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04 series ccts Aug 8 2008

04 series ccts Aug 8 2008 - 33 1EXPERIMENT 4 SERIES...

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1EXPERIMENT # 4: SERIES CIRCUITS (Note, this is a two period lab in order to allow the lecture to catch up with the lab if necessary.) PRE-LAB INSTRUCTIONS RESISTORS IN SERIES Components are connected in series for two main reasons. One, is to control current flow and the second, is to develop a definite voltage for some specific purpose. The resistance of a piece of wire is calculated, using the formula; R = ( ρ l / A) where l is the length of wire. It is clear that doubling the length will increase the resistance by a factor of 2. Connecting two identical pieces of wire (resistors) one after the other or in series gives an effective resistance double that of one piece. The effective resistance (R T ) of a circuit is given by R T =R 1 +R 2 +R 3 +.... Rn. The effective resistance (R T ) of the circuit and the supply voltage E, determines the current that will flow I=E/R T . The amount of power that is safely dissipated by a resistor is determined largely by its physical size, and is called its power rating. Typical resistor power ratings are: 1/8 W, 1/4 W, 1/2 W, 1 W, for carbon composition types. Wire wound types can be made to dissipate larger amounts of power. Since P = I 2 R the maximum current that can safely be carried is I = (P/R) 1/2 e.g. a 10k , 1/4 W resistor will safely carry a current of : I max = (0.25W/10,000 ) 1/2 = 5mA without overheating. Exceeding this current for a significant period will cause overheating and a probable increase in the ohmic value of the component. "VOLTAGES IN A SERIES CIRCUIT" One of the principal reasons for using a series circuit is the need to create a certain voltage at some point. This voltage may be required for "biasing a transistor" (turning it “on”) or to cause the operation of second circuit when the desired voltage is reached. In paraphrasing Kirchhoff's Voltage Law (KVL), we can say that the sum of the voltages dropped around any current path is equal to the total source voltage. E 1 + E 2 + E 3 ....E n = V 1 + V 2 + V 3 + . ...V n EE148 33

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When there is no current flow through a resistor, there will be no voltage dropped across that resistor, and the potential is the same at either end. (The voltage drop V = I*R.) It can be shown that the voltage drops in a series circuit have the following relationship: V 1 = E *(R 1 / R T ) and V 2 = E *(R 2 / R T ) etc. In the case of two resistor arrangement: V 2 = E *(R 2 /R T ) where R T = R 1 + R 2 This is the voltage divider principle. Voltages in series are proportional to the individual resistance values i.e. the larger resistor will have a larger voltage across it. A potentiometer can also be used in a voltage division role, the slider can be varied
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04 series ccts Aug 8 2008 - 33 1EXPERIMENT 4 SERIES...

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