Basic DC Theory BASIC DC CIRCUIT CALCULATIONSWhen any two resistors are unequal in a parallel circuit, it is easier to calculate R_{T} by multiplyingthe two resistances and then dividing the product by the sum, as shown in equation (2-11). Asshown in equation (2-11), this is valid when there are only two resistors in parallel.(2-11)R_{T}R_{1}R_{2}R_{1}R_{2}Example: Find the total resistance of a parallel circuit which has one 12W and one 4Wresistor.R_{T}R_{1}R_{2}R_{1}R_{2}(12)(4)12448163WIn certain cases involving two resistors in parallel, it is useful to find an unknown resistor, R_{x},to obtain a certain R_{T}. To find the appropriate formula, we start with equation (2-10) and let theknown resistor be R and the unknown resistor be R_{x}.R_{T}RR_{X}RR_{X}Cross multiply: R_{T}R + R_{T}R_{X} = RR_{X}Transpose: RR_{X} - R_{T}R_{X} = R_{T}RFactor: R_{X} (R - R_{T}) = R_{T}RSolve for R_{x}:R_{X}R_{T}RRR_{T}Rev. 0 Page 35 ES-02

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