Editing Understanding the math of electrical loads

=Formulas= ==Ohm's law== Ohm's Law defines the relationships between power (P), voltage (E), current (I), and resistance (R). One ohm is the resistance value through which one volt will maintain a current of one ampere. To do any sort of work on stereo systems, automotive wiring or related tasks, Ohms law is going to be used frequently. Learning and understanding it is important. The pie chart below makes this easy- keep a copy of it handy. You can also use the chart to help you with current draw, wire sizes, and about anything else electrical while installing. *'''I-''' Current is what flows on a wire or conductor like water flowing down a river. Current flows from negative to positive on the surface of a conductor. Current is measured in (A) amperes or amps. </p> *'''E-''' Voltage is the difference in electrical potential between two points in a circuit. It's the push or pressure behind current flow through a circuit, and is measured in (V) volts. *'''R-''' Resistance determines how much current will flow through a component. Resistors are used to control voltage and current levels. A very high resistance allows a small amount of current to flow. A very low resistance allows a large amount of current to flow. Resistance is measured in ohms. *'''P-''' Power is the amount of current times the voltage level at a given point measured in wattage or watts. <br>[[File:Ohmslawpie.jpg|thumb|400px|left|Ohm's law pie chart]]<br style="clear:both"/> ==Using the formula== To remember the basic formula: '''P = I X E''', just think of a tasty pie. '''P''' ('''''power''''' in watts) = '''I''' ('''''current''''' in amps) X '''E''' (electromotive force, or '''''volts'''''). ===Examples=== As an example, to calculate how many watts an amplifier is supplying to a speaker, you measure the voltage at the speaker and square that voltage, then divide the result by the impedance of the speaker. So if you measured 10 volts at the speaker and you have a 4 ohm speaker, the answer would be 25 watts. So, how much current at 12 volts does a pair of 55 watt headlights consume? Algebra lets you turn the formula around to find the unknown. 110 watts = I X 12 volts, whip it around you get 110 watts divided by 12 volts = 9.17 amps. If you have a battery rated at 65 amp/hour you can see the time at the above load of 9.17 amps to complete battery exhaustion would be about 7 hours (not counting the Puekert effect, which in simple terms means the larger the current draw on a battery is, the lower the a/hr of the battery). Now comes some fun calculations, hp and watts, both units of work. Let's say an aftermarket, high performance starter motor manufacturer claims he has a 4 hp starter. The amp draw of this starter can be calculated as: 1 hp = 746 watts, so 4 hp = 2984 watts divided by 10 volts (your 12 volt battery under the load of the starter) = 298.4 amps. ==Wiring audio amplifiers== If wiring a 2 kilowatt (2000 w) stereo amplifier, listen up. First and most important, do not try and wire this any distance or use the vehicle's starting battery. Odyssey [http://www.crankshaftcoalition.com/wiki/Auto_battery#Absorbed_Glass_Mat_.28AGM.29_batteries AGM (absorbed glass mat) batteries] are your best bet for monster amplifiers due to their dual personality of being both a deep cycle battery and the highest cranking amp battery per pound of battery weight you can buy. Mount the AGM battery right with the audio amplifier so that your leads are heavy and very short in total length. Now you can run that easy-to-work-with 12 ga wiring from your alternator or starting battery back to the AGM for charging. This is why the above method is the ONLY acceptable method, and yes- it involves math again. Your stereo amp, if in fact is a ''true'' 2000 watt unit, is possibly a Class A amp, so efficiency is about 50%. So, 4000 watts input, again divided by 10 volts (remember the voltage drop under heavy loads) = 400 amps. Do you think 20 feet of 8 ga is going to carry this? Nope. Now, a little 15 lb Odyssey battery will put out 680 amps for 5 seconds. If you want to blast your tunes for more than 5 seconds to be truly obnoxious, don't worry, you can. That current draw is at peak output and that doesn't happen often or for only a few milliseconds at a time. With the engine running and sending 15-20 amps back to the AGM battery, you're good to go. Another misconception is large amplifiers burn out your tweeters, but believe it or not it is small amps that burn out the tweeter. When an underpowered amp is turned up so loud that the amp is clipping the wave form, you burn out the tweeter. Clipping is a distortion that muddies the sound and is caused by the amp running out of output voltage swing and clipping the top and bottom of the waveform. Looking at clipping on a scope, instead of seeing a nice completed sine wave the tops and bottoms are a horizontal line which is DC, no longer a frequency. What keeps a tweeter healthy is the fact that its voice coil is moving in and out 8000 to 20000 times a second and that motion keeps the voice coil from burning up due to the moving air keeping the voice coil in an acceptable temperature range. So when the underpowered amp clips at high volume settings, the tweeter is seeing DC so a lot of heat energy is made that needs to be dissipated but because it is DC, the little sucker isn't moving, so it soon overheats and is smoked. Radios in new cars no longer will clip because what you think is the volume control potentiometer is in reality just a shaft with a slotted light chopper wheel and as you turn up the volume, each time the light from a led passes through a slot to a light sensor on the other side of the wheel, it "requests", if you will, the ramping up of volume- but now the audio system can employ control circuitry that stops the volume increase short of clipping, thus saving the speakers and warranty claims. ==Jump starting== Back to you adults who don't care about the need to blast tunes. Here's a tip on jump starting. Assuming you have a good starting battery that you ran flat for some reason, do your jump this way. You all know how to hook the cables up but after that I see a common mistake and that is cranking the engine right away and when it doesn't spin fast enough you yell out to friend to speed up his engine. Under certain conditions you could damage your friend's charging system. When getting a jump, just let it idle while you chew the fat for about 10 minutes. By doing this, when you go to start your car, better than 90% of the load will now come off your battery. I don't mind anyone copy and pasting this article but we all should get in the habit of, if using original material like this for this forum, at least credit the forum it came from or better yet look up at your URL address window and link back to it here. That covers the legal and directs traffic here and possible new members. ==References== *[http://www.archtoolbox.com/representation/abbreviations/99-electabbrev.html Electrical abbreviations] from archtoolbox. *[http://www.crutchfield.com/S-5MVuWF3SPfx/learn/learningcenter/car/amplifiers_glossary.html Stereo glossary] from Crutchfield. *[http://www.crutchfield.com/learn/learningcenter/car/amplifiers/shopping_guide.html Car amplifier shopping guide] from Crutchfield. *[http://tbuckets.lefora.com/2011/02/04/understanding-the-math-of-electrical-loads/#post2 Understanding the math of electrical loads] from Tbuckets forum. *[http://www.wikihow.com/Jump-Start-a-Car How to jump start a car], wikiHow article. [[Category:Electrical]] [[Category:Ignition]]

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