Craftsman 28906 Professional Shop Manual page 23

CRAFTSMAN
.
Kirchhoff's current law:
Kirchhoff's current law deals with nodes. Nodes
are the junction of two or more wires or the junc-
tion of a wire to a component.
Kirchhoff's current law states that what ever cur-
rent goes into a node must come out.
As an example: Three wires are connected with
a wire nut. One wire has 5 amps going into the
connection.
The sum of the currents coming out of the other
two wires must equal 5 amps. That could be 3
amps in one wire and 2 amps in the other or it
could be 2.5 amps in each wire, but the total
coming out must be the same as the current
going in. See Figure 7.26.
5 Amps
Node
___,_ _ 3 Amps
2 Amps
.
Kirchhoff's voltage law:
Kirchhoff's voltage law deals with voltage drops.
A voltage drop is the amount of voltage used up
or "dropped" by resistance in a circuit. Ohm's law
states that V = IxR, every component in a circuit
has resistance, even the wires. To push current
through resistance, it takes voltage. Kirchhoff's
voltage law states that the sum of all the voltage
drops equals the source voltage.
As an example, imagine a circuit that has a 12V
battery that produces 4 amps of current power-
ing a light bulb that creates 3 W of resistance.
The wires are assumed to have 0 W resis-
tance*. The light bulb uses 12 volts (4 amps x 3
ohms = 12 volts). The battery produces 12 volts
that equals the 12 volts used by the light bulb.
See Figure 7.27.
4 Amps flow
X
3 _) resistance
12 Volts
Figure 7.26
Figure 7.27
NOTE: * If the proper size wire is used and
there is no corrosion in the wire, the resistance
will be too small to worry about.
20