Question Deck Flashcards
Q B1 E
From the eq 1/RT = 1/R1 + 1/R2 it is clear that the very large resistance will give a very small value.
Thus we can approximate the resistance to be equal to be R2 .
The larger the value for R1, the more accurate the approximation.
Using the percentage difference eq you can find the error.
Q B2
a)
The relationship is non linear for Vout vs R2/R1 !
Q B2 - b)
How you approach solving this and what is the answer?
You would approach this by calculating the resistance through the light bulb.
Since you know the power, you can use P = V2/R rearranged in the form, R = V2/R.
Calculating the resistance you can see that it is lower, and thus it would short circuit** R2 and the potential divider wouldn’t be maintained, in other words, the potential divider would not be **stiff.
How do you convert this expression so that it can written in the phasor form?
You can bring the minus in, and it will change the signs of the terms inside the brackets, this is a feature of sines.
Next using the identity cos(x) = sin(90-x)
Similarly, sin(x) = cos(90° - x).
Q. B2 - d)
- Why is it fairly easy for the Wheatstone bridge to detect changes?
- Since it is a null measurement (no offset) all you have to do is keep increasing the sensitivity of the voltmeter to keep measure smaller imbalances.
- Precautions?
- Keeping temperature stable in the room
- Using low current to avoid heating
- Using low contact resistance
- And a very stable power supply
- Do you think you could detect such small changes with a multimeter
- No, it’s hard to detect small changes with a multimeter.
State Kirchoffs laws
1st law: The sum of the currents going into a node of a circuit must be equal to zero, ΣI = 0.
2nd Law: That the sum of voltages around a closed loop is zero.
Wheatstone Bridge
How would you analyze the current through a wheatstone bridge.
First, know that the arrows at the first node the current arrows need to be pointing away from it, and for the last node of the wheastone the current needs to be pointing towrads it.
For the current in the middle, if the wheatstone is not balanced the current needs to be able to keep both nodes on either side at zero.
State the Superposition Theorem
The current that flows in a branch of a circuit is the sum of the currents that would flow if each source was considered separately.
How would you calculate the Rth of this wheatstone bridge?
take the parallel resistance for both sides and then add them in series.
This is visually illustrated in the image.
How would you calculate the Vth for this circuit?
You have to take into account the potential drop of the whole branch.
Norton’s theroem:
Any linear network of voltage/current sources can be replaced by an ideal current source In in parallel with a resistor.
If IN is flowing between the two terminals, how would you define the current through the rest of the circuit?
Since IN is flowing between the terminals, we know that the node has to output IN in that direction.
We also know that the current in the other branch must be different because of current division.
Lastly, we know that the current in the second node needs to add back up to the initial current flowing into the first.
So the current in 2v branch is I1 and the current flowing into the first node is I1 + IN .
How do you convert the supply circuit below into Thevenin?
Convert the 10v and 6v into Norton
Combine the two current sources, you add them, and combine the resistors in parallel. Convert into Thevenin.
Convert the other current source into voltage source, convert to Thevenin.
Combine the voltage sources and resistors, taking into account direction of voltage source.
Add the load and calculate current from there.
How would you solve this ?
In this case, you are going to need to create two different equations.
One nodal analysis eq for V1
and another for Vy, the current is equal to the voltage drop across the resistor, divided by the resistance.
In a purely inductive circuit the voltage and current waveforms are not in phase. Which one lags?
in an inductive circuit, current “LAGS” voltage.
Out of the current and voltage waveforms, what lags what?
Voltage lags behind current.
CIVIL mnemonic
One way to memorise these current/voltage (I/V) relationships in capacitors(C) and inductors (L) is to consider the positions of letters in the word CIVIL. The first three letters CIV indicate that in a capacitor (C), V lags (comes after) I, and the last three letters VIL indicate that I lags (comes after) V in an inductor (L).
what does -j * -j equal to ?
-1 !
negatives cancel out!
What is the cutoff freqency defined as?
The cutoff frequency is defined as the freuqency at which the transfer function magnitude is decreased by 1/√(2) from its maximum value.
How do you convert signal frequency into radians per second?
multiply frequency by 2pi.
What is the phase angle of this eq in radians
(-60/360) * 2pi
Solve e).
It is important to note that a negative sign can be taken out of a cosine, and this does not add a negative sign outside of the cosine.
What are the 2 simple identities to convert between cosine and sine?
Using the concept of phasor combine the following sinusoidal functions into a single trigonometric expression.
How do you convert a phasor into complex number form?
Draw a simple diagram of a four-pole, three-phase synchronous machine, ….
Draw a circuit to represent the synchronous reactance model of a synchronous generator (neglecting winding resistance). By means of a phasor diagram, show that the power delivered by a synchronous generator is given by:
Important things to remember?
Turn into phasor.
Current division is the impedance of your branch over the impedance of all branches..
What should you think of if you see this sort of question?
How would you solve it
You can see that it is a filter,
Also, they want you to find Vo, so you should think of using Vo/Vi
which can be equal to the impedance of the output branch over the impedance of the whole circuit.
Draw a diagram to illustrate the process of digitizing an analogue signal.
How would convert a twos complement to a decimal ?
Take the top value as negative and treat the rest the of the values as positive and then add them.
How are the hexadecimal values above 9 defined?
EA3B16
How to convert Hexadecimal to decimal
Treat the bits as if they were binary, just multiply the value of the placeholder by the value of the bit.
How to approach this exercise?
Write V2 in terms of R3 and R4
Using Ix = Iy , relate the voltages V1, V+(or V-) and Vout.
The inverting input. It is called a virtual earth because although its potential is the same as the earth, no current can flow into or out of it.
Do b)
A baseband communication system transmits random binary data at 1Mbit/s. Define how you might measure the bandwidth of such a signal. State any assumptions made.
