Physics 310 - Dr. Boye
Safety, Troubleshooting, Switches / Relays & Soldering
         

A. Safety

B. Troubleshooting

C. Switches / Relays

D. Soldering

 
 
A. Safety
Safety can not be overemphasized when working with electronics.  Two areas of great importance are EYE Safety and ELECTRICAL Safety; it is very important to use safe practices in these areas as it takes only a split second to lose an eye or be electrocuted.  Although it is extremely unlikely that one would even come close to suffering an eye injury or electrocution in this class; now is a good time to develop safe work habits that you will use for a lifetime.

EYE Safety - Always wear eye protection when working on energized equipment, soldering or even in being in the vicinity of operating machinery.  Eye protection should always have side shields (wrap-around style glasses) - standard prescription glasses provide some protection but are inadequate from a safety standpoint as missiles can still enter the eye from the side.  I have witnessed several incidents of electronic equipment exploding and parts coming off of operating machinery; on two occasions I had debris bounce off of my safety glasses at high speed.  I have also seen old/defective solder that had impurities in it explode when it was heated, resulting in my burns on my hands.  Always wear safety glasses when working on energized equipment.

ELECTRICAL Safety - Electricity should always be approached with respect.  When used properly electricity provides an infinite number of uses that have made life much easier and more productive in the past hundred years or so; when used improperly it can easily kill or maim as well as causing fires.  When working on an energized circuit make sure that all jewelry (including rings and necklaces are removed).  Not only does metal jewelry make an excellent conductor for electricity to arc, it can also get physically snagged on equipment which may result in the loss of a finger.  Never physically touch a component in a circuit without first verifying that there is no power to the circuit (many types of equipment have multiple sources of power so care must be made to ensure that ALL power sources are off).  Additionally, some components, such as capacitors, can retain an electrical charge long after power to the equipment has been turned off.  The best source of verification that a circuit is dead is to test it yourself with a multimeter for AC and DC voltages (NEVER rely on the word of another person that power has been turned off - ALWAYS check it yourself).  Always wear safety glasses when working on energized equipment.

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B. Troubleshooting
Generally speaking, troubleshooting any system or circuit is first and foremost based on common sense.  This applies whether one is troubleshooting an electronic circuit, debugging a computer program, working on a plumbing system, working on an industrial electrical circuit or any number of other types of circuits/systems.  There are several different ways of organized troubleshooting but they all boil down to basically the following:

1.  Verify that a problem exists.  You have to know how the equipment works It is not uncommon for most equipment trouble to be caused by the operator's incorrect understanding of the equipment's operation.  Sometimes a user may be trying to get a piece of equipment to perform in a manner in which it is physically impossible to do or they may have the equipment set up wrong.  Probably 80% of problems can be avoided by proper operation and using the equipment in a manner for which it was designed.  It is almost impossible to effectively troubleshoot a piece of equipment if you do not know how it is supposed to work.

2. Isolate the trouble to the smallest possible component.  This process can be achieved by swapping known good components or modules until the problem area is isolated to its smallest possible component.  If possible, take the suspected bad module/component and plug it into a known good piece of equipment and see if the unit then fails; if it does then that component has been proven bad.

3. After the bad component has been found, it is an excellent idea to try and ascertain why the component failed:  the failure could be caused by the age of the component, the component could have been faulty to begin with, the component could have been damaged by incorrect usage of the equipment, a power spike could have caused the failure or any other number of causes.

4. Replace the component and verify that the equipment now operates normally again.  It is always a good idea to keep a log of component failures to see if a failure trend exists in certain pieces of equipment.  It is not unusual to run across equipment that is just designed badly.  It is always an excellent idea to ensure that equipment users are thoroughly trained in the operation of any equipment that they use.

Things I have learned in 20+ years of troubleshooting:  relays and switches are the most common failure items mechanically speaking; semiconductors (IC's, transistors, diodes, etc.) are the most common failure items electrically speaking - passive components (resistors, capacitors, inductors, etc.) do fail also but not at the rate that semiconductors do.  Always check the inputs to a piece of equipment before assuming the equipment is broken (this includes power connections, timing signals, etc.).  After a piece of equipment has been determined to be faulty with all known good inputs always check the power supplies (including AC ripple on DC supplies) to ensure that they are within tolerance.

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C. Switches / Relays
1. Switch Basics - there are numerous types of switches, but it is helpful to understand some of the common types of switches.  Many switches are often referred to as SPST (Single Pole - Single Throw), DPST (Double Pole - Single Throw), etc.  A Pole is a contact set, a SP switch was one contact set and a DP has two contact sets for example.  It may be easier to understand by looking at the Switch Basics diagram that is hyperlinked at the beginning of this paragraph.  A Throw indicates the number of conducting positions; again, looking at the Switch Basics diagram will make this easier to understand.  If a circuit requires more than two throws then a rotary switch is used.  Rotary switches can have many throws AND poles.  Rotary switches can have multiple wafers which allows additional poles and throws. 

It is also important to understand the concept of NO (Normally Open) and NC (Normally Closed) when dealing with switches.  NO and NC switches are binary (two-state) switches; they are either closed (made) or open (broken).  A NO switch is statically open; when it is activated it closes the circuit.  NO switches are also called Make switches.  A NC switch is statically closed; when it is activated is opens the circuit.  NC switches are also called Break switches.

When choosing a switch for an application it is important to consider several things.  The most important is that the switches voltage/current rating is sufficient for your circuit (for example: a switch may be rated 125V@4A Or 250V@8A).  Pushbuttons come in either Maintained or Momentary; your application will determine which is the proper choice.  Lastly, it needs to be determined how many poles/throws are required.

2. Relay Basics - relays are electromechanical devices that operate by running current through a coil.  This current creates a magnetic field that pulls the contacts from a NC to the NO position. It may be easier to understand relays by looking at the Relay Basics diagram that is hyperlinked at the beginning of this paragraph.  Relays can have from one set of contacts to many sets of contacts.  A relay contact has an input and two outputs, a NO and NC position.  When the relay coil is not energized the input passes straight through to the NC output; when the relay coil is energized the input passes through to the NO output.  Relay coils can be either AC or DC with 120VAC/240VAC and 12VDC/24VDC coils being common place.

 

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D. Soldering
The most important thing in soldering is safety.  Two very important rules are Always wear safety glasses when soldering and Never solder on a live circuit (a circuit that is energized).  Soldering also has to be learned by practice and can not be learned out of a book but a few key things are helpful to know before soldering:

1. Have the proper size soldering iron for the job at hand.  If you have to solder on a SMT (Surface Mount Technology) circuit a low wattage iron is needed; if too much heat is used it will destroy the components and the circuit board.  Soldering heavy gauge wires usually requires a soldering gun while soldering a power transistor may require a 45 watt iron.  If possible, it is best to have an adjustable soldering iron to set the heat (power) for the job to be done.

2. There are many types of solder available and not all of them work well with electronics; in fact some solder types can damage electronic components.  It is best to use a 60/40 or 63/37 type of solder for electronic work (60/40 is 60% tin and 40% lead; 63/37 is 63% tin and 37% lead).  Solder is used by many outside of the electronics field including plumbers and craftspeople that work with stained glass; what works for a plumber joining copper tubing is not necessarily good to use for electronics work!

3. In addition to the soldering iron and solder there are a number of tools that make soldering easier: vises to hold work, braided solder wick & solder suckers to remove excess/unwanted solder, lead bending tools, clip-on heat sinks to protect components from heat damage during soldering as well as many other soldering tools.

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