5/25 Apparent Power and Power Factor

Today, we talked about effective or RMS value, apparent power, power factor, and complex power.
The effective value of a periodic current is the dc current that delivers the same
average power to a resistor as the periodic current.
The effective value of a periodic signal is its root mean square (RMS) value.

The apparent power (in VA) is the product of the RMS values of voltage and current. 

The power factor is the cosine of the phase difference between voltage and current. It is also the cosine of the angle of the load impedance.

Here is the power summary:

The picture below is the relationship between effective value and max value.

We did some example of how we use the equation to find what we want.

Then, we did lab.

Apparent Power and Power Factor

Pre-lab

The picture above is when RL=10ohms.

The table below is what we calculated for different resistance value.

The actual resistor we used are 10.8ohms, 47.1 ohms, and 99.3ohms.

The picture below is the basic set up for this lab.

Result:

RL = 10 ohms

The RMS value of load voltage is 627mV.

The RMS value of load current is 18.7mA.

The phase difference is 64.44 degree.

RL = 47 ohms

The RMS value of load voltage is 625.1mV.

The RMS value of load current is 10.68mA.

The phase difference is 36.83 degree.

RL = 100 ohms

The RMS value of load voltage is 660.1mV.

The RMS value of load current is 6.22mA.

The phase difference is 12.6 degree.

The table below is the comparison of experimental value and calculated value.

From the graph above, we successfully verify the apparent power equation Irms*Vrms = Power, and the power factor is cos(V-I).

Summary

Today, we learned many new things: effective or RMS value, apparent power, power factor, and complex power. By learning these terms, we can deal with power in AC circuit, and know how power is viewed in actual world--the importance of power factor in electricity consumption cost.

5/23 Op Amp Relaxation Oscillator

Today, we continued on the AC circuit analysis with op amp.
First, we did a example of the circuit with op amp with AC voltage supply. (I did not take a picture.)
An oscillator is a circuit that produces an ac waveform as output when powered by a dc input. Oscillator is a device which transform DC signal to AC signal. Without the existence of oscillator, AC signal will not exist.

In order for sine wave oscillators to sustain oscillations, they must meet the Barkhausen criteria:

1. The overall gain of the oscillator must be unity or greater. Therefore, losses must be compensated for by an amplifying device.

2. The overall phase shift (from input to output and back to the input) must be zero.

(From Day 23 AC Op Amps , Oscillators, AC Power.pdf)

Then, we did lab.

Op Amp Relaxation Oscillator

Pre-lab

We were planning to create a signal with the frequency 254 Hz (which is the last 3 digits of my student id). We need to have R=18k to produce this frequency.

The picture above is the basic set up for this lab. We used one 12k ohms resistor and two 3k ohms resistors to produce 18k ohms resistance.

Using Everycircuit, we have successfully proven that the circuit will provide an oscillating signal.

The picture is the result of our lab. Our measured value of frequency in our oscillator is 1/0.003740 = 267.3Hz. Comparing to the theoretical, we had a good result with a small percentage error:  5.23%.

Summary

We learned what op amp can do in a AC circuit, and how to use a op amp to produce oscillation. By doing the lab, we successfully implement what we learned before the lab, and we got a small percentage error of frequency from what we wanted to get (254Hz).

5/18 No Lab. Sinusoidal Circuit Analysis

Today, we had no lab, but we did Sinusoidal Steady State Analysis.
Here are the steps to analyze AC Circuits:
1. Transform the circuit to the phasor or frequency domain.
2. Solve the problem using circuit techniques (nodal analysis, mesh analysis, superposition, etc.).
3. Transform the resulting phasor to the time domain.
(From Day 22 Sinusoidal Circuit Analysisc.pdf)

Basically, it is just what we did before, but with complex number.

The picture above is an example of using nodal analysis in AC circuit.

The picture above is an example of using mesh analysis in AC circuit.

Summary

We learned how to analyze AC circuit by using what we learned before, like nodal analysis, mesh analysis, superposition theorem, source transformation, and Thevenin’s and Norton’s theorems. Nothing new.

5/16 Impedance

We talked about impedance and admittance, which can be seen as resistance in AC circuit.
The impedance Z of a circuit is the ratio of the phasor voltage V to the phasor
current I, measured in ohms ().

The table below summarizes RLC impedances and admittance. 

The picture below is the example of how we calculate the impedance of the capacitor and the alternating current going through it.

Then, we did lab.

Impedance

Pre-lab

By calculating the impedance of each cases(100ohms, 1mH, 100nF), we can find the current of each cases with 5kHz frequency.

The picture above is the basic set up for this lab.

Result:

The picture above is the graph of VR(t) (yellow) and ouput V(t) (blue), and i(t) (orange) with 100 ohms resistor and 5kHz waveform.

The picture above is the graph of VR(t) (yellow) and ouput V(t) (blue), and i(t) (orange) with 1mH inductor and 5kHz waveform.

The picture above is the graph of VR(t) (yellow) and ouput V(t) (blue), and i(t) (orange) with 100nF capacitor and 5kHz waveform.

We record all the data with different frequency but we didn't do the pre-lab of 1kHz and 10kHz.

Here is the table of all the data we get.

We can only compare the data of 5kHz

The table below is the Comparison of the current and voltage:

Summary

Today, we learned impedance, which is the resistance of AC circuit. By using impedance of the circuit and the voltage function, we can calculate the current function going through each components. Also, we learned how to determine the current function by looking at the graph. From the lab, we can see that the experimental data and the calculated data match with each other. The lab ended up to be successful. 

5/11 Phasors: Passive RL Circuit Response

Today, we talked about sinusoids and phasors.
A sinusoid is a signal that has the form of the sine or cosine function.

where
Vm = the amplitude of the sinusoid
ω = the angular frequency in radians/s
ωt = the argument of the sinusoid

A sinusoid can be expressed in either sine of cosine form. With these identities, it is easy to express both as either sine or cosine with positive amplitudes.

By using these relationships, we can transform a sinusoid from sine form to cosine form or vice versa.
The picture below is the graphical approach.

To add A cos ωt and B sin ωt, we note that A is the magnitude of cos ωt while B is the magnitude of sin ωt.
The magnitude and argument of the resultant sinusoid in cosine form is readily obtained from the triangle.


Next, we talked about phasors, which are more convenient than sinusoids to work with sine and cosine functions.
A phasor is a complex number that represents the amplitude and phase of a sinusoid.


To deal with the complex numbers calculations, the following operations are important.

We did a little example of how to transform rectangular form to polar form.

Also, we did a example of the calculation of polar form.

Next, we introduced how to determine which one the leading direction and lagging direction are.

Then, we did a example of two sinusoids function addition.


The derivative v(t) is transformed to the phasor domain as jωV
The integral of v(t) is transformed to the phasor domain as V/jω
These are useful in finding the steady-state solution, which does not require knowing the initial values of the variable involved.
The picture below is the example of the transformation of the derivative of v(t) and the integral of v(t) to the phasor domain.

The table below summarizes the time-domain and phasor-domain representations of the circuit elements.

Then, we did lab.

Phasors: Passive RL Circuit Response
Pre-lab

The picture above is the amplitude gain and phase difference between the input voltage and the input current with different frequency.

The picture below is the basic set up for this lab.


Result:

With the cutoff frequency, the phase difference is (16.87)/(135)*360 = -44.9degree, and the gain is 13.28/668.3 = 0.01987


With ten times the cutoff frequency, the phase difference is (3.102)/(13.5)*360 = -82.72degree, and the gain is 2.155(mA)/223.2(mV) = 0.009655


With one tenth of cutoff frequency, the phase difference is (0.03)/1.45 = -7.448degree, and the gain is 0.001675/1.013 = 0.0016535


The picture above is the comparison of experimental and theoretical data.
From the data, we can see that we successfully finished the lab, especially on the phase difference.

Summary

We learned about sinusoids and phasors, which are different ways to deal with AC circuit. Also, we learned how to determine the phase difference and gain according to the graph. By comparing the data, we can see the experiment matched what we expected.

5/9 RLC Circuit Response

Today, we talked about Schmitt Trigger, whihc is a logic input type that provides hysteresis or two different threshold voltage levels for rising and falling edge. This is useful because it can avoid the errors when we have noisy input signals from which we want to get square wave signals. (From Day 19 2nd Order Circuits Contc.pdf)
The picture below is what the operational amplifier based schmitt trigger looks like:

Next, we introduced the non-symmetrical schmitt trigger.

The picture below is what the the non-symmetrical schmitt trigger looks like.

Then, we talked about the step response of a series and parallel RLC circuit.

Review questions:

Next, we did lab.

RLC Circuit Response

Pre-lab

The picture is the differential equation relating Vin and Vout for the system.

By using the equation, we can determine the damping ratio and natural frequency of the circuit.

The damping ratio is 1563.8 and natural frequency is 10105.8Hz.

The picture below is the basic set up for this lab.

Result:

The damping ratio from the graph is 0.25, rise time is 2.15*10^-4, DC gain is 8.

After lab, we talked more about the second order circuits.

Summary

We learned about what the Schmitt Trigger is, and reviewed about RLC circuit which is second order circuit.  In order to recover the transmitted analog

signal, the output is smoothed by letting it pass through a “smoothing” circuit, as illustrated at right. An RLC circuit may be used as the smoothing circuit.

5/2 Series RLC Circuit Step Response

Before, we considered the circuits with only one element (a capacitor or a inductor), and the circuits are first-order.
Today, we talked about the circuits with two storage elements which is known as second-order circuits because their responses are described by differential equations that contain second derivatives.
The main idea of this kind of problems is to get the value of  v(0), i(0), dv(0)/dt, di(0)/dt, i( ∞ ), and v( ∞ ). By determining these value, we can solve the problems easily.
Next, we talked about the source free series RLC circuit.
 
By determining  α  and ωo, we can see what type of this circuit is.
1. If α > ω 0 , we have the overdamped case.
2. If α = ω 0 , we have the critically damped case.
3. If α < ω 0 , we have the underdamped case.
We did a example of the source free series RLC circuit.


Series RLC Circuit Step Response
Pre-lab

This is the prediction of differential equation, damping ratio, natural frequency, and damped natural frequency of this lab.

The picture below is the basic set up for this lab.

The graph below is the result for this lab.

The  rise time of the graph is 1.241ms.
The overshoot is 3.5ms.
The oscillation frequency is 23485Hz.
Out estimated damping ratio, natural frequency and damped natural frequency is in the pre-lab.
Our estimated DC gain is 2.383 (in the beginning of the transient).

Summary

From today's lecture, we learned the source free series RLC circuit and the source free parallel RLC circuit. By determining α and ω 0 , we can determine which case the circuit is. From the lab, we learned how to determine the rise time, overshoot time, and the oscillation frequency. And from the graph, we can see that there is a sudden change when we apply the voltage to the circuit. An automobile ignition system takes advantage of this feature. By creating a large voltage (thousands of volts) between the electrodes, a spark is formed across the air gap, thereby igniting the fuel.