Kirchhoff's Voltage Law; How The System Is Wired Together - MTD 700 Series Professional Shop Manual

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Series 700 Lawn Tractor

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 = I x R, 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 powering a light bulb that
creates 3 Ω of resistance. The wires are assumed to have
0 Ω resistance*. 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 7.41.
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.

How the system is wired together

The Rules
All circuits have some basic rules that must be followed:
1. All circuits must have at least one voltage source. It could be a battery, an altenator or both.
2. All circuits must have a load. A circuit without a load is the same as shorting out the power source. Typical
loads could be:
• lights
• a motor
• a solenoid
3. All circuits must have a complete path back to the voltage source. This is also known as having continuity.
NOTE: On outdoor power equipment, the frame of the machine is frequently used as the return path to the bat-
tery. This is referred to as grounding the machine. Any point on the frame should be the same as the
negative post of the battery (Electrically) unless there is a bad connection between the battery and the
frame or between the frame and the component or cable that is assumed to be grounded to it.
4. Most circuits have additional components like switches and fuses.
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12 Volts
4 Amps
Figure 7.41
4 Amps flow
X
3 Ω resistance
= 12 Volts