Electrical System for All Caterpillar Products Caterpillar

Ohms Law


D3400 01T

Introduction to Ohm's Law

In 1827 a mathematic reasoning to electronics called Ohm's Law was established by George Simon Ohm. Ohm's Law is a fundamental law of electricity, that relates the quantities of voltage, current, and resistance in a circuit.

Basic Direct Current Circuit Elements

Illustration 1g01070291

A circuit is a path for electric current. Current flows from one end of a circuit to the other end when the ends are connected to opposite charges. The ends are usually called power and ground. Current flows only in a closed circuit or a completed circuit. Current cannot flow if there is a break in the circuit. Every electrical circuit should contain the following components:

  • Power Source

  • Protection device (fuse or circuit breaker)

  • Load (light)

  • Control Device (switch)

  • Ground

These devices are connected together with conductors in order to form a complete electrical circuit.

General Rules of Ohm's Law

Ohm's Law states that current flow in a circuit is directly proportional to circuit voltage and inversely proportional to circuit resistance.

This means that the amount of current flow in a circuit depends on how much voltage or how much resistance there is in the circuit.

Since most Caterpillar electrical circuits that are on mobile equipment use a 12 volt source or 24 Volt source, the amount of current will be determined by how much resistance is present in the circuit.

Remember, current flow does the work. Voltage is only the pressure that moves the current. Resistance is opposition to the current flow.

The rules that are needed to understand, predict, and calculate the behavior of electrical circuits are grouped under the title Ohm's Law. You can derive the following general rules from the Ohm's Law equation.

Assuming the resistance does not change:

  • As voltage increases, current increases.

  • As voltage decreases, current decreases.

Assuming the voltage does not change:

  • As resistance increases, current decreases.

  • As resistance decreases, current increases.

Ohm's Law Equation

Ohm's Law can be expressed as the following algebraic equation:

  • E stands for electromotive force (voltage).

  • I stands for intensity (amperage).

  • R stands for resistance (ohm's).

If you know two parts of the Ohm's Law equation, you can calculate the third part. For example:

To determine voltage, multiply current times resistance.

To determine current, divide voltage by resistance.

To determine resistance, divide voltage by current.

Ohm's Law Solving Circle

Illustration 2g01070294

The Ohm's Law solving circle is an easy way to remember how to solve any part of the equation. To use the solving circle (Illustration 2), cover any letter that you do not know. The remaining letters give you the equation for determining the unknown quantity.

Voltage Unknown

Illustration 3g01070300

In this circuit, the value of the source voltage is unknown. The resistance of the load is 2 ohms. The current flow through the circuit is 6 amps. Since the voltage is unknown, the equation to solve for voltage is current times resistance. So, multiplying 6 amps times 2 ohms equals 12 volts. Therefore, the source voltage in this circuit is 12 volts.

Resistance Unknown

Illustration 4g01070303

In this circuit, the value of the resistance is unknown. The current flow through the circuit is 6 amps and the source voltage is 12 volts. Since the resistance is unknown, the equation to solve for resistance is, voltage divided by current. So, 12 volts divided by 6 amps equals 2 ohms. Therefore, the resistance in the circuit is 2 ohms.

Current Unknown

Illustration 5g01070307

In this circuit, the current is unknown. The resistance of the load is 2 ohms and the source voltage is 12 volts. Since the current is unknown, the equation to solve for current is voltage divided by resistance. So, 12 volts divided by 2 ohms equals 6 amps. Therefore, the current flow in this circuit is 6 amps.

Metric System of Measure

When you measure something, you find a number to express the size or the quantity of the item that is being measured. Numbers are used to express the results of simple calculations. In addition to using numbers, there is always a unit, or an expression to describe what the number means. In the study of electrical systems, the metric system is used for units of measurement.

When you work in the metric system, there are only a few basic units that you need to be familiar with. Basically, you simply multiple or divide the basic unit by factors of 10 if you need a larger or smaller measuring unit. These factors of 10, or multiples of 10, have special names in the metric system. The names are used as prefixes. The names are attached to the beginning of the names of basic units.

The following is an example of a metric prefix:

  • 1500 Volts of electricity would be expressed metrically as 1.5kV.

  • The equation would be stated in power of 10 as 1.5 × 103 or 1.5 × 1000 = 1500.

  • The prefix k is equal to 1000, so the equation for 1500 volts is therefore stated as 1.5kV.

Electrical applications and electronic applications work with either very large quantities or very small quantities. These quantities are expressed in metric prefixes.

The metric system units make up an internationally recognized measuring system that is used throughout the world. The measuring system is called the International System of Units (SI). The following units are the most common units in the study of basic electrical theory: Mega (millions), Kilo (thousands), Milli (thousandths) and Micro (millionths).

The following table lists some of the more common prefixes. The table lists the standard abbreviations and the powers of 10.

Illustration 6g01070311

The entire metric system will not be covered in this course. Only the metric prefixes that are most commonly used in measuring electrical properties and electronic properties are covered in this course.

Base Units

Base units are standard units. These units are without a prefix. Volts, ohms, and amperes are the primary base units that are used in electronics. Prefixes are added to base units in order to change the unit of measurement.


Mega stands for one million and is abbreviated with a capital M. One megohm equals a million ohms. To convert any value from megohms to ohms, move the decimal point six places to the right. For example, 3.5 megohms would convert to 3,500,000 ohms.


Kilo means one thousand and is abbreviated with a k. A kilohm is equal to 1,000 ohms. To convert any value from kilohm to ohms, move the decimal point three places to the right. For example, 0.657 kilohms would convert to 657 ohms.


Milli stands for one thousandth and is abbreviated by the lower case letter m. A milliampere is one-thousandth of one ampere. To convert any value from milliamperes to amperes, move the decimal point three places to the left. For example, 0.355 milliamp would convert to .000355 amp.


Micro means one millionth and is abbreviated by the symbol µA. Microampere is equal to one millionth of an amp. To convert any value from microamperes to amperes, move the decimal point six places to the left. For example, 355 microamperes would convert to .000355 amp.


Power is a measure of the rate at which energy is produced or is consumed. In an engine, the output horsepower rating is a measure of the ability to do mechanical work. In electronics, power is a measure of the rate at which electrical energy is converted into heat by the resistive elements within a conductor. In an electrical circuit, resistance uses electrical power. Remember that many kinds of devices can have resistance. Devices that offer electrical resistance include conductors, insulators, resistors, coils, and motors. Many electrical devices are rated by how much electrical power they consume, rather than by how much power they produce. Power consumption is expressed in watts.

746 watts = 1 horsepower

The unit of measurement for power is the watt. Power is the product of current multiplied by voltage. One watt equals one amp times one volt. In a circuit, if voltage or current increases, power increases. If current decreases, power decreases. The relationship among power, voltage, and current is determined by the Power Formula. This is the basic equation for the power formula:

P = I × E, or Watts = amp × Volts

You can multiply the voltage times the current in any circuit and find out how much power is consumed. For example, a typical hair dryer can draw almost 10 amps of current. The voltage in your home is about 120 volts. Multiplying 10 volts by 120 volts shows that the power produced by the hair dryer would be approximately 1200 watts.

The most common application of a watts rating is the light bulb. Light bulbs are classified by the number of watts they consume. Common examples of items with wattage ratings are audio speakers, some motors, and most home appliances.

Resistor Ratings

Resistors are rated by how many ohm's of resistance they create and by how many watts they can handle. Common ratings for carbon-composition resistors are 1/4 watt, 1/2 watt, 1 watt, and 2 watts.

A resistor converts electrical energy to heat. As the resistor works, the resistor always generates some heat. If a resistor is forced to handle more watts than the resistor was designed for, the resistor will generate excessive heat. When substantially overloaded the resistor may fail prematurely.

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