Lab Report

by My Name

Student ID #

Electronics Experiment

Teaching Assistant: TA Name

Chem 374-005

Group #6

12-6-96

Performed: 11-28-96

Lab Partner: Their Name

Abstract

In this experiment various electronic properties were explored through the use of such instruments as a DMM, an oscilloscope, a prototyping board, a frequency generator, a variable power supply, various resistors and capacitors, and a diode. Using these components, voltage dividers, high and low pass filters, and various instructional circuits were built and tested.

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.

Experimental

The procedure given in a handout entitled Electronics Experiment was followed, from experiment 0 to experiment IV, as well as experiment VI. Data for experiment V was provided on a separate handout entitled Data Recorded by EWF.

Results

While resistors a current limiting devices, and capacitors store charge. The more charge a capacitor has stored, the greater its resistance to current flow. Diodes control the ability of current to flow in a particular direction.

Voltage dividers take advantage of proportional voltage drops in a series resistive circuit to provide reduced voltages to one or more sources. The use of diodes in an AC circuit allows the positive and negative portions of the signal to be isolated, which is one of the first steps in the conversion of AC to DC. Circuits containing resistors and capacitors are capable of attenuating signals based on their frequency.

The tolerances of the components were not noted during the course of the experiment as it was not asked for in the procedure. However, as most resistors have a tolerance in the range of 5-15%, a tolerance of 10% is assumed for all components.

Actually measured at 471.2, a difference of 0.26%. Within tolerance.

Actually measured at 1.0121 k, a difference of 1.2%. Within tolerance.

Actually measured at 32.84 k, a difference of 1.5%. Within tolerance.

Actually measured 476.1 k, a difference of 1.3%. Within tolerance.

No capacitors were available with precise markings on them, so tolerance determinations can not be made.

Discussion

In experiment IV, the voltage seen across the capacitor was a triangular wave, although the applied wave was square. This is a result of the capacitor's charging and discharging effects. Since the applied wave was never positive or negative for the capacitor to come close to becoming completely charged, the charging effect is seen as a linear voltage drop. The exact charging time, and the resulting shape of the voltage drop seen across a capacitor, is a function of the applied voltage and the capacitance.

A specialized type of resistor, called a thermistor, is available for the electrical measurement of temperature. A thermistor is a resistor made of materials that are sensitive to temperature, and will have a typical range of resistance from 10 k to 1000 k and a temperature range of about -50C to 200C.

Spectrophotometers are based on photoresistors, a type of resistor that varies with the amount of incident light. By passing light through a sample with an indicator in it, absorption and transmittance values can be determined by measurement of the photoresistor's resistance. A typical photoresistor will have a resistance that ranges from 10 k to 1000 k. The resistance of these resistors usually varies with light intensities from moderately bright to very bright.

While an input impedance of 1 M would have very little real effect on the behavior of most circuits, 100 pF is large enough a capacitance to change the circuit being studied. If this instrument were used to study high frequencies, attenuation would cause voltage measurement problems, and measurements of capacitances would require a correction for the capacitance introduced to the circuit by the probe itself. For a square wave at 5 MHz, the capacitance would most likely be slightly visible as a rounding of the leading edges of each half wave. Measurements of waves in the vicinity of 20 MHz and higher would have a more pronounced effect.

An input impedance of only 50 would change the circuit being measured by such a large amount that accurate measurements would be largely impossible. It would be equivalent, from the circuit's point of view, to connecting a 50 resistor to the point of measurement to ground. For a 5V source, this is a current drain of 0.1 A. At this level of impedance, corrections are a must if any values are to be taken at all.

If a probe of 50 impedance and 100 pF were used to study the high and low pass circuits described earlier an effect would be apparent event at 500 Hz, largely because of the low impedance. In the low pass filter, the probe would create a voltage divider and reduce the applied voltage to the capacitor significantly. In addition, the effect of its 100 pF would be added to the capacitance of the capacitor, yielding an actual system capacitance of 0.0501 F. While the shape of the waveform would be similar, its amplitude would be much smaller.

In the high pass filter, the input's biggest effect would be its parallel effect on the resistor, effectively reducing its resistance to 10 . Again, the waveform would posses a similar shape, but a much smaller amplitude.

References

A class handout entitled Electronics Experiment

A class handout entitled Data recorded by EWF