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ohms law?


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klctexas 
Copper - Posts: 111
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Location: Texas, United States
Posted: July 17, 2008 at 7:36 AM / IP Logged  
I am having a little trouble understanding Ohms Law. I keep coming up with incorrect answers, maybe someone could help me out? If we had an amplifier rated for 600w @ 1 ohm @ 12.5v, then it should follow that 600 * 1 = 600 the square root of 600 is like 24 something, not 12.5. What am I doing wrong?
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i am an idiot 
Platinum - Posts: 13,666
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Posted: July 17, 2008 at 10:27 AM / IP Logged  
The 12.5 is simply what the supply voltage going into the amp was at the time of rating.  It has nothing to do with the equation. 
KPierson 
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Posted: July 17, 2008 at 11:23 AM / IP Logged  

The 12.5vdc is on the input of the amp.

The 600W and 1 ohm are on the output - there is not direct correlation as it isn't a straight forward circuit.

Amplifiers aren't 100% efficient anyway, so you can't tell how much power the amp will output even if you knew the voltage in and the current in, UNLESS you know the exact efficiency of the amp.

Kevin Pierson
klctexas 
Copper - Posts: 111
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Posted: July 17, 2008 at 3:00 PM / IP Logged  
Ok, well real world situations aside, how would I know how to do this problem? Situation: the amp I was looking at said it was rated for 600w on 1 channel at 1ohm, with an input voltage of 12.5 vdc. I'm not wanting the amp or anything, I just thought to myself "I wonder what power it would put out (theoretically) with input at 14.4 or 14 vdc?" can't seem to come up with the right answer? how would I know what voltage to use? Voltage squared divided by resistance equals power, but how would I know what voltage to use?
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Fry: And then the battle is not so bad?
Soldier: Oh, right. I forgot about the battle.
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: July 17, 2008 at 9:32 PM / IP Logged  
There is no way to know for sure if the power supply voltage will affect the output or not unless the manufacturer gives you that information.  It is certainly not a straight Ohm's Law calculation.  Rated power at 12.5 volts means just that: the manufacturer designed the amplifier to operate at 12.5 volts.  With higher supply voltage it will still produce at least the rated power.  That's all you can know for sure (and then only if the manufacturer is telling you the truth.)
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klctexas 
Copper - Posts: 111
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Posted: July 18, 2008 at 8:09 AM / IP Logged  
So wth? Does Ohms Law not have a purpose or what? I made this thread so that the experts here could tell me properly how to use the laws, now Im just even more confused. How do you use Ohms Law? Could someone make up a situation with Ohms Law to demonstrate please.
Soldier: This is the worst part. The calm before the battle.
Fry: And then the battle is not so bad?
Soldier: Oh, right. I forgot about the battle.
KPierson 
Platinum - Posts: 3,527
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Posted: July 18, 2008 at 8:21 AM / IP Logged  

Ohm's law does have a purpose - to show the relationship between voltage, current, and resistance in a circuit.

The trick is you have to have all the right information to apply Ohm's law.  You don't have that information here.

Again, the voltage going in to an amp can not be used to calculate the power out of the amp, as they arn't technically the same circuit.  There is a gain involved, and there are also efficiency issues that need to be accounted for.

Kevin Pierson
DYohn 
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Joined: April 22, 2003
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Posted: July 18, 2008 at 8:24 AM / IP Logged  

Ohm's Law applies in every case in every electrical circuit.  That's why it's a LAW.  You simply can't use it all by itself to figure out the kind of issue you describe because you are talking about a power supply.  Power supplies can be designed to do all sorts of things (like regulate their output no matter what the input might be) which is why you simply can't do an Ohm's Law calculation to figure out what you want to figure out.

I recommend you do some reading (or go take a class) on basic electricity and basic electrical circuits.  A good understanding of the basics is necessary for anyone who wants to deal with automobile electrical systems, including sound systems, alarms, etc.  Google the term "Basic electricity" and also try the links on the left column of this site under "Mobile Electronics Basics."

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jeffwhiteman 
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Posted: July 18, 2008 at 9:29 AM / IP Logged  
I would also google simple transistor audio amplifier. this will explain how a transistor takes a regulated dc supply voltage to properly bias it. then an ac current (audio) can be applied for amplification. there are calculations for determining the hfe or gain of the circuit
klctexas 
Copper - Posts: 111
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Joined: March 10, 2008
Location: Texas, United States
Posted: July 21, 2008 at 10:18 AM / IP Logged  
Okay, I have read up on this stuff already, you guys are not helping me. Please just give me an example of how one would use Ohms Law to figure something out. You all claim that this law helps you determine relationships between voltage, resistance, etc, but no one has shown me. Obviously I cannot figure this out by simply reading. Its not like I'm coming into the electrical world today, and trying to understand Ohms Law right off the bat.
Soldier: This is the worst part. The calm before the battle.
Fry: And then the battle is not so bad?
Soldier: Oh, right. I forgot about the battle.
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