Day 2 - Resistance, Conductance, Branches, Loops, etc.

Topics Discussed

On day two of the ENGR 44 course, we were reintroduced to the concepts of resistance, Ohm's Law, and loops. Additionally, we reinforced old topics and reinforced new ones. This included an overview of power, as well as an introduction to nodes and branches. Specifically, we also discussed how to identify independent loops and how this could lead us to discovering the number of branches and nodes and vice-versa. We also saw a brief demonstration of voltage potential in the beginning of class, and how certain loops, even when powered, will not cause a voltage change to, say, a light bulb in another loop of the circuit. (Fig. 1)

Fig. 1
It was found that attaching the lower battery had no effect on the light bulbs.

In class, we discussed what made a material have a resistance and concluded that it was based on a combination of material type, length, and cross-sectional area. We found that resistivity varied directly with material type and the corresponding coefficient of resistivity, that length varied directly as well, and that cross-sectional area varied inversely. We also discussed the topic of "hot" and "cold" resistance, and completed a problem involving a tungsten filament light bulb which utilized the concept (Fig. 2)

Fig. 3

Dependent Sources and MOSFETS

Fig. 4

In lab, we attempted to construct a circuit that had a dependent source of current. This dependent source was achieved by use of a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which essentially would act as a voltage controlled current source (VCCS). We used a 130 ohm resistor in the circuit to offset the current that would pass through the MOSFET, ensuring no damage would happen to it or to the circuit. We used a 4.5 V voltage source, and the MOSFET would be used to regulate how much current would go through the circuit. The circuit can be seen in Fig. 4. 

Using the analog device to control the current, we recorded the corresponding voltages at the gate of the MOSFET to the current with which we were measuring on our DMM. A table and graph show our results in Fig. 5. 

Fig. 5

Fig. 5

Summary

In this lab, we found that there is an initial baseline where the MOSFET will essentially not allow any current into the circuit. As we increased voltage, we uncovered a range in which the MOSFET would greatly change voltage. Although not recorded, it was found that at roughly 5 V, the MOSFET would again level out and not show any change in current. Using the slope of the data we created, although limited, gave us a value for "g", which is essentially the parameter of our VCCS. "g" was found to equal 10.4 V/mA. Unfortunately, our graph did not fully verify the leveling out at the upper range of voltage for the MOSFET.