem in the best possible manner. Most problems can be solved in number of ways. An engineer’s task is to find the best methodology.  To determine the best method one has to determine all the methods that may help in solving the problem. The pros and cons of these methods are discussed, evaluated and then the theme plan is developed based on some facts and/or preliminary experiments.

 

 

 

Example: For the given problem there are two approaches: to make the report stereotypical or to keep the format flexible and innovative to keep the readers interest alive.

 

4.       Implementation

Once the modus operandi has been selected what’s the next task? Of course GET TO WORK. So the next heading is naturally the implementation. Now here the details of the methodology adopted, the design process, equations, the technical details all come in. Most often the problems faced during implementation are also discussed. Two other subheadings can be.

a.       Datasheet

b.      Tables

Hey I said datasheets. But it did not mean copy paste the whole data sheet. It is just to mention the technical details from datasheet and the design steps you have taken for its to meet those requirements. Most text book methods of report writing prefer appending the datasheets at the end of report as annexure.

5.       Results

Was the project without any results? No. So state the RESULTS.

So in the last you got to discuss what did u achieve? How many targets were accomplished? What was the outcome? Get down to start and REVIEW your work!

6.       Recommendations

Now after the completion of your work you are capable of making recommendations for future work. Guiding others of the possible choices they have and the probable solutions to the problems faced by you. Recommendations might also include few solid suggestions for the supervisors, in case of students, the professors and lab demonstrators. However, be sure not to over suggest! It may cost you some marks

7.        References

Technical papers, research work and documentation are incomplete without proper references. Remember copying text from somewhere without referencing is plagiarism – a serious crime! Moreover, citing different technical papers, books, thesis or reports may increase the worth of your work, but keep a balance and do not include useless references.

Controllers for Dummies

Welcome to this page: Its about controllers and dummies! Can dummies controll devices like controllers? NO! So what are u here for? To controll controller and NOT being a dummy. So lets get to bussiness. The brief description of course are as below:

---

 

 

The course comprises tutorials on different important concepts involved in robotics with regards to the NERC competition. The controller emphasized in the course is 8051; the programming language followed is C. It is assumed that the readers have at least studied the basic electric circuits and basic programming courses. The modules of this course are as follows:

 

The course comprises tutorials on different important concepts involved in robotics with regards to the NERC competition. The controller emphasized in the course is 8051; the programming language followed is C. It is assumed that the readers have at least studied the basic electric circuits and basic programming courses. The modules of this course are as follows:

Electric Circuits Review – review the basic circuit elements.

Digital Logic Review – touches upon the basic yet crucial concepts of DLD that are a pre-requisite for hardware and controllers

Electronics Review – briefly goes through the BJT theory and emphasis is on the use of transistor as switch.

Memory and architecture – builds upon the previous concepts and jumps on the memory and system architecture review.

Introduction to 8051 MCU – a brief introduction to controllers, familiarize the reader with 8051 microcontroller.

SFRs – introduces the 8051 programming and describes the special function registers of 8051 mcu.

Timers – explains the working and applications of timers in 8051 mcu.

Interrupts – explains the theory and practice of interrupts with some solid examples.

PWM – the classical way of control used in NERC, Pulse width modulation is explained along with ‘how to generate pwm’

Sensors – sensors are the crucial part of robot as far as line tracking as is concerned. So they have been explained in detail.

H-bridge – This is what makes your motors running in both directions. A comprehensive tutorial is all u want!

 A tutorial for lab report writing is also included with the package. So don’t miss it!

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