(Day 7) BJT Curve Tracer

This lab was aimed at analyzing the collector current vs collector voltage characteristics of the BJT. Our analysis focused on the behavior of the BJT when given a step voltage source and a triangle voltage supply. Below is our prediction of what Vout would look like for each given a voltage divider scenario.

Below is the circuit we built containing a 100 ohm resistor, 100 k ohm resistor, two curve tracer voltages, and then the probes for the oscilloscope readings.

Here are the two successful set ups of the variable voltages supplied to the circuit.

Our oscilloscope received the signals below for the output and input voltages.

When we extracted the data from the Analog Discovery and plotted it, we plot almost identical graphs to what we measured using our oscilloscope. This lab was particularly difficult to understand and set up efficiently.

(Day 6) Group Quiz and Mesh Analysis III

Our day 6 started off with a group quiz with the following circuit and correct answer. Using mesh analysis and MATLAB we derived our answers.

Mesh Analysis

The purpose of this lab was to be able to use mesh analysis to predict the given circuit's behavior and compare that to our measured results. Our focus was primarily on the current I1 that runs through the 1.8 kilo-ohm resistor and the voltage, V1, across the 22 k-ohm resistor. Pre-lab calculations are below.

Below is the successful circuit.

Below is the measure of voltage across the 22k Ohm resistor.

The percent error for the current and voltage were negligible.

(Day 5) Nodal Analysis

The purpose of this lab was too be able to analyze, build, and test a multiple-source circuit. As part of our pre lab we had to analyze the circuit on the whiteboard using a new method of nodal analysis. Nodal analysis is a favorable method because it results in a simpler system of equations compared to KVL or Mesh Analysis. Below are our calculations.

The voltage V1 we calculated to be 2.424 V. The picture below shows the reading of our actual circuit.

Again for V2, we calculated it to be 4.424 V. Below is our measurement.

The three voltage sources and ground in the back of the circuit, along with the three different resistors.

Our error was minimal and negligible.

(Day 3) Creating a Night Light and the Hot Dog Circuit

For our Day 3 warm up we were given a hot dog circuit. Some of the LEDs ran in series with the hot dog, while the others ran in parallel. It turns out that the LEDs in series were the ones that lit once current flowed through the hot dog, which slowly cooked until it burned and started smoking (as you may see in the picture below). Like college students, current flows down the path of least resistance, which are the LEDs in series.

Dusk to Dawn Light Lab:

In this lab, we essentially created a night light. Using a photocell, a BJT transistor, and an LED we created the circuit pictured below. 

The photocell we used changes its resistance depending on how much light it is receiving. With high amounts of light, the resistance of the photocell is very low and allows current to pass through, drawing the concentration away from the rest of the circuit. At low levels of light, the resistance is high forcing the current to flow towards the BJT and the LED, allowing it to light up.

The BJT (Bipolar Junction Transistor) was very useful in stepping down the current to the necessary value required for the LED to light up. 

Here the circuit is complete, but there is not enough resistance from the photocell for the LED to be able to light up. As I lower my sleeve over the photocell, the changes will take effect.

Below is a successful capture of the "night light" working. Enough light was blocked by my sweatshirt sleeve to simulate low ambient light conditions and power the LED.

(Day 4): Temperature Measurement System- Thermistors in a Circuit

Thermistors are circuit elements that vary in resistance as temperature increases or decreases. Resistance and temperature are inversely proportional to each other in a thermistor.

The purpose of this lab was to design a circuit with a voltage output that varies by at least 0.5 V as the temperature of the thermistor increased from room temp to body temp (25 C and 37 C). Our first step was to theoretically solve for resistance values that would allow us to achieve the desired change in Voltage. Those calculations are shown below.

The next step was to create a circuit like the one drawn in the top left corner of the theoretical solutions. The next picture shows the voltage source, thermistor, resistor, and voltmeter successfully connected in a circuit.

Below is the successful trial of changing the voltage by a minimum of .5 V when the temperature of the thermistor is changed from room to body temperature.T

While we weren't successful on the first 2 trials, we finally found the resistor R = 5.6 kilo-Ohms from our supply of D12 resistors that would allow us to achieve the desired results. The process was slow and steady, but fulfilling nonetheless.

Summary of our trials and error calculations.

MATLAB Review

MATLAB is one of the best software applications for numerical computations. The following is a review of its functions.

Just a few of the practice exercises to familiarize ourselves with Matlab and the syntax it uses:

Here is an example of creating vectors, matrices, and different math functions using them.

This is an example of using Matlab's plotting functions. These include plotting multiple graphs in one figure, changing the look of each graph, and holding plots in order to add more to the figure later.

Solving Simultaneous Equations

Here I used Matlab in order to solve a system of linear equations in a script file. We were given a circuit with two loops and asked to find the value of the net current that flows through the middle resistor. I set up the system of equations by hand, and once I had the coefficients of the system I was able to create the two matrices above. Using Matlab's Gaussian Elimination function '\', I was able to solve for the current in each loop. I then assigned (the current through R3) currentI3 the difference between the two currents. Correct answer -0.186 A.

Plotting Exponentials

Adding Sinusoids

(Day 2) Voltage-Current Characteristics; Dependent Sources and MOSFETS

Voltage-Current Characteristics

The objective of this lab was to learn how to measure the conductance of some element in a circuit. Conductance is the inverse of resistance, G = I/V.

These are the values of current we measured as a result of the varied voltage we supplied:

The slope of the graph below of Voltage vs. Current is the measured value of conductance.

The best fit line for the measured data has the equation:

I = .0091V - 6.5 X 10 ^ -5

For the resistor that we measured conductance of, G = m (slope) = .0091 mho. Theoretical value for this resistor is 1/100 or .01 mho because it is the inverse of 100 ohms. This means that there was:

(.01 - .0091)/.01 X 100% = 9% error.

Dependent Sources and MOSFETS

Our objective was to measure and understand how MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) interact with and alter the current in a circuit. The MOSFET is what's known as a Voltage Controlled Current Source (VCCS) meaning that depending on how much voltage is applied to the MOSFET, it will magnify the current produced.

In order to better understand this, we placed the MOSFET in series with a voltage source, an ammeter, and a 100 ohm resistor and then proceeded to vary the voltage supplied. A simple diagram of the circuit and the calculated Voltage threshold for the circuit is below.

The graph below shows measured values and can be interpreted to understand how MOSFETs function. The transistor has a voltage threshold value equal to 1.5 Volts. Only once the voltage source provides 1.5 V is there any change in the current supplied by the transistor. When V > 1.5 V, the current supplied by the transistor increases exponentially.

At 1.5 < V < 2.5 is when we observe the linear phase of the current supply. This is when the effects of adding even a small amount of voltage will result in a magnified supply of current. This is what makes the MOSFET so useful.