How PLCs Work

A programmable logic controller is a specialized computer used to control machines and processes. It therefore shares common terms with typical PCs like central processing unit, memory, software and communications. Unlike a personal computer though the PLC is designed to survive in a rugged industrial atmosphere and to be very flexible in how it interfaces with inputs and outputs to the real world.
The components that make a PLC work can be divided into three core areas.

• The power supply and rack
• The central processing unit (CPU)
• The input/output (I/O) section

PLCs come in many shapes and sizes. They can be so small as to fit in your shirt pocket while more involved controls systems require large PLC racks. Smaller PLCs (a.k.a. “bricks”) are typically designed with fixed I/O points. For our consideration, we’ll look at the more modular rack based systems. It’s called “modular” because the rack can accept many different types of I/O modules that simply slide into the rack and plug in.

The Power Supply and Rack

So let’s start off by removing all our modules which leaves us with a naked PLC with only the power supply and the rack.

The rack is the component that holds everything together. Depending on the needs of the control system it can be ordered in different sizes to hold more modules. Like a human spine the rack has a backplane at the rear which allows the cards to communicate with the CPU. The power supply plugs into the rack as well and supplies a regulated DC power to other modules that plug into the rack. The most popular power supplies work with 120 VAC or 24 VDC sources.

The CPU

The brain of the whole PLC is the CPU module. This module typically lives in the slot beside the power supply. Manufacturers offer different types of CPUs based on the complexity needed for the system.
The CPU consists of a microprocessor, memory chip and other integrated circuits to control logic, monitoring and communications. The CPU has different operating modes. In programming mode it accepts the downloaded logic from a PC. The CPU is then placed in run mode so that it can execute the program and operate the process.

Since a PLC is a dedicated controller it will only process this one program over and over again. One cycle through the program is called a scan time and involves reading the inputs from the other modules, executing the logic based on these inputs and then updated the outputs accordingly. The scan time happens very quickly (in the range of 1/1000th of a second). The memory in the CPU stores the program while also holding the status of the I/O and providing a means to store values.

I/O System

The I/O system provides the physical connection between the equipment and the PLC. Opening the doors on an I/O card reveals a terminal strip where the devices connect.

There are many different kinds of I/O cards which serve to condition the type of input or output so the CPU can use it for it’s logic. It's simply a matter of determining what inputs and outputs are needed, filling the rack with the appropriate cards and then addressing them correctly in the CPUs program.

Inputs
Input devices can consist of digital or analog devices. A digital input card handles discrete devices which give a signal that is either on or off such as a pushbutton, limit switch, sensors or selector switches. An analog input card converts a voltage or current (e.g. a signal that can be anywhere from 0 to 20mA) into a digitally equivalent number that can be understood by the CPU. Examples of analog devices are pressure transducers, flow meters and thermocouples for temperature readings
Outputs
Output devices can also consist of digital or analog types. A digital output card either turns a device on or off such as lights, LEDs, small motors, and relays. An analog output card will convert a digital number sent by the CPU to it’s real world voltage or current. Typical outputs signals can range from 0-10 VDC or 4-20mA and are used to drive mass flow controllers, pressure regulators and position controls.

Programming a PLC

In these modern times a PC with specially dedicated software from the PLC manufacturer is used to program a PLC. The most widely used form of programming is called ladder logic. Ladder logic uses symbols, instead of words, to emulate the real world relay logic control, which is a relic from the PLC's history. These symbols are interconnected by lines to indicate the flow of current through relay like contacts and coils. Over the years the number of symbols has increased to provide a high level of functionality.
The completed program looks like a ladder but in actuality it represents an electrical circuit. The left and right rails indicate the positive and ground of a power supply. The rungs represent the wiring between the different components which in the case of a PLC are all in the virtual world of the CPU. So if you can understand how basic electrical circuits work then you can understand ladder logic.
In this simplest of examples a digital input (like a button connected to the first position on the card) when it is pressed turns on an output which energizes an indicator light.

The completed program is downloaded from the PC to the PLC using a special cable that’s connected to the front of the CPU. The CPU is then put into run mode so that it can start scanning the logic and controlling the outputs.

Definition of a PLC

What is a PLC?

A Programmable Logic Controller, or PLC for short, is simply a special computer device used for industrial control systems. They are used in many industries such as oil refineries, manufacturing lines, conveyor systems and so on. Where ever there is a need to control devices the PLC provides a flexible way to "softwire" the components together.
The basic units have a CPU (a computer processor) that is dedicated to run one program that monitors a series of different inputs and logically manipulates the outputs for the desired control. They are meant to be very flexible in how they can be programmed while also providing the advantages of high reliability (no program crashes or mechanical failures), compact and economical over traditional control systems.

A Simple Example

Consider something as simple as a switch that turns on a light. In this system with a flick of the switch the light would turn on or off. Beyond that though there is no more control. If your boss came along and said I want that light to turn on thirty seconds after the switch has been flipped, then you would need to buy a timer and do some rewiring. So it is time, labor and money for any little change.

A PLC Saves the Day

Now consider the same device with a PLC in the middle. The switch is fed as an input into the PLC and the light is controlled by a PLC output. Implementing a delay in this system is easy since all that needs to be changed is the program in the PLC to use a delay timer.

This is a rather simple example but in a larger system with many switchs and lights (and a host of other devices) all interacting with each other this kind of flexibility is not only nice but imperitive. Hopefully a light bulb has now turned on over your head.

Contoh aplikasi menggunakan relay

Harry Porter's Relay Computer

Click on each photo for an enlargement

Last updated: 23 April 2006

(More pictures follow below) Features of the Arithmetic Logic Unit: Two 8-bit inputs (from B and C registers) 8-bit result (onto data bus) 3-bit function code input Functions: Add, Increment, And, Or, Xor, Not, Shift-left, Nop Carry output (from Add, Increment) Zero-detect output Features of the Register Unit: 8 Registers (8-bits each) Register Names: A, B, C, D, M1, M2, X, and Y Data Bus (8 LEDs and 8 Switches) Features of the Program Control Unit: Program Counter (16 bits) Instruction Register (8 bits) Jump Target Register (16 bits) Increment Unit (16 bits) Increment Register (16 bits) Address Bus (16 LEDs and 16 Switches) Features of the Sequencer Unit: Clock (Using capacitors for delay) Finite State Control (24 states) Instruction Decoding Main Memory (32K x 8bits, static RAM chip) General Features: Data Bus (8 bits) Address Bus (16-bits) All relays are the identical part (Four-Pole-Double-Throw, 12 Volts) 415 Relays 111 Switches 350 LEDs Max Power Consumption: Estimated 12 Amps @ 13.5 Volts (160 Watts) For more information, click here (Last Updated, 23 April 2006) Back to Harry's homepage: click here Send email to Harry: harry@cs.pdx.edu

The Birth of the PLC

The Original Challenge

The early history of the PLC is fascinating. Imagine if you will a fifty foot long cabinet filled with relays whose function in life is to control a machine. Wires run in and out of the system as the relays click and clack to the logic. Now imagine there is a problem or a small design change and you have to figure it all out on paper and then shut down the machine, move some wires, add some relays, debug and do it all over again. Imagine the labor involved in the simplest of changes. This is the problem that faced the engineers at the Hydra-matic division of GM motors in the late 1960's.
Fortunately for them the prospect of computer control was rapidly becoming a reality for large corporations as themselves. So in 1968 the GM engineers developed a design criteria for a "standard machine controller". This early model simply had to replace relays but it also had to be:

• A solid-state system that was flexible like a computer but priced competitively with a like kind relay logic system.
• Easily maintained and programmed in line with the all ready accepted relay ladder logic way of doing things.
• It had to work in an industrial environment with all it's dirt, moisture, electromagnetism and vibration.
• It had to be modular in form to allow for easy exchange of components and expandability.

The Race is On

This was a tall order in 1968 but four companies took on the challenge.

1. Information Instruments, Inc. (fully owned by Allen-Bradley a year later).
2. Digital Equipment Corp. (DEC)
3. Century Detroit
4. Bedford Associates

Bedford Associates, run by Richard Morley, won the contract and quickly formed a new company around the technology called MODICON after Modular Digital Control. By June of 1969 they were selling the first viable Programmable Controller the "084" (their 84th project) which sold over one thousand units. These early experiences gave birth to their next model the "184" in 1973 which set Modicon as the early leader in programmable controllers.
Not to be outdone, the powerhouse Allen-Bradley (all ready known for it's rheostats, relays and motor controls) purchased Information Instruments in 1969 and began development on this new technology. The early models (PDQ-II and PMC) were deemed to be too large and complex. By 1971 Odo Struger and Ernst Dummermuth had begun to develop a new concept known as the Bulletin 1774 PLC which would make them successful for years to come. Allen-Bradley termed their new device the "Programmable Logic Controller" (patent #3,942,158) over the then accepted term "Programmable Controller". The PLC terminology became the industry standard especially when PC became associated with personal computers.

PLCs Versus Other Types of Controls

A PLC is not the only choice for controlling a process. Sticking with only basic relays may be of a benefit depending upon your application. Yet, on the other hand, a computer might be the way to go. The PLC vs. PC debate has been going on for a long time. More often though it doesn't come down to an "either or" situation but involves a mix of technologies.

PLC vs. Relay

When I first started programming PLCs it was still questionable if a PLC was necessary over just relay control. With PLC prices going down, size shrinking, and performance of PLCs improving over the years this has become less of a battle. Yet the designer has to ask themselves if a PLC is really overkill for their application. Some questions should be asked.

• Is there a need for flexibility in control logic changes? Will there be frequent control logic changes? Will there be a need for rapid modification?
  A lot companies believe they will never change a design but more often then not ideas and goals do change and modifications will need to be made. Do you want to do that in hardware (relays) or software (PLC)?
• Must similar control logic be used on different machines?
  It's so much easier to download a program then build another panel.
• Is there a need for future growth?
  A PLC can easily accept a new module in a slot or get an expansion base.
• Is there a need for high reliability?
  PLCs are seen as more robust over individual components.
• Is downtime a concern?
  Any change or troubleshooting on a relay system means the system might have to go offline. Changes in a PLC can often be made online with no downtime.
• Are space requirements important?
  Based on the number of relays a PLC can be a real space saver.
• Are increased capability and output required?
  PLCs can be faster then their mechanical counterparts.
• Are there data collection and communications required?
  Only possible with a PLC or computer.
• What are the overall costs?
  There's a certain price point comparison but in these days it's very low in favor of a PLC.

PLC vs. Dedicated Controller

A dedicated controller is a single instrument that is dedicated to controlling one parameter such as a PID controller measuring a temperature for heating control. They have the advantages of an all in one package, typically with display and buttons. This can be a very good thing to use in simple applications. A PLC these days can compete price wise and functionally with these controllers especially if you more then one controller is needed. PLCs offer a greater degree of flexibility too because the can be programmed to handle all sorts of different scenarios.

PLC vs. PC (Personal Computers)

The PLC vs. PC debate has been going on for years and I'm not going to attempt to give the definitive answer. They both have their pros and cons. What often happens is that the two are used for their strengths in different parts of the factory.

	PLC	PC
Environment	The PLC was specifically designed for harsh conditions with electrical noise, magnetic fields, vibration, extreme temperatures or humidity.	Common PCs are not designed for harsh environments. Industrial PCs are available but cost more.
Ease of Use	By design PLCs are friendlier to technicians since they are in ladder logic and have easy connections.	Operating systems like Windows are common. Connecting I/O to the PC is not always as easy.
Flexibility	PLCs in rack form are easy to exchange and add parts. They are designed for modularity and expansion.	Typical PCs are limited by the number of cards they can accommodate and are not easily expandable.
Speed	PLCs execute a single program in sequential order. The have better ability to handle events in real time.	PCs, by design, are meant to handle simultaneous tasks. They have difficulty handling real time events.
Reliability	A PLC never crashes over long periods of time. ("Never" may not be the right word but its close enough to be true.)	A PC locking up and crashing is frequent.
Programming languages	Languages are typically fixed to ladder logic, function block or structured text.	A PC is very flexible and powerful in what to use for programming.
Data management	Memory is limited in its ability to store a lot of data.	This is where the PC excels because of it's hard drive. Any long term data storage, history and trending is best done on a PC.
Cost	Just too hard to compare pricing with so many variables like I/O counts, hardware needed, programming software, etc.

Hybrids of PLC/PCs are common now (e.g. WinPLC). This type of hardware tries to mix the two platforms using the strengths of both. So the CPU might be able to run Windows CE or Linux in a rack that can accept common I/O modules.

Off the Shelf vs. Build Your Own

For some manufacturers the choice these days is coming down to buy a PLC or make your own. The benefits of PLCs have become so widely known that manufacturers looking to cut cost can engineer their own solutions and build them more cost effectively. An example of this is the Divelbiss "PLC on a Chip" with the accompanying EZ LADDER programming software.

How to Install Electrical Wiring

BASIC PRINCIPLES OF GOOD WIRING Before beginning any electrical repair, shut off the power. Remove the fuse or trip the breaker for the circuit you will be working on in your service panel. Use a neon tester to be sure the power is off. If there is any doubt, you can remove the main fuse or trip the main breaker. Remember: Removing the main fuse or tripping the main breaker will usually shut off the power to the entire house. Electrical wires are color coded to prevent wiring errors. White wires almost always connect to other white wires or to chrome terminal screws on switches and receptacles. Some wiring devices–such as receptacles–are back-wired by pushing the bare wire end into spring grip holes. These wiring devices are plainly labeled to show which color goes into each spring grip hole. Switches are nearly always connected into black wires in cables. The only exception is where a cable is extended, making it necessary for the white wire to play the role of the black wire. When this is necessary, the white wires should be painted black to prevent future wiring errors. Study the wiring diagram. This will help you understand the basic principles of good wiring. Also, find a good electrical how-to book. It's one book every homeowner should keep on hand for ready reference. Most home wiring is complete with either No. 14 gauge or No. 12 gauge wiring. No. 14 is the smallest wiring permitted under most codes. Always use the same size cable for a continuation of any extended wiring circuit.

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CONNECT NEW WIRING TO LAST OUTLET IN CABLE New wiring should be connected to the last outlet in a run of cable. To locate the last outlet in the run, shut off the current. Remove the cover plates from each outlet on the circuit. The last outlet in the run has wires connected to only two of the four terminal screws. The two unused terminal screws on the last receptacle serve as a starting point for wiring to a new outlet.

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ATTACHING CABLE FOR NEW WIRING Shut off the power to the circuit you will be working on at the service panel. Loosen the screws holding the receptacle in the box and remove it, as shown. Attach the the earth wire (the bare or green) to the chrome terminal. The yellow (or green in some instances) wire should be connected to the receptacle and the box maintaining the equipotential bonding on the earth system. The earth wires should only be connected to the correct screw terminals on the recepticle to the brass terminal on the receptacle and to the box, if the box is metal. Use care to match the size of the original cable. If No. 12 wire is used, continue with No. 12. If No. 14 wire is used, use No. 14 for continuing the cable. The size of the cable is usually stamped on the side of the cable. New wiring can be connected to continue the run beyond the last receptacle. Note that the new wires are pulled through knockout plugs in the back of the outlet box.

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ADDING NEW WIRING FROM A JUNCTION BOX New wiring can also be tied into a junction box, unless the wiring in the junction box is already at maximum capacity. Before tying in at a junction box, always trace the cables leading to the box to check the voltage. Be sure you are not connecting a 120-volt outlet to a run of wire providing 240 volts for larger appliances. To tie in new wiring at a junction box, first shut off the current at the service panel. Locate the main supply cable coming into the junction box from the service panel. Locate the supply wire by tracing the white wires. All white wires in the junction box will be attached to the white wire on the supply line. Knock out the unused plug on the junction box and run the new line from the box as illustrated. Be sure to use a cable clamp to secure the cable to the junction box.

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TYING IN NEW WIRING AT A CEILING LIGHT You can tie in new wiring at a ceiling light if the light is not controlled by a switch. Shut off the current at the service panel. Tie white wires to white wires and black wires to black wires, as illustrated. Connect the ground wires as illustrated. If you are using a metal box, attach them to the box as well as the light fixture. Knock out an opening in the outlet box, and continue the new wiring as illustrated.

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ALWAYS MATCH CONNECTORS TO TYPE OF CABLE USED Some boxes come with built-in connectors. Armored cable connectors have inner rims to hold fiber bushings at the end of the cable. Nonmetallic cable connectors are designed to grip the installation around the cable with a two-screw clamp. Regardless of the type of cable used, always leave about 6" to 8" of wiring in the box to allow plenty of wire for making easy connections. You can tighten the nut on either type of cable connector by placing a screwdriver in the notch and tapping the screwdriver lightly.

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MAKE ALL CONNECTIONS IN APPROVED BOXES   Always remember that connections must be made in an approved box. Never connect one cable to another by an open-line splice. All switch, outlet, and junction boxes must be positioned so they are always accessible. You can easily remove knockout plugs with a nail punch, screwdriver or metal rod.

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RUNNING NEW CABLE BETWEEN MULTIPLE FLOORS Drill a hole through the floor from bottom to top, as illustrated. Be sure the hole is drilled into the recessed area behind the wall rather than in the open. Be sure to use a bit that's large enough to permit free passage of the wiring cable. Run the cable through the newly drilled hole to the desired location for the new receptacle or switch. Bring the cable through the opening by using a weight on the end of a string and a wire with a hook on the end. Using this same technique, you can add one outlet to another by drilling up through the floor, pulling the cable under the floor, and then running it to the desired position on the opposite wall. The same wiring can be pulled through for either receptacles or switches.

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ADDING NEW WIRING FROM BOXES IN CEILING If your home has an unfinished attic, it may be easier to add new wiring by attaching it to boxes in the ceiling. In this way, gravity works for you rather than against you. Attach the cable to the box as previously described. Cut a hole in the wall at the desired location for the switch or receptacle, and run the cable from the box in the ceiling to the new outlet location. Bring the new cable through the wall and ceiling by cutting and drilling holes in and through the wall, the 2x4 plate, and the ceiling. A special fish tape is available for these types of jobs.

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ADDING NEW WIRING ON THE SAME WALL You can connect new cable from an existing outlet to a new outlet on the same wall by running it inside the wall. Mark the approximate location of the new outlet. Using a stud finder locate and mark the wall studs. Start one stud before the existing outlet and end one stud after the new outlet. Mark the exact location of the new box. Make it the same height as the existing box. Do not locate it over a stud. Using a drywall or keyhole saw, cut the opening for the new box. Using a utility knife and a drywall saw, cut a strip of drywall about 3" wide out of the wall, below the outlets. Start at the center of the first stud you marked and end at the center of the last stud; watch for nails as you cut. Carefully remove the drywall strip. Using a hand or circular saw, make two cuts 1" apart and 3/4" deep in each of the exposed studs. Using a hammer and a chisel, remove the wood between the two saw cuts. Be sure the power is off to the existing outlet. Remove the cover plate and the receptacle. Remove one of the knockouts in the bottom of the box. Run the new wire behind the wall and up through the knockout in the box. Tighten the clamp and attach the wires. If the box does not have a clamp, place a wire clamp on the new cable. Tighten the screw to hold the clamp on the wire. Be sure the nut is off the wire clamp and run the wire up to the box as before. Feed the threaded end of the clamp up through the knockout, replace the nut and tighten. Replace the receptacle and the cover plate. On the new box, remove one of the knockouts in the bottom of the box. If the box you are using is a self-clamping box, insert the box into the wall and tighten. If not, insert the box into the wall, insert a Madison hanger on each side of the box, and bend the tabs over into the box to tighten. Finish running the wire from the existing box through the notches and up behind the wall into the box as before. Clamp the wire and install the receptacle as in the figure. Install the cover plate, turn on the power, and test the circuit with a neon tester. Shut off the power again to safely finish the project. Nail metal cable protectors to the exposed studs over the notches. Replace the drywall strip you removed earlier. Use the spackling compound and drywall tape to complete the installation. Cable can be pulled from an existing box on one wall to a new outlet on the opposite side of the same wall. Attach a cable to the existing receptacle in the box as previously described. Allow ample slack in the cable to permit easy connection to the new box to be installed on the opposite wall. Bring the cable through the new opening with a wire, as illustrated in. Connect the cable to the new box, attach the desired receptacle, and mount the box to the wall with box supports if it is not near a stud.

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TOOL AND MATERIAL CHECKLIST

• Two-Wire Cable
• Switches
• Screwdriver
• Extra-Long Bit
• Conduit
• Fish Tape
• Outlet Boxes
• Electrical Tape
• 1/4" Drill
• Cable Connectors
• Pigtails
• Hand or Circular Saw
• Madison Hangers
• Drywall Tape
• Cable Protector Plates
• Switch Boxes
• Side Cutter Pliers
• Wire-nuts
• Chisel
• Drywall or Keyhole Saw
• Three-Wire Cable
• Receptacles
• Brace
• Ripping Bar
• Wire
• Neon Tester
• Stud Finder
• Hammer
• Spackling Compound

Check your state and local codes before starting any project. Follow all safety precautions. Information in this document has been furnished by the National Retail Hardware Association (NRHA) and associated contributors. Every effort has been made to ensure accuracy and safety. Neither NRHA, any contributor nor the retailer can be held responsible for damages or injuries resulting from the use of the information in this document.

How to Solder

Follow these tips and instructions on how to work with solder to help you save time, money and effort. In this document you will find information about:

• How to Prepare for a Soldering Job
• How to Solder Various Metals
• Soldering Flat Pieces of Metal

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HOW TO PREPARE FOR A SOLDERING JOB There are many types of soldering pencils, guns and irons that are adequate for home use. Most home-use soldering tools are heated electrically. There are soldering tips that can be used with your propane torch. There is even a small refillable butane gas-powered soldering tool. The proper soldering tool depends on your project. The propane torch is for jobs requiring a high heat source like sweating copper fittings. The gun is for soldering tasks requiring a little more control of the amount of heat and where it is going, such as joining wires, while the pencil is for intricate soldering jobs requiring even less heat but more control, like circuit-board repairs. Before soldering with any pencil, gun or iron, be sure the tip is thoroughly cleaned. Use a light or medium file to remove any corrosion that is built up on the tip of the soldering point. The tip of a soldering tool should be clean at all times. Clean the tip after each use to eliminate much of the need for filing the tip. The shape of the tip of a soldering tool is also important. The modified chisel tip as illustrated is ideal for most soldering jobs. The tip of the soldering tool should be small enough to reach into tight places but blunt enough to ensure that heat is transmitted all the way down to the point. Before beginning the soldering job, apply a thin, even coat of solder to all sides of the tip. This coating process is referred to as "tinning". Tinning should be done frequently while you are soldering. To apply an even coat of solder on all sides of the tip of the pencil, gun or iron, hold a length of core-type solder against the hot tip. With the solder against the tip, rotate the soldering tool so all sides of the tip are covered evenly. Always be sure your soldering tool is at maximum heat. You cannot get a proper soldering job with a pencil, gun or iron that does not melt the solder quickly. Also, be sure the material you are soldering is completely clean. Dirt, grease or any foreign matter limits the holding power of solder. Any material to be soldered should be scraped, sanded or treated with a soldering flux before you apply the solder. Always do your soldering on a flat, even surface. For safety, it is best to work on a fireproof surface. A kitchen-type cleaning pad or a piece of steel wool is a handy cleaning device for the point of your soldering tool while you are soldering. This pad or piece of steel wool can be stapled or tacked to the work surface where you are soldering. An occasional wipe across the cleaning pad keeps the point clean at all times. Tack two crossed finish nails into a scrap piece of wood to make an ideal holder for your soldering pencil or iron. These nails keep the pencil or iron off the flat surface, hold it in place and keep the point of the pencil or iron clean while you are doing the job. Always apply heat with the point of the soldering tool held flat against the metal to be soldered. Do not try to transmit heat with only the tip–the tip is for shaping or forming. Keep the soldering point hot at all times. If either the solder or the metal to which the solder is applied is not kept hot enough, you will get a poor soldering joint. Although solder is also sold in a solid bar, core-type solder is most commonly used. One type of solder has a rosin core while the other has an acid core. Always use a rosin-core solder (this has a rosin flux in the center) for soldering electrical wiring and metals like tin and copper. Use an acid-core solder (this has an acid flux in the center) for soldering more difficult metals, such as galvanized iron. When you use an acid-core solder, the surface to which the solder is applied should be washed after each soldering to remove the corrosive effect of the acid. A special type of solder is required for soldering stainless steel.

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HOW TO SOLDER VARIOUS METALS It is important that all metal to be soldered is thoroughly clean. Solder simply will not adhere to dirty or oxidized metal surfaces. Clean any flat surfaces which are to be soldered with steel wool, a file, emery cloth, etc. It's important to take time to clean the surface thoroughly. Scrape any wire to be soldered with the back of a knife or any flat piece of metal. If the wire is extremely dirty, dip it into a flux. Do not touch the wire with your hands after it has been cleaned. Natural oils in the skin may cause the solder not to stick. Although the core of solder contains flux, additional flux may be required on extremely difficult soldering jobs. Liquid flux can be brushed on the metal if required. You will need flux if you are soldering with bar solder, which does not contain a core of flux. If solder remains on the tip of the pencil, gun or iron for any period of time, the flux boils out and must be replaced. If you find it difficult to get solder to stick on galvanized metal or any other hard-to-solder surface, add some flux. This will normally improve the sticking capacity of the solder. If you are attempting to solder any coated surface, such as enamelware, you must chip away the coated area before applying the solder. Solder will not stick to coated surfaces. When soldering electrical wire, separate the wires to be soldered and scrape them clean. Each section of the wire should then be "tinned" or coated with a thin layer of solder. Apply this thin coating of solder by holding the wire on the hot tip of the soldering tool and feeding the rosin-core solder from the top. You will need a small bench vise or some other holding device to provide a "third hand" for soldering jobs of this type. After the wires have been thoroughly tinned, twist them together. After the wires have been twisted together, apply a small amount of flux to the exposed wire to remove any oil that might have been left on the wiring during the twisting process. A small paper cup makes an excellent holding device for soldering small pieces of wire. Make a slot in each side of the cup to hold the wire in a firm position. Also, fill the bottom of the cup with water. This will make the cup more stable and reduce the chances of a flame-up. Note in Figure that the splices in the wire are located at different positions. This eliminates the danger of electrical shorts and lessens the amount of buildup when the soldered spots are taped for insulation. When the wires have been twisted together and fluxed, they are ready for soldering. Hold the hot soldering tool under the joint to be soldered and feed the solder from the top. Let the solder melt and run down until the joint is thoroughly covered. Allow the soldered joint to cool completely before applying any pressure. After the solder cools and becomes hard, test it to make sure the soldered joint is secure. Always use a rosin-core solder for soldering electrical wiring. NEVER use an acid-flux solder for soldering electrical wire. Joints soldered properly should look somewhat like those illustrated. A joint that is properly twisted and soldered is as strong as any uncut section of the wire.

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SOLDERING FLAT PIECES OF METAL You should solder most flat metals, such as copper and tin, with a rosin-core solder. Use acid-core solder only on galvanized iron and other hard-to-solder metals. To get a good bond on two pieces of flat metal, apply a thin layer of solder to both edges. After applying this thin layer of solder to the edges to be soldered together, place the tinned edges one over the other and press them firmly in place with the broad side of the hot soldering iron. As you apply pressure with the soldering iron, feed additional solder into the joint from the side. A little experience will enable you to "sweat" the edges and solder the two pieces of metal together easily, quickly and firmly. Heat that is applied to flat pieces of metal can cause the metal to warp and bow up or down. This makes soldering difficult. When soldering two pieces of metal, hold them firmly in position with a screwdriver or some other blunt object while soldering. If you do a lot of soldering, you may find a small C-clamp or some other permanent holding device helpful on jobs of this type. Knowing how to solder is helpful for many home repair jobs. The soldering pencil, gun or iron and core-type soldering make it possible for you to repair gutters, electrical wiring, sheet metal or almost any other type of metal object. Always be sure to clean the point of the soldering tool on the cleaning pad or steel wool before putting it away. An empty tin can makes an ideal holder for a hot soldering pencil or iron. If you do not use a tin can, be sure to lay the hot soldering pencil or iron in a safe position until it cools to prevent a fire hazard.

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TOOL AND MATERIAL CHECKLIST

• Soldering Pencil, Gun or Iron
• Rosin-Core Solder
• Steel Wool
• Vise
• File
• Flux
• Cleaning Pad
• Vise-Type Pliers
• Propane Torch
• Tin Snips
• Knife
• Paper Cup
• Acid-Core Solder
• Work Gloves
• Pliers
• Small Brush