Volvo VNL Service Manual page 14

Group 30 Electrical System—VNL, VNM
Parallel Circuits
In parallel circuits, electrical devices are connected by
parallel wires. The current divides; part of it flows into one
device, part into another. The voltage remains the same
across each branch of the circuit as though each branch
was connected directly to the source voltage.
With circuits in parallel, each circuit can be switched on
and off by itself since each receives current independently
of the other circuits. The current divides across each
branch of the parallel circuit; the sum of the current in
each of these branches is the total current in the circuit.
The total resistance in parallel circuits is less than any of
the individual resistances.
Ohm's Law in parallel circuits
In the circuit below, one resistor is 2
4 . The source voltage is 12 volts. Current must be found
for each path individually, as follows.
Apply Ohm's Law to find current across the first resistor:
4
I = V R
4
I = 12 volts 2 = 6 amps
And for the second resistor:
4
I = 12 volts 4 = 3 amps
Now, to find the total current, add the current from each
of the circuit branches:
3 amps + 6 amps = 9 amps
Using Ohm's Law, the total circuit resistance can also be
found:
4
R = V I
4
R = 12 volts 9 amps = 1.33
Another way to find total resistance in a parallel circuit
with 2 resistors is to divide the product of the 2 resistors
by the sum:
4
R = (2 x 4 ) (2 + 4
Notice that the total resistance of 1.33
of the individual resistors in the circuit.
12
, and the other is
4
) = 8 6 = 1.33
is less than either
Typical Parallel Circuit
Design and Function
W3000494
W3000565