Prelab Report

by My Name

Student ID #

Electronics Experiment

Teaching Assistant: TA Name

Chem 374-005

Group #6

11/21/96

Lab Partner: Their Name

Introduction & Theory

Electronics is the study and manipulation of the flow of electricity through circuits. As it turns out, the flow of electricity may be expressed quite precisely with mathematic expressions. As a result, a tremendous number of formulas, methods, and constants exist to describe, explain, and predict the behavior of nearly any electronic circuit.

Probably the most basic concept in electronics is the fact that electricity flows. Electrical charge is a result of the movement and location of electrons. If the number of electrons in a conductor is reduced, neighboring electrons rush in to take their place, other electrons rush to take their place, and so on. The result is somewhat similar to the flow of liquids through a pipe. Just as water flows from high pressure (excess electrons) to low pressure (few electrons), electrons (and the accompanying flow of electricity) can be made to move by changing electron densities.

A battery creates a flow of electricity by means of a chemical process that creates areas of excess and few electrons in different areas. As a result, electrons tend to flow (if a conductive path is provided) from the negative end of the battery to the positive end. The DC power supply used in this experiment creates the same effect, but uses a wide variety of concepts and components beyond the scope of this experiment. As a result, the specific "electron pump" used is of little consequence, and may be generally termed a voltage source.

Voltage (symbolized by E) is often likened to the pressure in a fluid system. Similarly, high voltages (also called potentials) are found between areas of very high and very low concentrations of electrons. Another basic property of electrical circuits is current (also called amperage). The current in a circuit is very similar to the rate of flow in a fluid system. Current is usually measured in amperes (often shortened to amps) and is symbolized with the letter I. Together, these two properties may be used, with the proper tools, to create a wide variety of circuits.

A basic mathematical tool in electronics is Ohm's Law. Discovered by George Simon Ohm (1787-1854), it defines a relationship between voltage, current and another electronic property, resistance. Resistance is the amount which a material prevents the flow of electrons, and is measured in units of Ohms (symbolized by ). The resistance of a circuit does not usually depend on the flow of electrons through it, but the materials the circuit is made of. By varying the type and amount of material electricity flows through, the value of the resistance may be set. As a result, the resistance of most circuits (and parts of circuits) is usually a constant. These three properties may then be related by Ohm's law:

Wattage is defined as the product of voltage and current and is expressed in Watts (W).

Along with Ohm's law, this allows a great understanding of many circuits. As an example, with the basic material presented so far, the resistance of a 100 W light bulb can be determined. As the voltage is 120 V (standard American household voltage), the current flow through the bulb is 0.83 amps. Ohm's law may then be solved for R, resulting in a value of 115 . This resistance applies when the filament is at operating temperature, and a measurement at room temperature would certainly be different.

All components of a circuit have some resistance, but often the amount is negligible. The resistance of wires, closed switches, and the like is usually of no concern and may be ignored. The components of a circuit that do posses considerable resistance are called resistors, especially if that is their primary purpose. As an example, a component known as a capacitor stores electrical charge, and also has a resistance associated with it. However, unless one is speaking specifically of the resistance of the capacitor, it would not be called a resistor.

Capacitors posses a property called capacitance (symbolized C), which is measured in units of Farads (in honor of Michael Faraday), and most frequently expressed in microfarads. A farad is an extremely large unit, and capacitors with a capacitance of close to one farad are extremely rare. Capacitors generally store current with two or more conductors with an insulating material between them (a dielectric). The capacitance of a capacitor is a function of the surface areas between the conductors and the nature of the dielectric material.

Goal

The goal of this experiment is to study to properties of basic electronic circuits using modern components and measurement devices.

Apparatus

The circuit will be built on a prototyping board (also called a breadboard) with a variable power supply. A function or signal generator capable of producing low voltage (<20 V) square and sine waves will be used to provide input. A DMM (Digital Millimeter) and an oscilloscope will be used to make measurements. Also, the following electronic components will be used:

The attached sheets show the schematics for the various circuit configurations used.

Experimental Observables

Voltage, current, and waveform transformations will be measured with a Digital Millimeter (DMM) and an oscilloscope.

Experimental Method

Using a prototyping board with a variable DC power supply, a DMM, a function generator and an oscilloscope, various electronic circuits will be built and measured to determine several electronic properties.

Experimental Procedure

Using the DMM, make accurate measurements of all resistors to be used.

Experiment I

With the power supply turned off, connect a 470 resistor between the positive and negative strips of the prototyping board. Set the power supply to 15 volts DC. Activate the power and measure the voltage drop across the resistor. With the power off, disconnect the end of the resistor connected to ground and place the DMM between the resistor and ground and measure the current. Voltage is always measured in parallel and current is always measured in series. Replace the resistor with a 1 k resistor and repeat.

Experiment II

Place a 470 resistor in series with the 1 k resistor and connected to ground. Take voltage measurements across both resistors and current measurements at each resistor.

Experiment III

Replace the 470 resistor with a diode with the band going toward ground. Set the voltage to 5 V DC. Take voltage measurements across both the resistor and the diode as well as current measurements at both the resistor and the diode. Repeat after reversing the diode (ie putting the non-banded end toward ground).

Experiment IV

Turn off the power supply and attach a function generator to the breadboard. Use a Tee connector to feed a signal to one channel of an oscilloscope (with x10 probes) and also to the positive strip of the prototyping board. Set the function generator to sine wave output with Vpp = ~5-10 V (record exact value). Record any observations. Reverse the diode and repeat. Compare the results.

Experiment V

Replace the diode with a 0.05 F capacitor and a 470 k resistor in series. With the power supply set to 5 V DC, measure the voltage across the circuit and the current through it. Monitor the voltage and current at the capacitor qualitatively as a function of time (do not use the autoscale feature of the DMM while making this voltage measurement). Remove and reconnect power via a power lead (to avoid rise and fall times associated with the power supply) to gather qualitative measurements.

Replace the resistor with a 33 k resistor, and connect a function generator in place of the power supply. Set the function generator to a square wave at ~5 Vpp and ~10 Hz (record exact values). Use an oscilloscope to compare source signals with the waveform across the capacitor and the then the resistor. Record the waveform and the rise time at the capacitor and the fall time at the resistor. Record the output as the frequency is raised from 10 Hz to 5 kHz in 1 kHz increments.

Experimental Precautions

Preliminary Calculations

Using Ohm's law (eq. 1) the current in experiment I can be calculated to be:

with the 470 resistor, and similarly 15.0 mA with a 1 k resistor.

As series voltage drops are equal to the voltage across the entire series multiplied by the ratio of the resistance over which the resistance is measured to the total resistance of the series, eq. 3 and eq. 4 give the voltage drops across the resistors in experiment II.


As the current in a series circuit is the same at every point, Ohm's law (eq. 1) can be used to determine a current of 10.2 mA.

In experiment III, the diode is effectively a short circuit when forward biased and an open circuit when reverse biased. Using Ohm's law and the argument for eqs. 3 and 4, the voltage across the resistor is 5.00 V and 0 V across the diode, with a current of 4.33 mA when the diode is forward biased. When reversed biased, the voltage drop is 0 V across the resistor and 5.00 V across the diode and no current flows.

The Experiment IV has a resistance and a capacitance value, from which a f1 value can be calculated as follows:

Similarly, f1 for experiment V can be determined to be 0.0032 -1F-1.

Safety

Although the power supply is limited to low voltages and currents, the possibility of fire or burns is possible in the case of a short circuit or overload. In the event of a burn, cover the wound and, in the unlikely event it is severe, seek medical attention. Use no salves or oils. In the case of fire, first shut off the power, then use a fire extinguisher suitable for electronic fires (NOT WATER!).

References

D. P. Shoemaker, C. W. Garland, J. W. Nibler, Experiments in Physical Chemistry, 6th ed., chap. 17, The McGraw-Hill Companies (1996).

The Staff of Buck Engineering Co. Inc., Practical Electronics, 1st ed., Buck Engineering Co. Inc.