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Unformatted text preview: OUT ) SOLUTION V OUT = f*12V *= 12V * R 2 /(R 1 +R 2 ) (R 1 and R 2 make a voltage divider) So R 2 /(R 1 +R 2 ) = f (1) To calculate the power drain, The source current is I = 12V/(R 1 +R 2 ) so the source power supplied is P S = I*12V = 144V 2 /(R 1 +R 2 ) = 0.1 Watt Therefore (R 1 +R 2 ) = 1440 (2) to get exactly 100 mW. Combine (1) and (2) to get R 2 = f*1440 R 1 = 1440  R 2 . As an example, suppose f = 0.65. R 1 R 2 V OUT Then we want an output voltage of 7.80V with R 2 = 936 and R 1 = 504 . To stay within the power limit of 100 mW, we must pick the resistors that have (R 1 +R 2 ) > 1440. (This makes sure the current is restricted below the max allowed.) The closest values satisfying this are R 2 = 910 and R 1 = 560 ; R 1 +R 2 = 1470. Then the output voltage is V OUT = 12V* 910/(1470) = 7.429V instead of the desired 7.80V. Percentage Error = 100 x (7.429  7.80)/7.80 =  4.75%....
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This note was uploaded on 07/17/2011 for the course E E 240 taught by Professor Shankar during the Spring '10 term at University of Alberta.
 Spring '10
 Shankar
 Volt

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