E=volts but volts as in what, DC or AC (battery volts, or the volts you get when you meter speaker terminals). It seems to me that it would be AC volts.

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- 11-14-2006 #1
## Question about ohms law?

E=volts but volts as in what, DC or AC (battery volts, or the volts you get when you meter speaker terminals). It seems to me that it would be AC volts.

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- 11-14-2006 #2
## Re: Question about ohms law?

Ohms Law equations are valid for DC or AC.

- 11-14-2006 #3
## Re: Question about ohms law?

That's confusing as hell. What if you want to find say wattage....volts x amps.....what voltage would I use?? The only time I seem to come back to a matching anwser is when using the AC volts.

**Alarm:**Viper 5902

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- 11-14-2006 #4
## Re: Question about ohms law?

AC is alternating current so that's to speakers, DC is direct current so that is battery voltage, hope that helps ya out

- 11-14-2006 #5

- 11-14-2006 #6
## Re: Question about ohms law?

So when do I know when to use DC and AC to figure out what I am looking for? Currently I am trying to figure out how many amperes one of my amps draw. I could do...I=P/E. Which would be say 1800/14.4(DC) which means 125 amps of draw (seems a bit high). Or I could do 1800/60(AC) which means 30 amps of draw (seems more realistic.

It seems to me AC is the sopposed voltage because randomly throwing in DC messes up the whole chart and every equation. Ex. if you use the equation I = Square Root of (P / R) with the same info, the square root of 1800/2 = 30 amperes. I.E. AC volts matches, DC does not.

Now, furthermore of an example, I want to find what Ohms I am running at. I will use the equation R = E 2 / P.

With AC volts, 60squared/1800 = 2ohms. With DC volts, 14.4 squared/1800= a VERY low impendence.

Also, with the equation E = Square Root of (P x R), the square root of 1800x2 = 60, not 14.4. Now with this equation, I would have to be running 103 watts of power at 2 ohms to get a voltage of 14.4.

What am I missing?Last edited by the727kid; 11-14-2006 at 11:03 PM.

**Alarm:**Viper 5902

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- 11-14-2006 #7
## Re: Question about ohms law?

Watts = Volts * Amps

1800 = 14.4 * X

1800/14.4 = X

X= 125A

Plug it back in, works just fine. You are missing efficiency of the amp though

- 11-14-2006 #8
## Re: Question about ohms law?

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- 11-14-2006 #9
## Re: Question about ohms law?

Where are you getting 60 (AC)??? Your battery doesn't produce AC power. Your alternator does, but it's converted to DC before it's used anywhere in the vehicle. The unrealistic number is probably the 1800, but if that's the right wattage number then 125 amperes of current wouldn't be out of line.

It seems to me AC is the sopposed voltage because randomly throwing in DC messes up the whole chart and every equation. Ex. if you use the equation I = Square Root of (P / R) with the same info, the square root of 1800/2 = 30 amperes. I.E. AC volts matches, DC does not.

Now, furthermore of an example, I want to find what Ohms I am running at. I will use the equation R = E 2 / P.

With AC volts, 60squared/1800 = 2ohms. With DC volts, 14.4 squared/1800= a VERY low impendence.

Also, with the equation E = Square Root of (P x R), the square root of 1800x2 = 60, not 14.4. Now with this equation, I would have to be running 103 watts of power at 2 ohms to get a voltage of 14.4.

What am I missing?

First example: you want to figure out amperage through the power wire, using a wattage rating of 1800 and a voltage of 14.4VDC. If these numbers are correct, then using I=P/E, 125 amps is the correct amperage. (Leave aside that you're assuming output power is equal to input power, which isn't true because amplifiers aren't 100% efficient). Your 60VAC reading at the speaker terminals isn't relevant to the current draw at the main power input. On the other hand, let's use the same equation to find the amperage at the OUTPUT. I=1800/60 so you're running 30 amps through the speaker wires. Same equation, both answers are correct though one is for DC and one is for AC. You're just assuming that the DC answer is wrong because the 125 amps answer doesn't "feel" right to you, but why do you think amplifiers require much larger power wire than speaker wire?

Second example, calculating impedance from power and voltage. Of course the DC calculation gives you a crazy answer, because 14.4VDC isn't present at the speaker terminal. Garbage in, garbage out: if you plug in the wrong number, you'll get the wrong answer. But guess what: if your amp was producing 1800 watts, and only 14.4 volts were present at the speaker terminals, you can bet your speakers would have to be a VERY low impedance.

Bottom line, in every example you're mixing up power line voltage with speaker terminal voltage, then blaming the DC answer because it doesn't seem right. For power line calculations, use 14.4VDC. For speaker output calculations, use 60VAC. In both cases, your answers will be correct if your other numbers are accurate.

- 11-14-2006 #10
## Re: Question about ohms law?

"For power line calculations, use 14.4VDC. For speaker output calculations, use 60VAC."

Thanks, I get it now. That's all I needed to hear to understand it lol. I see what I was doing wrong now. Appreciate it.

**Alarm:**Viper 5902

**HU:**Stock

**Front Stage:**

Tweeters: Image Dynamics CD1-Pro Mini Horns

Mids: Sundown Neo Pro 8"

Subs: (2) 15" Dayton HF ran IB

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Processor: JBL MS-8

- 11-16-2006 #11

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