UNIVERSITY OF CALIFORNIA BERKELEY EE40 Fall 2009 Lab 1 Introduction to Circuits and Instruments Guide 1 Objectives The electronic circuit is the basis for all branches of electrical engineering In this lab basic electronic circuit theory electronic and photonic devices will be introduced and employed Fundamental testing equipment will be used to measure and characterize simple circuitry In the hands on lab you will apply these basic theories to the devices and components provided to design simple circuits 2 Basic circuit theory and devices In this section some basic circuit theory will be presented first You will need to apply this to the following hands on lab to design your circuit Simple electronic and photonic devices such as resistors light emitting diodes LEDs and speakers microphones will also be discussed and used in the lab Before doing the lab please read through this section carefully and complete the prelab to test your understanding of the material presented here 1 Ohm s Law V IR Current denoted I and voltage denoted V are two major quantities that are used to study electronic circuits Current is the amount of charge passing through a certain area in a unit time period while voltage describes the electrical potential drop across any two nodes in a given circuit Ohm s Law states that the voltage V across an ideal resistor is proportional to the current I through the resistor The constant of proportionality is the resistance R of the resistor I V R V IR V Slope R I Figure 1 1 2 Series and parallel connections A circuit usually contains many devices connected in different fashions Two basic types of configuration are series and parallel As shown in the figure below when the devices are connected in series the current going through them is the same I I1 I2 and the total voltage across both devices is the sum of the voltage across each device V V1 V2 However for parallel connection the voltage across the devices is the same V V1 V2 since they share the same nodes across which the potential drop is measured and the total current running through all the devices is the sum of the current in each branch I I1 I2 V1 I1 V2 I2 Device 1 I Device 2 I I1 I 2 Series V V1 V 2 V V1 I1 Device 1 Parallel I I1 I2 V2 I2 Device 2 V V1 V2 I V Figure 2 Now let us examine the resistive circuits shown below I1 Series V1 I2 V2 V1 I1 I R1 Parallel R2 R1 V2 I2 V I R2 V a b Figure 3 2 In a based on Ohm s law V1 R1I1 V2 R2I2 And since this is a series connection I1 I2 I V V1 V2 Therefore V R1 R2 I Voltage divider circuit It is straightforward to get V1 R2 R1 V V and V2 R1 R2 R1 R2 Therefore when a voltage is applied to a series combination of resistances a fraction of the voltage appears across each of the resistances And of the total voltage the fraction that appears across a given resistance in a series circuit is the ratio of the given resistance to the total series resistance In b the two resistors are connected in parallel From Ohm s law V1 R1I1 V2 R2I2 And since this is a parallel connection V1 V2 V I I1 I2 Therefore I V R1 V R2 R1R2V R1 R2 Current divider circuit It is straightforward to get I1 R2 R1 I and I 2 I R1 R2 R1 R2 Therefore the total current flowing into a parallel combination of resistances divides and a fraction of the total current flows through each resistance And the fraction of the total current flowing in a resistor is the ratio of the other resistance to the sum of the two resistances 3 Ideal voltage and current sources An ideal voltage source supplies a constant voltage across its output terminals no matter how much current is going through it Likewise an ideal current source will supply constant current out no matter what the voltage across it is The circuit symbol of the ideal voltage or current source is shown in the figure below ideal voltage source V ideal current source I Figure 4 3 4 Resistor The resistor is the most basic and widely used component in electronic circuits And the relation of the voltage and the current of a resistor in a circuit will follow Ohm s law A typical resistor is color coded to indicate the resistance value There are two types of color coding 4 band code and 5 band code As can be seen the 5 band code has one more digit resolution than the 4 band code The following chart provides the color code for both 4 band and 5 band resistors To decode the color bands and calculate the corresponding resistance value one needs to follow the steps below a Find the tolerance band It is located at one end of the resistor and far away from the rest of the color bands It gives the accuracy of the actual resistance to the value that is labeled b Start from the other end and use the color code map to identify the color band This will be the first digit the most significant digit of the resistance value c Then similarly decode the second and the third band for 5 band resistor only Write down all the digits in order from left to right d The last band is the multiplier Use the decoded digits to multiply the decoded multiplier to get the resistance value 5 Light emitting diode LED A diode is a basic but very important device that has two terminals the anode and the cathode Current can ONLY flow from anode to cathode and NOT the other way around The circuit symbol for a diode is shown in the a figure below indicating the polarity The simple piecewise linear model approximates a 4 diode s I V characteristic and is shown in b When the voltage applied across the diode is higher than a certain threshold value VTH the diode is on and the voltage across it stays fixed at the threshold value However if the voltage is lower than VTH the diode is off and there is no current going through the device The actual diode I V characteristic is shown in c Even though it does not have a sharp turn on at a well defined threshold voltage the simple piecewise linear model is still a good approximation in most applications A light emitting diode is a diode that will emit light when it is on The color of the light depends on the material used to fabricate the device circuit symbol Piecewise linear I V characteristic Real I V characteristic I Anode I Cathode VTH I a b V V c Figure 5 The polarity of an LED device is often denoted by the length the two legs For example the LED that will be used in this lab is shown in the figure below The long leg is the …
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