Electrical System for All Caterpillar Products Caterpillar

Solid State Electrical Components


D3400 01T

Introduction to Solid State Electrical Components

Modern electronic circuits use solid state components. Solid state electrical components are found in most Caterpillar machines.


Some elements, such as copper, are good conductors. Other elements are poor conductors, but good insulators. There are other elements that are neither good conductors nor good insulators. If an element falls into this group, but the element can be changed into a useful conductor, the element is called a semiconductor. Silicon and germanium are the most commonly used elements for semiconductors.

Examples of semiconductors include diodes, transistors, and integrated circuits (ICs). Semiconductors are used throughout most Caterpillar machines. Semiconductors often replace mechanical switches.

All semiconductors are solid state devices. A solid state device is one that can control current without moving parts, heated filaments, or vacuum bulbs. There are other solid state devices that are not semiconductors. A transformer is not a semiconductor.

How Semiconductors Work

Illustration 1g01072606

Pure semiconductors have tight electron bonding. There is no place for electrons to move. In this natural state, these elements are not useful for conducting electricity.

However, semiconductors can be made into good conductors through doping. Doping is the addition of impurities. The impurities affect the free electrons in the semiconductor. Depending on which impurity is added, the resulting material will have either an excess of free electrons or a shortage of free electrons.

If the added material creates an excess of free electrons, the semiconductor is negative or N type. If added material creates a shortage of free electrons, the semiconductor is positive or P type.

Semiconductors are made from a sandwich of at least one slice of N type material and at least one slice of P type material. These slices are mounted inside a plastic or a metal housing. The area where the N type material and P type material meet is called the PN junction.

Current Flow through Semiconductors

The flow of electricity through a semiconductor is different from other electrical devices. Usually, you define the movement of electricity as the movement of free electrons that bump each other from the negative terminal of the voltage source through the conductor and toward the positive terminal. When you discuss semiconductors, you describe not only the flow of electrons, but also the flow of holes. Holes are spaces in an electron shell to which an electron will be attracted.

The flow of electrons is relatively easy to visualize. You can think of a flow of marbles through a channel, for example. The flow of holes is slightly harder to visualize.

Illustration 2g01072607

Think of the same channel, that is filled with marbles, as shown in Illustration 2. One marble moves ahead, leaving a hole in the marbles place. The next marble moves into the position vacated by the first marble. At the same time, the hole can be said to be moving from the position that the first marble had held to the position that the second marble had held. As marbles move in one direction in the channel, holes can be said to be moving in the opposite direction.

With no voltage applied to a semiconductor, the free electrons at the PN junction are attracted to the holes in the P type material. Some electrons drift across the junction in order to combine with holes. Similarly, holes from the P type material can be said to be attracted to the free electrons in the N type material. Although, holes are not particles themselves the holes can be visualized as crossing the PN junction in order to combine with electrons.

Depletion Region

As long as no external voltage is applied to the semiconductors, there is a limit to how many electrons and holes will cross the PN junction. Each electron that crosses the junction leaves behind an atom that is missing a negative charge. Such an atom is called a positive ion. In the same way, each hole that crosses the junction leaves behind a negative ion. As positive ions accumulate in the N type material, the ions exert a force (a potential) that prevents any more electrons from leaving. As negative ions accumulate in the P type material, the ions exert a potential that keeps any more holes from leaving. Eventually, this results in a stable condition that leaves a deficiency of both holes and electrons at the PN junction. This zone is called the depletion region.

Barrier Voltage

When voltage is applied to a PN semiconductor electrons flow from the N side, across the junction, and through the P side. Electrons will flow in this manner if the semiconductor is configured in the circuit to allow electricity to flow. Holes flow in the opposite direction. The effect of the PN junction on current flow in a circuit depends on where it is placed and on the order of the P and N type materials.

The voltage potential across the PN junction is called the barrier voltage. Doped germanium has a barrier voltage of about 0.3 volts. Doped silicon has a barrier voltage of about 0.6 volts.


The simplest kind of semiconductor is a diode. A diode is made of one layer of P type material and one of N type material. Diodes allow current flow in only one direction. On a schematic, the triangle in the diode symbol points in the direction that current is permitted to flow by using conventional current flow theory.

Diodes are used for many purposes in electrical circuits. These purposes include illumination, rectification, and voltage spike protection.

Anode and Cathode

Illustration 3g01072609

Current flows from left to right in Illustration 3. You can indicate this by a positive (plus) sign to the left and a negative (minus) sign to the right of the diode. The positive side of the diode is the anode and the negative side is the cathode.

There is an easy way to remember the names anode and cathode. Associate anode with A+ (the positive side) and cathode with C- (the negative side). The cathode is the end with the stripe. Current flows through a diode when the anode terminal is more positive than the cathode terminal.

Diode Bias

Illustration 4g01072611

The term bias is used to refer to a diode's ability to allow or to prevent the flow of current in a circuit.

A forward biased diode is connected to a circuit to allow the flow of electricity. This is done by connecting the "N" side of the diode (the cathode) to the negative voltage, and the "P" side (the anode) to the positive voltage. When the diode is connected in this way, both electrons and holes are being forced into the depletion zone. This connects the circuit. Current flows in the direction of the arrowhead indicating that the diode is forward biased.

When a forward biased diode is connected to a voltage source the diode acts as a switch that closes a circuit. You can think of the voltage as forcing both electrons and holes into the depletion region, which allows current to flow.

A diode will not conduct (current flowing) until the forward voltage (bias) reaches a certain threshold. The threshold voltage is determined by the type of material that is used to construct the diode. A germanium diode usually starts conducting when the forward voltage reaches approximately 300 millivolts, while a silicon diode requires approximately 600 millivolts.

A diode is limited to the amount of current that can flow through the junction. The internal resistance of the diode will produce heat when current is flowing. Too much current produces too much heat, which can destroy the diode.

Illustration 5g01072613

A diode that is connected to voltage so that current cannot flow is reverse biased. This means that the negative terminal is connected to the "P" side of the diode, and the positive terminal is connected to the "N" side. The positive potential is on the cathode terminal and, as such, current is being blocked against the arrowhead.

When voltage is applied to this circuit, the electrons from the negative voltage terminal combine with the electron holes in the P type material. The electrons in the N type material are attracted toward the positive voltage terminal. This enlarges the depletion area. Since the holes and electrons in the depletion area do not combine, current cannot flow.

When a diode is reverse biased, the depletion region acts like an open switch, blocking current. When the negative terminal is connected to the P material, holes are attracted away from the depletion region. When the positive terminal is connected to the N material, electrons are attracted away from the depletion region. The result is an enlarged zone that contains neither holes nor electrons that cannot support current flow.

Diode Leakage Current

A very small amount of current can flow through a reverse biased diode. If the supply voltage becomes high enough, the atomic structure inside the diode will break down, and the amount of current that flows through the diode will rise sharply. If the reverse current is large enough and lasts long enough, the diode will be damaged by the heat.

In summary, if a diode is forward biased, the diode acts like a small resistance, or a short circuit. If the diode is reverse biased, the didode acts like a very large resistance or open circuit.

Zener Point

Illustration 6g01072614

The applied voltage at which the diode fails is called the maximum reverse voltage or Zener Point. Diodes are rated according to this voltage. Circuits are designed to include diodes with a rating that is high enough to protect the diode and the circuit during normal operation.


The following diodes are used in electrical circuits:

  • Voltage regulation (using Zener diodes)

  • Indicators (using LEDs)

  • Rectification (changing AC current to DC current)

  • Clamping to control voltage spikes and surges that could damage solid state circuits (acting as a circuit protector)

Zener Diodes and Voltage Regulation

A Zener diode is a special kind of diode that is heavily doped during manufacture. This results in a high number of free electrons and electron holes. These additional current carriers permit reverse current flow when a certain reverse bias voltage is reached. This is called the avalanche point or Zener point. In forward bias, the Zener diode acts like a regular diode.

One common Zener diode will not conduct current in the reverse direction if the reverse bias voltage is below six volts. If the reverse bias voltage reaches or exceeds six volts, the diode will conduct reverse current. This Zener diode is often used in voltage control circuits.

For an example of Zener diodes, look at a charging system. Zener diodes are shown inside the alternator. These diodes act as a safety mechanism to limit the output of the stator. The Zener diodes in the alternators are rated in order to turn on at approximately 28 volts.

Light Emitting Diodes (LEDs) & Illumination

Illustration 7g01072615

Another type of diode that is commonly used is a Light Emitting Diode (LED), which is used for indicator lamps. Like all diodes, LEDs allow current flow in only one direction. The difference is that when forward voltage is applied to an LED, the LED radiates light. Many LEDs that are connected in series can be arranged to light as numbers or as letters in a display.

While most silicon diodes need about 0.5 volts or 0.7 volts to be turned on, LEDs need approximately 1.5 volts to 2.2 volts. This voltage results in currents that are high enough to damage an LED. Most LEDs can handle only about 20 to 30 mA of current. To prevent damage to an LED, a current limiting resistor is placed in series with the LED.

LEDs Versus Incandescent Lamps

In complex electrical circuits, LEDs are an excellent alternative to incandescent lamps. The LEDs produce much less heat and need less current to operate. The LEDs also turn on and off more quickly.

Diodes as Rectifiers

Illustration 8g01072617

Rectifiers change alternating current (AC) to direct current (DC). Several diodes can be combined in order to build a diode rectifier, which is also called a rectifier bridge.

Rectifier and Generator

The most common use of a rectifier in Caterpillar electrical systems is in the alternator. The alternator produces alternating current (AC). Because electrical systems use direct current (DC), the alternator must convert the AC to DC. The DC is then provided at the alternator 's output terminal.

Alternators use a Diode Rectifier Bridge to change AC current to DC current.

Study Illustration 8 in terms of conventional theory. First, you must understand that the stator voltage is AC. That means the voltage at A alternates between positive and negative.

When the voltage at (A) is positive, current flows from (A) to the junction between diodes (D1) and (D2). Notice the direction of the arrows on each diode. Current cannot flow through (D1), but current can flow through (D2). The current reaches another junction, between (D2) and (D4). The current cannot flow through (D4), nor can current return through (D2). The current must pass through the circuit load because current cannot flow through (D4) or (D2). The current continues along the circuit until the current reaches the junction of (D1) and (D3) .

Note: The circuit load in this simplified example is a resistor. In a real charging system, the load would be the battery.

Even though the voltage that is applied to (D1) is forward biased, current cannot flow through D1 because there is positive voltage on the other side of the diode. There is no voltage potential. Current flows through (D3), and from (D3) to ground at (B). When the stator voltage reverses so that point (B) is positive, current flows through (D4) and (D1) to ground at (B) .

Whether the stator voltage at point (A) is positive or is negative, current always flows from top to bottom through the load (R1). This means the current is (DC).

Illustration 9g01072618

The rectifiers in generators are designed to have an output (positive) and an input (negative) diode for each alternation of current. This type of rectifier is called a full wave rectifier. In this type of rectifier, there is one pulse of (DC) for each pulse of (AC). The (DC) which is generated is called full-wave pulsating (DC), as shown in Illustration 9.

Diodes In Circuit Protection

Illustration 10g01072619

Electromagnetic devices like solenoids and relays have a unique characteristic that can cause voltage spikes if not controlled. The coil that is in the electromagnetic device sets up a magnetic field as the current flows through the device. When the circuit is abruptly opened and the supply voltage is removed, the collapsing magnetic field actually generates it's own voltage potential. The voltage potential may be high enough to damage some circuit components, especially sensitive solid state controllers.

To protect against sparks or surges, Clamping Diodes are added in parallel with the coil. While voltage is applied to the circuit, the diode is reverse biased and does not conduct electricity. When voltage is removed and the induced current is flowing, the diode is forward biased and does conduct electricity. The current flows in a circular path through the diode and coil until it dissipates.

Induced current can cause problems other than sparks. The computers in today's Caterpillar machines make decisions based on circuit voltages. The computers make the wrong decisions if electromagnetic devices cause abnormal voltages.

Testing Diodes

When a diode is functioning properly in a circuit, the diode acts as a large voltage drop in one direction, and as a very small voltage drop in the other. Unfortunately, testing diodes is not always this simple.

In fact, there are four possible ways in which you can test diodes:

  • Take the diode out of the circuit. Sometimes this is not possible.

  • If the diode is in a series circuit, the diode can be tested with the circuit power off.

  • If the diode is in a series circuit, the diode can be tested with the power on. For a typical silicon diode, the forward biased voltage drop should be approximately 0.6 volts.

  • If the diode is in a parallel circuit, the diode must be tested with an analog meter, not with a digital meter.


A diode is only one type of semiconductor. By combining several kinds of semiconductor material, you can create transistors. Like diodes, transistors control current flow. Transistors can perform practically all the functions that were once performed by vacuum tubes, but in much less space and without creating as much heat. Transistors are used in many applications, including radios, electronic control modules and other solid state switches.

Transistor Types

There are many kinds of transistors. Transistors can be divided into two major groups: bipolar and unipolar (also called Field Effect Transistors, or FETs). While there are several differences between the two types, the most important difference is this:

  • Bipolar transistors vary current to control voltage.

  • FET transistors vary voltage to control current.

Bipolar transistors are more common in Caterpillar electrical circuits.

Transistor Construction

Illustration 11g01072621

Like diodes, transistors contain a combination of N type and P type material. However, transistors contain three materials instead of two. The three materials are arranged so that N type and P type materials alternate (either as an NPN or a PNP group). This means that diodes have two leads while transistors have three. Illustration 11 is a symbolic representation of transistor construction.

Emitter, Base, and Collector

In Illustration 11, the material on the left is called the emitter. The material that is sandwiched in the middle is the base. The material on the right is the collector.

The symbols on the top of Illustration 11 are the schematic symbols for a transistor. The arrow indicates current flow direction this is conventional theory, and is always on the emitter. The arrow points in a different direction depending on whether the transistor is PNP or NPN.

FETs also have three sections. These sections are referred to as the gate, the source, and the drain .

  • The gate approximates the function of the base.

  • The source is similar to the emitter.

  • The drain is similar to the collector.

Basic Function

A transistor works by using the base to control the current flow between the emitter and the collector. When the transistor is turned ON, current can flow in the direction of the arrow only. When the transistor is OFF, current cannot flow in either direction.

Base Paths

It is important to realize that the base leg of a bipolar transistor controls the flow of current. Current flow through the base accounts for only a small amount of the total current flow (typically around 2% of the total). Current flowing through the base also allows current to flow from emitter to collector.

PNP or NPN Transistors

There is an easy way to identify the kind of transistor without thinking about the movement of electrons or of electron holes. The arrow always points toward the N material and away from the P material. For a PNP transistor, the arrow points inward toward the base. For an NPN transistor, the arrow points away from the base.

In Caterpillar electrical circuits, NPN transistors are much more common than PNP.

Transistor Operation

When you are trying to understand how a transistor functions in a specific circuit, there are two facts you must remember. First, an NPN transistor is turned ON by applying voltage to the base leg. NPN is turned OFF by removing voltage from the base leg. This is very similar to the operation of a relay, which is turned on and off by applying and removing voltage to the coil.

Second, the current through the base circuit is always much smaller than the current across the collector circuit. Changing the base current a little results in a big change in the collector current. The current that flows through the emitter circuit is always the largest current of all. In fact, the emitter current must be equal to the base current that is added to the collector current. The current in the emitter circuit is split between the base circuit and the collector circuit.

Solid State Relays

Illustration 12g01072623

In some circuits, it is desirable to have transistors function like relays. For example, in Illustration 12, a switch with a very small current that controls a light consumes a large amount of current. This solid state relay has the following advantages over a mechanical relay:

  • The solid state relay can switch faster.

  • The solid state relay is smaller.

  • The solid state relay will not wear out.

Transistor relays are different from mechanical relays. A mechanical relay acts as a switch that turns current completely on or completely off. A transistor varies the current flow according to the amount of current that is flowing through the base.

Resistors in Transistor Circuits

Resistors are used with transistors for several purposes. For example, using resistors, the voltage that is supplied to a transistor can be precisely controlled. This produces precise output currents. Resistors that are used in this way are placed on the base circuit.

The second function is transistor protection. If resistors or other resistances are not placed in the emitter and collector parts of the circuit, high currents can destroy the transistor.

Transistor Terminology

There are many terms that describe the characteristics of a specific transistor. For example, transistor current gain describes how much bigger the collector circuit current is than the base circuit current. If a transistor has a gain of 100 and a base current of 10 mA, then the current in the collector circuit is 100 multiplied by 10, which equals 1000 mA, or 1 A.

Transistors have many other ratings that are similar to those for diodes. Transistors can be rated according to the following conditions:

  • How fast the transistor can turn on and off.

  • How much heat the transistor can handle.

  • How much current leaks through a transistor when the transistor is supposed to be turned off.

Other Applications

Transistors are useful as switching devices. If you see a transistor in an electrical circuit, the transistor is likely functioning as a switch. Transistors can also be used to amplify or to oscillate current, or as dimmers.

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