Day 6

Today we had a fiesta at the beggining of class, worked a little more on mesh analysis with a lab, and learned about diodes and transistors. 

Here we had our 10 minute fiesta where we used mesh analysis to find the currents in the circuit. 

We then worked on mesh analysis that included a current supply. Mesh analysis allows us to negate that branch when we use KCL. Then use junction rule to get the third equation for 3 equations and 3 unknowns.

The objective of the Mesh Analysis III lab is to use mesh analysis to determine I1 and V1 as shown below, then build the circuit and compare with measured values. 

Through mesh analysis, we calculated I1 = 0.260mA and V1 = 2.46V. 

Here is the schematic of our circuit. The green and yellow wires were used to connect the DMM to measure current. The meter shows a negative current because the polarity of the meter was accidentally switched.

Here we are measuring the voltage across the 22k ohm resistor.

As shown in the third picture of this post, our percent error of current and voltage were quite small. Due to the way the DMM measures current and its efficiency, there is always a larger uncertainty in current measurements than voltage measurements which getting a larger error on current seems expected. Also, we assumed the resistances used are exact as marked.

Here we show which type of analysis makes the math easier based on the diagrams shown above.

Here we learned how to redraw the circuit to model the transistor in order to find the current going in the base of the BJT and the voltage across it. Transistors are modeled as shown in the upper right corner of the above picture.

Day 5

Today we learned about nodal analysis and mesh analysis of DC circuits. 

Here we learned how to use nodal analysis to find voltages at the nodes. We selected the supernode as labeled and the reference node at the bottom branch of the circuit. 

The objective of this nodal analysis lab is to use nodal analysis to find voltages V1 and V2 as shown in the picture, then compare the results with measured voltages. 

Circled in green are the predicted voltages (should say "voltages predicted across..." Instead of "voltages measured across..."). The voltages measure are circled in black. 

Here is a picture as we measured V1 with the multimeter. The value is negative because the polarity is backwards. 

%error for V1 = 0% and %error for V2 = 1.04%. Any error in out measurements we believe may be due to our resistors not having the exact resistance. Since our error is small, we can conclude that nodal analysis does indeed work efficiently to predict voltages. 

Here we learned how to use mesh analysis to predict currents in the circuit pictured above. 

Day 4

Today we did a lab on thermistors and learned how to use nodal analysis to solve circuits.

Here was our first class fiesta where we attempted to use KCL and KVL to find currents and resistances.

The objective of this lab is to design a temperature measurement system using a thermistor. Using the resistance curve for our thermistor, see that at room temperature the resistance is 11k ohms and at body temperature it's 7k ohms. We are to pick a resistance value that will give is a minimum delta voltage of 0.5V. 

In order to get a change of .5V, we calculated that we would need the resistance values that are boxed. Since those exact resistance values are not available to us, we are to pick resistance values that are close.

Here  we are measuring the thermistor's resistance as it reaches body temperature. We measured a resistance at 37°C to be 11.54k ohms and 25°C to be 8.0k ohms. This results in a 5% and 14% error respectively.

As shown above, we used resistance values that were close to our calculated resistances in order to get the 0.5 delta voltage. We found the 12k ohm resistor to give us the change in voltage that we were looking for. We believe the calculated resistances to have a large uncertainty since we are not sure of the actual temperatures that the thermistor sees, as well as the efficiency of the thermistor (which can be apparent when we measured the resistance of the resistor at body and room temperatures respectively. 

Here we learned and practiced nodal analysis using V1 and V2 as our nodes and the bottom branch as the reference.

Day 3

Today we learned abut voltage/current dividers, a lab using a bipolar junction transistor (BJT) as a switch, and how DMM's work to measure voltage and current.

The objective of this lab is to create a circuit that will make an led turn on when there is a small amount of ambient light, and turn off when there is a high ambient light intensity. This circuit used a LDR to vary the resistance with respect to the amount of light it perceives, and a BJT (2N3904) to act as a switch. We will assume the LDR resistance value at a high amount of light is 5k ohms and 20k ohms with low light.

The picture above shows the diagram of the circuit we built. The 10 ohm resistor in the picture is actually a 10k ohm resistor. As shown above, we calculated the voltage across the LDR to be 1.67V for high light levels and 3.33V for low light levels.

This is the voltage measured at a high light level.

This is the voltage that we measured at a low light. The measured values are off by about one volt at high light levels and half a volt at low light levels. We believe this to be due to our original assumption that the LDR is at 5k and 20k ohms respectively. As for using the BJT as a switch, if works quite fine. Check the link below for a video of the circuit in action!

We learned how the voltmeter measures voltage by a connection as a parallel to the circuit. Such that the original circuit not be affected by the voltmeter, the voltmeter has a very high resistance which is what is represented by the equation above. R_m is the internal resistance of the meter.

As shown in the picture, we learned how the DMM measures current.

Dependent sources and MOSFET Lab

In this lab we will examine the behavior of a MOSFET for different voltages across the MOSFET. 

This is the diagram we followed for the circuit set up. The resistor used measured 97.9 Ohm's in our DMM. 

Here is how we hooked up our circuit according to the diagram shown in the first picture. 

We increased our variable voltage source (connected across the MOSFET) by 0.1V

And measured current in the circuit accordingly. As noted by our table, the MOSFET has a threshold voltage of about 1.4V where there is no current that flows through the circuit. The current then increases greatly per 0.1V increment  until it levels out at about 2.9V then remains close to constant. 

Resistors and Ohm's Law lab

In this lab we explored the characteristics of resistors. 

We grabbed a 100 ohm resistor and used our DMM to measure it's resistance. 

Next we set up the circuit to measure current and voltage as we had voltage increments of .2 volts. Voltage measured across the resistor was consistent with the displayed voltage in waveform software. 

Solder less breadboards Lab

This is the resistance we measured across the same row in the breadboard. 

This was the resistance measured in a row at opposite sides of the breadboard. A 1 as shown above indicates infinite resistance. 

This is the resistance measured at two arbitrary holes. 

Here we used a jumper wire two connect the two rows and measured it's resistance. 

Based on our results, this is what we concluded our circuit behavior to be respectively.