Basic DC Theory
BASIC DC CIRCUIT CALCULATIONS
Step 3:
Find the voltage across each component.
V1 = IR1
V1 = (0.6 amps)(40 W)
V1 = 24 volts
V2 = IR2
V2 = (0.6 amps)(60 W)
V2 = 36 volts
V3 = IR3
V3 = (0.6 amps)(100 W)
V3 = 60 volts
The voltages of V1, V2, and V3 in Example 2 are known as "voltage drops" or "IR drops." Their
effect is to reduce the available voltage to be applied across the other circuit components. The
sum of the voltage drops in any series circuit is always equal to the applied voltage. We can
verify our answer in Example 2 by using equation (2-4).
VT
V1
V2
V3
120 volts
24 volts
36 volts
60 volts
120 volts
120 volts
Parallel Currents
The sum of the currents flowing through each branch of a parallel circuit is equal to the total
current flow in the circuit. Using Ohm’s Law, the branch current for a three branch circuit
equals the applied voltage divided by the resistance as shown in equations (2-6), (2-7), and (2-8).
Branch 1:
(2-6)
I1
V1
R1
V
R1
Branch 2:
(2-7)
I2
V2
R2
V
R2
Branch 3:
(2-8)
I3
V2
R2
V
R2
Example 1:
Two resistors, each drawing 3A, and a third resistor, drawing 2A, are connected
in parallel across a 115 volt source (Figure 23). What is total current?
Rev. 0
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