Understanding the math of electrical loads
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| − | = | + | =Formulas= |
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| + | ==Ohm's law== | ||
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| + | 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. | ||
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| + | 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. | ||
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| + | You can also use the chart to help you with current draw, wire sizes, and about anything else electrical while installing. | ||
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| + | *'''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. | ||
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| + | <br>[[File:Ohmslawpie.jpg|thumb|400px|left|Ohm's law pie chart]]<br style="clear:both"/> | ||
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==Using the formula== | ==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'''''). | |
| − | ==Wiring amplifiers== | + | ===Examples=== |
| − | If wiring | + | 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. |
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| + | 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. | ||
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| + | 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). | ||
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| + | 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. | ||
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| + | ==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 [[AGM (absorbed glass mat) batteries|AGM 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. | 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. | ||
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==References== | ==References== | ||
| − | *[http:// | + | *[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/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://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. | ||
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[[Category:Electrical]] | [[Category:Electrical]] | ||
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