PSpice Tutorial: Part 1 Introduction and DC Analysis

As the name suggests, the lab was our first foray into the wonderful world of PSpice, a simulation tool for electrical circuits. In its latest iteration, PSpice is outfitted with a cutting edge GUI which makes use simple, even for lowly ENGR 44 students!

Once the program has been installed, the first order of business was to add certain libraries which were included. The default libraries include typical beginning electrical circuit stuff and I suspect that there are advanced libraries out there with all manner of objects to be inserted into an electrical circuit.

After adding the required libraries, we then proceeded to create a new project and after navigating multiple menus, we arrived at the final dashboard for a PSpice project. The most important thing about these PSpice schematics is to be careful in drawing the wires and objects. Many people in the class got errors when trying to run their simulations (myself included) due to wiring nodes that shouldnt exist and etc. Upon successful completion of a schematic and a simulation run, the following screen should appear.

It should also be noted that PSpice REQUIRES the setting of a GROUND to compute all other values relative to that point. Once complete, the simulation should display all calculated voltages at different points in the circuit. It will also display currents!

The following HW problem was made extremely easy with the application of PSpice. Needless to say, PSpice is an invaluable tool to an electrical engineer (or student of).

Nodal Analysis Lab

The Nodal Analysis Lab was designed to familiarize students with real world applications of the Node Voltage Technique. The idea behind it is to build a system which can sustain damage to one part of the circuit without bringing everything offline which has obvious uses in the real world. One simple way to think about how such a task is accomplished is the use of multiple voltage sources and loads. To test this out, students built the following circuit;

 

The following circuit is composed of several resistors which are meant to simulate the loads on a real world circuit. The values of the resistors starting from the left are; 100 , 1000, 220, 1000, and 220 Ohms. They are connected together in several nodes with power supplied from the left and right sides from two separate power supplies. Before building the circuit, students first calculated the theoretical values for the power supplies needed. Using the nodal equations, students were then able to calculate both the corresponding currents and power supplied by both batteries. 

 

Students simply used a 12 and 9 Volt setting respectively. The following is the measured values compared to actual values used.

Component	Nominal Value	Measured Value	Power or Current Rating
R1 (Ohms)	100	99	0.25 W
R2 (Ohms)	220	224	0.25 W
R3 (Ohms)	220	218	0.25 W
RL1 (Ohms)	1000	983	0.25 W
RL2 (Ohms)	1000	989	0.25 W
Vbat1 (Volts)	12	12.12	2 Amps
Vbat2 (Volts)	9	9.09	2 Amps

Here were the measurements taken when compared to the theoretical values.

Component	Nominal Value	Measured Value	% Error
Ibat1	17.5 mA	17.39 mA	0.63%
Ibat2	1.5 mA	1.5 mA	0%
V2	10.25 V	10.35 mA	0.98%
V3	8.67 V	8.75 mA	0.91%

As you can see, the experiment went very smoothly and nothing surpassed 1% error. A system with 2 separate power supplies can remain at least partially function if one is disabled. This measure of reliability is critical to real world applications in which electronic circuits may be subjected to the elements, or a copper tipped explosively formed penetrator punching its way through a military electronics system. By supplying two voltage sources, a system's operational capacity is not bottle necked at simply knocking out the voltage supply. In fact, by varying the voltage settings of the sources, the system can be controlled as demonstrated by the final portion of the lab. In order to get 0 current going across the RC2, a voltage of 9.9 V and 10.98 V are needed for VS1 and VS2 respectively.

Voltage Dividers Lab

The Voltage Dividers Lab is designed to familiarize engineers with the concept of an electrical system having multiple loads that can be switched on and off. One very basic example of this is an electrical fan with multiple speed settings. The idea is to provide multiple loads across a circuit and in this case use it to control the current going through any individual load.

The picture (courtesy of Nick Moran's Blog)  illustrates the basic concept of the circuit. Imagine a Voltage supply and resistor (resistor box) is connected in series with 3 resistors of the same resistance value in parallel to each other. These resistors would then be either connected or disconnected to modify the current going through any of them. In a real world example, one of these resistors could be replaced with the load and voila; simple way to vary the current across a load!

Here is the data we collected with varying configurations of resistors in parallel;

Config	EQ Resistance (Ohms)	VBUS (Volts)	IBUS (mA)	P Load (calculated Watts)
1 Load	1003	5.68	5.72	0.032
2 Loads	499	5.34	10.76	0.06
3 Loads	334	5	15.25	0.076

Overall the circuit tested in this voltage divider lab is a great way to control currents through a given system without being needlessly complex. The real world applications of this concept are numerous and can be found in countless modern circuits. Quite counter intuitively, the more loads placed in parallel, the less the equivalent resistance and hence the less strain on a battery because it would have to supply less current!

Introduction to Biasing

This lab consisted of 2 Light Emitting Diodes connected in parallel to each other to a current source. Unfortunately, the LEDs were not rated high enough to be able to withstand the kind of voltage being put out by a 9 V battery (simulated in this case by a power supply) so we had to connect two resistors (one before each LED) to lower the voltage. Here is the basic set-up that follows;

Current enters from the orange wire on the top right corner of the bread board, it is then split among 2 resistors and the resulting currents go through the respective resistors and into the LEDs (using the correct resistors we were able to ensure that the LEDs did not blow) and then back into the power supply through the green (right) and yellow wires. The other orange and green wires on the left were connected to the voltmeter

The red LED was rated to take 2V while the yellow LED was rated to take 5V. Unfortunately, the power supply puts out 9V so we had to put a resistor before each LED. Using Ohm's Law we were able to calculate the equivalent resistances of the LEDs which gave us the ratio of the current going through each. After knowing the currents going through each branch and the maximum voltage of the LEDs we were able to calculate the values of the resistors which were 175.8 Ohms and 350 Ohms for 1 and 2 respectively

Introduction to DC Circuits

The Introduction to DC Circuits Lab started off with a simple concept pictured below.

Real world application must take into account the natural resistivity of the material that conducting cables are made of.

In this set up, a current flows from the battery through the cable to the "load" then back to the battery again. In a perfect world, all resistance in this system would come from the load and up until now we've taken for granted that all theoretical cables are perfect conductors. The real world is not as forgiving however, and cables themselves (regardless of material) have inherent resistances.

Starting with the red cable (farthest right): Current flows from DC power supply to a resistor box then into a current meter and then into a 1000 Ohm resistor (which simulates the load). The voltage across the resistor is also measured by a voltmeter.

This comes into play when designing real life applications that require current to be transported over long distances; the conductivity and resistance of the material must be accounted for because it will make a difference in the overall system. To demonstrate this, we set up the following lab (pictured on the right).

What we found (aside from the fact that none of the equipment outputted the exact amounts stated on its labels; I.E 12V power supply actually put out 12.18V) was that there is a maximum length that exists for which the current could not effectively carried past that.