High-Tech - ETA International

y John Baldwin, CETsrOne of the reoccurring themes in electronics is the study of powerand energy. It may be confusing to lay persons what the differenceis between electrical and electronics occupations. Electricianstypically work on circuits and conductors that have currents higher than10 Amperes at low AC frequencies. Electronics technicians work withsmaller currents, but could be operating at very high frequency. Voltagescould be high or low in either field of work, but it is the current levelsthat tell the difference. Power is the product of current and voltage, soelectricians work with high power circuits and technicians are usuallyfound working with lower power values where smaller and more accuratemeasurements are often required. Both fields of work demand high skilllevels and sometimes those skills overlap.Back to Basics corner for Jan-Feb 2010 HTNby John Baldwin, CETsrAs wind turbines and solar arrays begin to replace conventional coal andnuclear One plants, of the technicians reoccurring themes may in find electronics their is career the study in of power these and new energy. fields. Itmay be confusing to lay persons what the difference is between electrical andIt is moreelectronicsefficientoccupations.to productElectricianspowertypicallyat largeworkfacilitieson circuitsratherand conductorsthan havemany small that have sites. currents Years higher ago, than many 10 Amperes small at towns low AC had frequencies. their own Electronics electricpower plant.techniciansToday,work withverysmallerfew townscurrents,havebut couldtheirbe operatingown plant,at verybuthighare fedfrequency. Voltages could be high or low in either field of work, but it is thethru the current maze levels of the that electrical tell the difference. power Power grid, is where the product it is of cheaper current and and voltage, moreeconomical so electricians to produce work with power high power at a circuits large and facility technicians and are transport usually found it overworking with lower power values where smaller and more accuratedistance.measurementsWind farmsareareoftenoftenrequired.locatedBoth fieldsin ruralof workareasdemandwherehigh skillfewlevelspeoplelive, but and most sometimes energy those is skills used overlap. in metropolitan areas, so infrastructuremust be in place to transport the energy where needed.As wind turbines and solar arrays begin to replace conventional coal and nuclearplants, technicians may find their career in these new fields. It is more efficient toAnalogies product can power be used at large to facilities describe rather than the have effects many of small maximum sites. Years transfer ago,many small towns had their own electric power plant. Today, very few townsof powerhavefromtheironeown plant,circuitbuttoareanother,fed thru theormazefromof theoneelectricalplacepowerto anothergrid, whereifdistances it is are cheaper great. and more Both economical situations to produce require power at an a large understanding facility ofbasics, whethertransport it overit isdistance.on a smallWind farmscircuitare oftenboardlocatedorinanruralelectricalareas wherelinefeworpeople live, but most energy is used in metropolitan areas, so infrastructure musttelecommunications be in place to transport long haul the energy trunk where over needed. land. Ohm’s Law and Watt’sLaw can be used to determine maximum power transfer and Kirchhoff’sAnalogies can be used to describe the effects of maximum transfer of power fromLaws canonebecircuitusedto another,to showorhowfrom onelossesplacecanto anotherresultif distancesfrom seriesare great.or parallelcombinations Both situations in either require large an understanding long haul situations of basics, whether or in small it is on a circuits. small circuitboard or an electrical line or telecommunications long haul trunk over land.Ohm’s Law and Watt’s Law can be used to determine maximum power transferThe classic and series Kirchhoff’s circuit Laws with can be R1, used R2 to and show R3, how can losses be can used result to from represent series or theline loss parallel and the combinations load a in long either haul large power long haul line situations circuit. or in In small the circuits. equivalentcircuit below,The classicR1seriesandcircuitR3 representwith R1, R2theand R3,conductorcan be usedortolinerepresentloss,theandlineR2is the load. loss and The the total load in power a long haul applied power is line equal circuit. to In the equivalent power dissipated circuit below, inthe lineR1lossesand R3(R1representand R3)the conductorplus theor linepowerloss, andusedR2 isortheconsumedload. The totalby thepower applied is equal to the power dissipated in the line losses (R1 and R3)load (R2). plus the Voltage power used drop or occurs consumed along by the the load distributed (R2). Voltage resistance drop occurs along of theconductors the distributed and subtracted resistance of to the what conductors is felt and across is subtracted the load. to what is feltacross the load.PSR1 = Line lossR3 = Line lossR2 = LoadIf a 100 ft long extension cord is used to run a high current devicerated at 10 Amps, such as a 120 V AC electric chainsaw at the endof the cord, the voltage drop across the line (on R1 and R3 above) willtake away power from reaching the device being used. If the wire sizeis 16 AWG, which has 0.4 Ohms per 100 ft, then the drop across eachconductor is V = I x R or 10 A x 0.4 Ω = 4 volts. Then there will be 8volts less being applied to the motor, or 112 volts available to the load,if 120 volts is applied at the power source.Suppose another 10 Amp load would be added in parallel to the end ofthe extension cord such as a hedge trimmer. Then there would be 20Amps being drawn, and the voltage drop on each line would increase to20 A x .4 Ω = 8 volts, or 16 volts total drop on both conductors (acrossboth R1 and R3). Then only 104 volts would be available at the endof the extension cord for both appliances to run. This may burn out amotor rated at 120 v if this arrangement is used over time at a lowervoltage. Less power is available for the load due to the line drop, andthe conductors heat up from the power dissipated in the resistance ofthe cord conductors.The series circuit becomes a series-parallel with the addition of theadded parallel load. It does not take much thought for circuits tobecome complex.All power sources have internal resistance (Ri) as well as the externalcircuit resistance (R1, R3 etc) and load resistance, R2 as shown in thecircuit or commonly referred to as RL. To get the most power to the load,it is important to keep the resistance of the conductors as well as theinternal resistance of the source as low a possible. In that way, the lossis minimized, so that maximum power can be transferred to the load.In another example, a car battery supplies power to the starter. The carbattery’s construction with lead plates and a strong acid solution makesit have very low internal resistance. The starter as a load also has very lowresistance as it is wound with very heavy copper wires. The conductorsthat feed the starter from the battery have to be lower resistance thaneither of the source or the load to minimize voltage drop. For example,to calculate the internal resistance of a power source, take the maximumcurrent it can produce into a dead short. Ri = V / Imax or typically is12 v / 500 A = .024 Ω for a car battery. The starter has about thatsame value of resistance to get the maximum power transferred fromthe battery. You can see why it is important to keep all connectionsclean and free of corrosion that would increase the slightest amountof resistance to cause the starter not to turn over, especially in coldtemperature winter months when battery efficiencies are lower. HTNJAN/FEB2010 | etainternational.org | 11