Basic DC Theory
BASIC DC CIRCUIT CALCULATIONS
When any two resistors are unequal in a parallel circuit, it is easier to calculate RT by multiplying
the two resistances and then dividing the product by the sum, as shown in equation (2-11). As
shown in equation (2-11), this is valid when there are only two resistors in parallel.
(2-11)
RT
R1R2
R1
R2
Example:
Find the total resistance of a parallel circuit which has one 12W and one 4W
resistor.
RT
R1R2
R1
R2
(12)(4)
12
4
48
16
3W
In certain cases involving two resistors in parallel, it is useful to find an unknown resistor, Rx,
to obtain a certain RT. To find the appropriate formula, we start with equation (2-10) and let the
known resistor be R and the unknown resistor be Rx.
RT
RRX
R
RX
Cross multiply:
RTR + RTRX = RRX
Transpose:
RRX - RTRX = RTR
Factor:
RX (R - RT) = RTR
Solve for Rx:
RX
RTR
R
RT
Rev. 0
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