Electrical System for All Caterpillar Products Caterpillar

Introduction to Basic Electrical Components

There are many different types of components that are used in electrical circuits.

Wires

Illustration 1	g01072288

Wires are the conductors for electrical circuits. Wires are also called leads. Most wires are stranded. The stranded wires are made up of several smaller wires that are wrapped together and covered by a common insulating sheath.

The following wires are found in Caterpillar machines:

• Copper is the most common type of wire. Copper wires are usually stranded.

• Fusible Links are circuit protection devices that are made of smaller wire than the rest of the circuit that is protected.

• Twisted/Shielded Cable is a pair of small gage wires that are insulated against RFI/EMI. This cable is used for computer communication signals.

Illustration 2	g01072291

Many wires are bound together in groups with one or more common connectors on each end. These groups are called wire harnesses. A harness may contain wires from different circuits and systems. An example would be the harness that plugs into the headlight switch assembly. The headlight switch assembly contains wires for the following lights: parking lights, taillights, high beam headlights and low beam headlights.

Some harness wires are enclosed in a plastic conduit. These conduits are split lengthwise in order to allow easy access to the harness wires. Other harness wires are wrapped in tape. Clips (plastic) and clamps (metal) attach harnesses to the machine.

Caterpillar electrical schematics provide wire harness locations in order to help you easily locate a specific harness on a machine. The features of Caterpillar electrical schematics will be covered later in the lesson.

Wire Gage

Illustration 3	g01072292

Electrical circuits and electronic circuits are engineered with specific size and length of conductors in order to provide paths for current flow. The size of a wire determines how much current the wire can carry. Wire sizes can be rated in two different ways. American Wire Gage (AWG) size is usually referred to as simply the gage of the wire. Wire sizes can also be rated by metric size.

When you repair machine wiring or you replace machine wiring, it is necessary to use the correct size and correct length for the conductors. Illustration 3 shows the typical resistances for the various size of conductors.

When you use the AWG, remember that smaller gage numbers denote larger wire sizes, and larger gage numbers denote smaller wire sizes. Metric wire sizes refer to the diameter of the wire in millimeters, so larger metric sizes translate to larger wires.

Soldering

While an electrical connection might exist between two crimped wires, it might be incomplete or faulty. Soldering creates a solid and a dependable electrical connection.

The soldering process depends on the molten solder that flows into all the surface imperfections of the metals in order to be soldered. When two pieces of metal are soldered together, a thin layer of solder adheres between the metals and completes the electrical connection.

Solder is a mixture of tin and lead. Solder usually contains a solder flux. The function of solder flux, is to eliminate oxidation during the soldering process. Flux also lowers the surface tension of the molten solder. This allows the molten solder to flow and spread more easily. The flux most commonly used in electrical wiring repair is rosin. Rosin is noncorrosive, reasonably non-toxic, and readily liquefied by heat. Rosin core solder is the only kind that should be used in electronic wiring repair. Never use acid core or use other solders that contain corrosive flux. The flux will rapidly destroy the connection's ability to conduct current.

When you solder, follow these guidelines:

• The soldering tool is used to heat the terminal or the clip. This will transfer heat by conductance to the wires, which will become hot enough to melt the solder. Do not heat the solder directly.

• Make sure that there are solder fillets between the core (conductor) and the terminal or the clip, but not on the insulator.

• If you use a clip, make sure that the solder covers the exposed conductor, and all of the clip.

• If you solder around a terminal, make sure the solder covers the conductor. Also, make sure the solder does not extend past the conductor. It may be helpful to tilt the terminal end of the wire that is being repaired slightly up in order to prevent solder from flowing onto the terminal.

• Do not apply so much solder that the individual wire strands are not visible.

• Do not allow the soldering tool to burn the terminal or the insulation.

• Do not leave sharp points of solder. The sharp points can tear the tape that is used to insulate the repair.

• Do not allow individual wire strands to protrude from the repair. Also, do not allow the wire strands to protrude over the insulator.

• Do not solder wires in a live circuit. Always disconnect power from the wires and then make the repair.

Illustration 4	g01072293

Soldering Tools and Preparation

Tools

The following tools are recommended for use when you prepare wires and connections. Also, when you solder wires or connections:

• Diagonal pliers, commonly called dikes, cutters or diagonals, are used for cutting soft wire and component leads. Diagonal pliers should not be used for cutting hard metals like iron or steel.

• Long nose pliers or needle-nose pliers are used for holding the wire so that the stripped end may be twisted around a terminal post or inserted into a terminal eye.

• A wire stripper, is used to remove insulation from the hookup wires. There are different types of strippers. These strippers range from the simple type found on diagonal pliers to the more automatic multisized strippers which can handle different wire diameters.

• A soldering iron is a standard tool in the industry that is used for connecting wires together. Soldering irons are rated by the amount of power the irons dissipate. So the soldering irons are rated indirectly by the amount of heat the irons can develop. One hundred watt guns and one hundred twenty five watt guns are the most popular sizes. The type of job determines which size iron should be used on the job.

• Heat sinks are used to prevent overheating during soldering. Heat sinks are used to prevent unsoldering of heat sensitive electronic parts. The heat sink is generally a clip that is attached to the lead between the body of the part and the terminal point at which the heat is applied. The heat sink absorbs heat. The heat sink reduces the amount of heat that is conducted by the component.

• When component leads are being removed from their holes, desoldering tools will simplify the job of cleaning solder from etched (pc) board solder holes. The holes must be free of solder before the terminals of a new component may be inserted.

Wire Preparation

Two or more wires that provide a conductive path for electricity must be electrically connected. This means that an uninsulated surface on one wire must be mechanically connected to an uninsulated surface on the other wire. To ensure that the wires will not separate, or the connection corrode, the wires are soldered at the junction.

Before wires may be connected and soldered the wires must be properly prepared. This involves stripping the insulation at the ends of the wire, providing terminal leads which may be connected to each other or to a terminal post or connector contact.

After you remove the insulation, examine the wire for nicks, cuts and discoloration. If the wire has a shiny look and is not nicked or damaged, no further preparation is needed. If the wire has a dull, dark appearance, the wire must be cleaned before soldering.

The final step before you solder the wire is to perform a task called tinning. If you use stranded wires, the wire should be twisted and placed on the tip of a heated soldering device. Heat the wire sufficiently so that the wire will melt the solder.

Mechanical Connections

Some of the more common connectors are posts, terminals and splices. Illustration 5 shows a connection to a terminal post. The wire should be secured to the post by preforming a three quarter turn to a full turn. Do not wind the wire around the post several times. It is inefficient and also causes problems if the connection needs to be desoldered.

Illustration 5	g01072294

Illustration 6 shows a typical connection to a terminal strip. Twist the wire in order to form a hook. Then insert the hook into the opening on the terminal strip.

Illustration 6	g01072295

If two wires are will be spliced, the recommended procedure is to twist each wire in the form of a hook. Combine the two hooks and apply the solder to the joint. It is not necessary to twist the wires together before soldering. Illustration 7 shows a hook splice connection.

Illustration 7	g01072298

When you connect heat sensitive components to a terminal post or to a terminal strip, it is recommended that a heat sink device be used. Illustration 8 shows a heat sink that is connected between a diode and a terminal post. The heat sink acts as a heat load, and therefore reduces the heat transfer to the diode.

Illustration 8	g01072299

Safety Precautions

The soldering gun or the soldering iron operates at temperatures that are high enough to cause serious burns. Observe the following safety precautions:

1. Do not permit hot solder to be sprayed into the air by shaking a hot gun, an iron or a hot soldered joint.

2. Always grasp a soldering gun or a soldering iron by the insulated handle. Do not grasp the bare metal part.

3. Do not permit the metal part of a soldering gun or a soldering iron to rest or to come in contact with combustible materials. An iron should always rest on a soldering stand when the iron is not in use.

Helpful Hints

Good soldering is part of a technician's skills. Solder connections must be mechanically strong, so that the connections will not vibrate loose. Loose connections will cause intermittent problems. Electrically, solder contacts must have low resistance for providing proper signal transfer.

The following rules are for basic soldering:

1. The soldering tip must be tinned and clean.

2. Metals must be clean before the metals are connected.

3. Support the joint mechanically where possible.

4. Pretin large surfaces before you solder the surfaces together.

5. Apply the solder to the joint, not to the gun or to the iron tip. Solder must flow freely and have a shiny, smooth appearance.

6. Use only enough solder to make a solid connection.

7. When additional flux is used, apply to the joint. Only rosin flux should be used on electrical connections.

8. Solder rapidly and do not permit components or permit insulation to burn or to overheat.

9. Use rosin-core solder or an equivalent. Do not use acid core solder for any electrical connections.

Connectors

Illustration 9	g01072301

The purpose of a connector is to pass current from one wire to another. In order to accomplish this, the connector must have two mating halves (plug or receptacle). One half houses a pin and the other half houses a socket. When the two halves are joined, current is allowed to pass.

With the increased use of electronic systems on Caterpillar machines, servicing connectors has become a critical task. Increased usage of electronic systems causes an increase in maintenance on the wiring, connectors, pins, and sockets. Another important factor that contributes to increased repair is the harsh environment in which the connectors operate. Connectors must operate in the following extreme conditions: heat, cold, dirt, dust, moisture and chemicals.

Illustration 10	g01072303

Pins and sockets have resistance and offer some opposition to current flow. Since the surface of the pins and sockets are not smooth (contain peaks and valleys) a condition known as asperity (roughness of surface) exists. When the mating halves are connected, approximately one percent of the surfaces actually contact each other.

The electrons are forced to converge at the peaks, thereby creating a resistance between the contact halves. Although this process seems rather insignificant to the operation of an electronic control, a resistance across the connector could create a malfunction in electronic controls.

Plating

In order to achieve a minimum resistance in the pins and the contacts, you need to be concerned with the finish, pressure and metal that is used in construction of the pins and the contacts. Tin is soft enough to allow for film wiping, but it has high resistivity. Copper has low resistivity, but it is hard. So in striving for minimum resistance and the reduction of asperity, low resistance copper contacts are often plated with tin.

Film wiping occurs when pins and contacts are plated with tin. When pins and contacts are mated together, the pins and contacts have a tendency to wipe together. Pins and contacts smooth out some of the peaks and valleys that are created by the asperity condition. Gold and silver are excellent plating material, but gold and silver are too costly to use.

Contaminants

Contaminants are one factor that contributes to resistance in connectors. Some harsh conditions that employ chemicals can cause malfunctions due to increased resistance.

Note: Connectors cause many diagnostic problems. It may be necessary to measure the resistance between connector halves when you are diagnosing electronic control malfunctions. Also, disconnecting and reconnecting connectors during the troubleshooting process can give misleading diagnostic information. Use breakout cables sparingly when you troubleshoot intermittent type electrical problems.

Types of Connectors

Several types of connectors are used throughout the electrical system and the electronic systems on Caterpillar machines. Each type of connector differs in the manner in which the connectors are serviced or are repaired.

The following types of connectors will be discussed in detail.

• Vehicular Environmental (VE) Connectors

• Sure-Seal Connectors

• Deutsch Connectors HD10 DT CE and DRC Series

VE Connectors

Illustration 11	g01072305

The VE connector was used primarily on earlier Caterpillar machine electrical harnesses when high temperatures, larger number of contacts, or higher current carrying capacities were needed.

The connector required a special metal release tool for removing the contacts that could damage the connector lock mechanism if the tool was turned during the release of the retaining clip.

Note: Do not use metal release tools that are listed in SEHS8038 for any other type of electrical connector.

After you crimp a wire to the contact, it is recommended that the contact be soldered in order to provide a good electrical contact. Use only rosin core solder on any electrical connection.

Specific information that relates to the process required for installing VE connector contacts (pins and sockets) is contained in the Special Instruction - Use of VE Connector Tool Group, SEHS8038.

This VE connector is no longer used on current product, but this connector may still require service by a field/shop technician.

Sure-Seal Connectors

Illustration 12	g01072308

Sure-Seal connectors are used extensively on Caterpillar machines. These connector housings have provisions for accurately mating between the two halves. Instead of using guide keys or keyways, the connector bodies are molded so that the connectors will not mate incorrectly.

Sure-Seal connectors are limited to a capacity of 10 contacts (pins and sockets).

Note: Part numbers for spare plug and receptacle housings and contacts are contained in the Special Instruction - Use of 6V3000 Sure-Seal Repair Kit, SMHS7531.

Use special tool 6V3001 for crimping contacts and stripping wires.

Sure-Seal Connectors require the use of a special tool 6V3008 for installing contacts. Use denatured alcohol as a lubricant when you install contacts. Special tooling is not required for removing pin contacts.

Any holes in the housings that are not used for contact assemblies should be filled with a 9G-3695 Sealing Plug . The sealing plug will help prevent moisture from entering the housings.

Deutsch Heavy Duty Series Connectors HD10

Illustration 13	g01072309

The HD10 connector is a thermoplastic cylindrical connector that utilizes crimp type contacts that are quickly and easily removed. The thermoplastic shells are available in nonthreaded and threaded configurations that use insert arrangements of 3, 5, 6 and 9 contacts. The contact size is no. 16 and accepts no. 14, no. 16 and no. 18 AWG wire.

The HD10 uses crimp type, solid copper alloy contacts (size no. 16) that feature an ability to carry continuous high operating current loads without overheating. The contacts are crimp terminated using a Deutsch Crimp Tool, 1U-5805 Wire Removal Tool part number .

Deutsch termination procedures recommend no soldering after properly crimped contacts are completed.

The procedure for preparing a wire and crimping a contact is the same for all Deutsch connectors and is explained in the Special Instruction - Servicing DT connectors, SEHS9615. The removal procedure differs from connector to connector and will be explained in each section.

DeutschTransportation (DT) Series Connectors

Illustration 14	g01072432

The DT connector is a thermoplastic connector that utilizes crimp type contacts that are quickly and easily removed. These connectors require no special tooling. The thermoplastic housings are available in configurations that use insert arrangements of 2, 3, 4, 6, 8 and 12 contacts. The contact size is no. 16 and accepts no. 14, no. 16 and no. 18 AWG wire.

The DT uses crimp type, solid copper alloy contacts (size no. 16) or stamped and formed contacts (less costly) that feature an ability to carry continuous high operating current loads without overheating. The contacts are crimp terminated with a Deutsch Crimp Tool, Caterpillar part number 1U5804.

The DT connector differs from other Deutsch connectors in both appearance and construction. The DT is either rectangular or triangular shaped and contains serviceable plug wedges, receptacle wedges and silicone seals.

The recommended cleaning solvent for all Deutsch contacts is denatured alcohol.

Note: For a more detailed explanation on servicing the DT connector, consult Special Instruction - Servicing DT Connector, SEHS9615.

Caterpillar Environmental Connectors (CE)

Illustration 15	g01072310

The CE connector is a special application connector. The CE Series connector can accommodate between 7 and 37 contacts. The 37 contact connector is being used on various electronic control modules.

The CE connector uses two different crimping tools. The crimping tool for no. 4 thru no. 10 size contacts is a 4C4075 Hand Crimp Tool Assembly, and the tool for no. 12 thru no. 18 contacts is the same tool that is used on the HD and DT Series connectors 1U5804 .

Note: For a more detailed explanation on servicing the CE connectors, refer to the Special Instruction - Use of CE/VE Connector Tools, SEHS9065.

Deutsch Rectangular Connector (DRC)

Illustration 16	g01072311

The DRC connector features a rectangular thermoplastic housing. The DRC connector is completely environmentally sealed. The DRC is best suited to be compatible with external and internal electronic control module.

The connector is designed with a higher number of terminals. The insert arrangements that are available are 24, 40, and 70 contact terminations. The contact size is no. 16 and accepts no. 16 and no. 18 AWG wire.

The connector uses crimp type, copper alloy contacts (size no. 16) or stamped and formed contacts (less costly) that feature an ability to carry continuous high operating current loads without overheating. The contacts are crimp terminated using a Deutsch Crimp Tool, 1U-5805 Wire Removal Tool part number.

The connector contains a clocking key for correct orientation and is properly secured by a stainless steel jackscrew. A 4 mm (5/32 inch) "HEX" wrench is required to mate the connector halves. The recommended torque for tightening the jackscrew is 25 inch pounds.

Note: The DRC uses the same installation and removal procedures as the HD10 series.

Switches

Illustration 17	g01072313

A switch is a device that is used to complete or to interrupt a current path. Switches are placed between two conductors (wires).

Some of the more common switches that are used on Caterpillar machines are listed below:

• Single-pole single-throw (SPST)

• Single-pole double throw (SPDT)

• Double-pole single-throw (DPST)

• Double-pole double-throw (DPDT)

Illustration 18	g01072314

There are many ways of actuating switches. The switches that are shown above are mechanically operated by moving the switch lever or the toggle. Sometimes, switches are linked so that the switches always open and close at the same time. In schematics, this is shown by connecting linked switches with a dashed line.

Other mechanically operated switches are limit switches and pressure switches. The switch contacts are closed or opened by an external means. A lever may actuate a limit switch or pressure may actuate a limit switch.

Some of the more common switches that are used on Caterpillar machines are listed below:

• Toggle

• Push-On

• Key Start

• Rotary

• Pressure

• Limit

• Rocker

• Magnetic

• Cutout

Some switches are more complex than other switches. Caterpillar machines use magnetic switches for measuring speed signals. Caterpillar electronic switches contain internal electronic components, such as transistors to turn remote signals on or off.

An example of a more complex switch that is used on Caterpillar machines is the key start switch. Illustration 19 shows the internal schematic of the "Key Start Switch". This type of switch controls the following functions: an accessory position "ACC", a run position "RUN", a start position "START" and an off position "OFF". This type of switch can control other components. Also, the Key Start Switch can deliver power to several components at the same time.

Illustration 19	g01072316

Circuit Protectors

Illustration 20	g01072317

Fuses, fusible links, and circuit breakers are circuit protectors. Excess current in a circuit causes heat. The heat, not the current, causes the circuit protector to open before the wiring can be damaged. This has the same effect as turning a switch "OFF".

Note: Circuit protectors are designed to protect the wiring and not necessarily other components.

Fuses and circuit breakers can help you diagnose circuit problems. If a circuit protector opens repeatedly, there is probably a more serious electrical problem that needs to be repaired.

Fuses

Illustration 21	g01072319

Fuses are the most common circuit protectors. A fuse is made of a thin metal strip or wire inside a holder made of glass or plastic. When the current flow becomes higher than the fuse rating, the metal melts and the circuit opens. A fuse must be replaced after the circuit opens.

Fuses are rated according to the amperage that the fuse can carry before the fuse opening. Plastic fuse holders are molded in different colors in order to signify fuse ratings. Fuse ratings are also molded into the top of the fuse.

A fusible link (not shown) is a short section of insulated wire that is thinner than the wire in the circuit that the link protects. Excess current causes the wire inside the link to melt. Like fuses, fusible links must be replaced after the fuse has blown.

You can tell if a fusible link is blown by pulling on the two ends. If it stretches like a rubber band, the wire must have melted and the link is no longer good. The insulation of a fusible link is thicker than regular wire insulation. The thicker insulation to contains the melted link after it blows.

Note: When you replace a fusible link, never use a length longer than 225 mm (9 inch).

Circuit Breakers

Illustration 22	g01072321

A circuit breaker is similar to a fuse. High current will cause the breaker to trip. This will open the circuit. The breaker can be manually reset after the overcurrent condition has been eliminated.

Some circuit breakers are automatically reset. These circuit breakers are called cycling circuit breakers. Circuit breakers are built into several Caterpillar components. An example of one component is the headlight switch.

There are also noncycling circuit breakers. This type operates with a heated wire that opens contacts until current flow is removed.

A cycling circuit breaker contains a strip that is made of two different metals. Current that is higher than the circuit breaker rating makes the two metals change shape unevenly. The strip bends, and a set of contacts is opened in order to stop current flow. When the metal cools, the metal returns to its normal shape. This closes the contacts. Current flow can resume. Automatically resetting circuit breakers are also called cycling circuit breakers. The circuit breaker cycles open and closed until the current returns to a normal level.

A PTC (for Positive Temperature Coefficient) is a special type of circuit breaker called a thermistor or a thermal resistor. PTCs are made from a conductive polymer. This material is in the form of a dense crystal, with many carbon particles that are packed together. The carbon particles provide conductive pathways for current flow. When the material is heated, the polymer expands, pulling the carbon chains apart. In this expanded state, there are few pathways for current.

A PTC is a solid state device. A PTC has no moving parts. When the PTC is tripped, the device remains in the open circuit state as long as voltage remains applied to the circuit. The PTC resets only when voltage is removed and the polymer cools.

Resistors

Illustration 23	g01072322

Sometimes it is necessary to reduce the amount of voltage or current at a specific point in a circuit. The easiest way to reduce the voltage or the current that is supplied to a load is to increase the resistance. This is done by adding resistors. Resistors come in two types: variable and fixed. Common uses for resistors in electrical circuits are the audio system and the climate control circuit. These components use several resistors that are wired to vary voltage.

Resistors are rated in ohms for the amount of resistance the resistors provide the circuit. Resistors are rated in watts for the amount of heat the watts can dissipate.

Illustration 24	g01072454

Illustration 24 shows the color code chart for identifying resistors. The rating of a resistor can be determined by looking at the bands of color on the resistor. The bands should be closer to one end of the resistor than the other end. The end with the color bands should be on your left as you read the bands. The bands are read from left to right.

The last color band indicates the resistor tolerance. This refers to how much the actual resistor value can vary from the specified rating. This rating is given as a percentage of the total rating. Some resistors have no band in this last position. Such a resistor has a tolerance of 20% of the resistance value.

Some circuits are designed with very precise resistance values and will not operate properly otherwise. For this reason, you should never replace a resistor with one of a higher tolerance.

Resistors and Wattages

Because a resistor resists current flow, electrical friction builds up. This creates heat that the resistor must be able to dissipate. Too much heat could change a resistor so that the rating and tolerance are no longer in the designed range.

Wattage is the amount of power that can be consumed by a resistor. The larger the wattage is, the more heat a resistor can withstand. Illustration 25 shows examples of resistor wattages.

Resistor Wattages

Illustration 25	g01072324

In order for a circuit to function properly, the resistors must have the correct wattage rating as well as the correct resistance rating. The resistors and other components could be damaged by additional current flow and heat if the resistance or wattage ratings are incorrect.

You can identify the wattage of a carbon-composition resistor by the size. The most common ratings are 1/10 watt, 1/4 watt, 1/2 watt, 1 watt, and 2 watts.

Variable Resistors

Illustration 26	g01072325

The kinds of resistors that have been discussed so far are fixed. This means that the resistors rating cannot be adjusted. Other resistors are variable (Illustration 26). This means that the resistor resistance can be changed by adjusting a control. The control moves a contact over the surface of a resistance. As current flows through a greater length of resistor material, the current decreases. As current flows through less resistor material, current increases.

The amount of variance and the number of resistance positions depend on how the variable resistor is constructed. Some variable resistors have only two different resistance values. Other variable resistors have an infinite range between the minimum values and their maximum values.

Variable resistors can be linear or non-linear. The resistance of a linear resistor increases evenly. When the control is set at one fourth of the travel, resistance increases to one fourth of the maximum. When the control is set to half of travel, resistance increases to half of its maximum.

There are many kinds of variable resistors. Some variable resistors are called rheostats, potentiometers or thermistors. Illustration 27 shows a schematic symbol for a rheostat.

Illustration 27	g01072326

A rheostat typically has two terminals. A rheostat allows current flow in one path. On Caterpillar machines, a rheostat is used to control the brightness of the instrument lights.

Another type of variable resistor is the potentiometer. The potentiometer allows two paths for current flow. The potentiometer can be controlled manually or controlled mechanically. Illustration 28 shows a potentiometer that is from a fuel system. The fuel sender measures a specific system resistance value which corresponds to a specific system condition. The output resistance is measured at the main display module and the value corresponds to the depth of fuel in the tank.

Illustration 28	g01072327

A potentiometer (pot) has three terminals and works by dividing the voltage between two of the terminals. Potentiometers can also be designed to work as rheostats.

Thermistors

Thermistors (thermal resistors) are a type of variable resistor that operate without human control. A thermistor is made of carbon. The resistance of carbon decreases instead of increasing at higher temperatures. This property can be useful in certain electrical circuits. Thermistor elements are used extensively in sensors on Caterpillar machines for measuring system temperatures.

Failed Resistors

Fixed resistors either work or do not work.. Fixed resistors work by passing the proper amount of current. A fixed resistor that has failed, will not allow current to pass or will allow too much current to pass.

Variable resistors, on the other hand, can exhibit a flat spot where the moving parts brush against one another and cause wear. This can become evident as lack of response through a portion of the resistor's travel.

Capacitor

Illustration 29	g01072328

A capacitor is a device that can store an electrical charge. A capacitor creates an electrical field that can store energy. The measurement of this energy storing ability is called capacitance. In Caterpillar electrical systems, capacitors are used for the following functions:

• To store energy

• As timer circuits

• As filters

Construction methods vary. A simple capacitor can be made from two plates of conductive material that are separated by an insulating material that is called a dielectric. Typical dielectric materials are air, paper, plastic, and ceramic.

Capacitor Energy Storage

In some circuits, a capacitor can take the place of a battery. If a capacitor is placed in a circuit with a voltage source, current flows in the circuit briefly while the capacitor charges. That is, electrons accumulate on the surface of the plate that is connected to the negative terminal. Electrons move away from the plate that is connected to the positive terminal. This continues until the electrical charge of the capacitor and the voltage source are equal. How fast this happens depends on several factors, including the amount of voltage that is applied and the size of the capacitor.

When the capacitor is charged to the same potential as the voltage source, current flow stops. The capacitor can then hold the charge when the capacitor is disconnected from the voltage source. With the two plates separated by a dielectric, there is no where for the electrons to go. The negative plate retains the accumulated electrons, and the positive plate still has a deficit of electrons. This is how the capacitor stores energy.

A charged capacitor can deliver stored energy just as a battery would. It is important to note that, unlike a battery, a capacitor stores electricity, but the capacitor does not create it. When a capacitor is used to deliver a suitable small current, a capacitor has the potential to deliver voltage to a circuit for as long as a few weeks.

Capacitor Measurements

Illustration 30	g01072329

Capacitors are rated in units of measurement called Farads. Farads are represented by the symbol F. These specify how many electrons the capacitor can store. The farad is a very large number of electrons. In the systems you use, you will see capacitors that are rated in microfarads (µF). A microfarad is one millionth of a farad.

In addition to being rated in farads, capacitors are also rated according to the maximum voltage that the capacitor can handle. When you replace a capacitor, never use a capacitor with a lower voltage rating.

Three factors combine to determine the capacitance of a given capacitor:

• The area of the conductive plates

• The distance between the conductive plates

• The material that is used as the dielectric.

Calculating Total Capacitance

The total capacitance of a circuit is dependent on how the capacitors are designed in the circuit.

When capacitors are in parallel, total capacitance is determined by the following equation:

• C_t = C1 + C2 + C3

When capacitors are in series, total capacitance is determined by this equation:

• C_t = 1/ (1/C1 + 1/C2)

Note: Always short across the terminals of a capacitor before you connect the capacitor to a circuit or meter. This discharges any residual charge that might be stored.