Watts=Amps?

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Forum Name: Relays
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URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=2622
Printed Date: June 15, 2024 at 12:00 PM

Topic: Watts=Amps?

Posted By: day952
Subject: Watts=Amps?
Date Posted: August 08, 2002 at 1:25 PM

How do I figure out how many watts a device draws based upon the wattage it puts out?

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day952

Replies:

Posted By: day952
Date Posted: August 08, 2002 at 1:26 PM

Correction, how many amps a device draws based on the wattage it puts out?

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day952

Posted By: HotRod53F100
Date Posted: August 08, 2002 at 8:14 PM

Hey Day952.....Power (watts) = I (current) X E (voltage) or

I (current) = (P Watts) divided by E (voltage)

example I (current) = 100W / 12V or 8.333 amps

Hope this helps......

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HotRod

Posted By: 2000vsm
Date Posted: August 08, 2002 at 11:04 PM

can you explain why the fuses in cars are so high? like for a small thing like a fuel pump which i'm sure does not run at 100watts... (the wire is really thin and it's like probably 5-10 watts most), the fuse for this is like 15amp?

Posted By: HotRod53F100
Date Posted: August 09, 2002 at 3:35 PM

Well, there might be a couple of things to concider. 1st when you size a fuse, the rule is 2X current draw + 10%. Second, anytime you are using an inductive load such as a motor, there is what is called an inrush current. This will cause a current that will momentarily be much higher than the normal running current draw. There may also be other things on the circuit with the pump.

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HotRod

Posted By: 2000vsm
Date Posted: August 12, 2002 at 8:34 AM

so how do i know which diode to use in a circuit, do i have to cut the wire and use an amp meter to measure it?

Posted By: sparkyssb
Date Posted: August 12, 2002 at 1:09 PM

ah..just use a big diode. 6 amps should probably be good. 6 Amps - 50PIV (Radio Shack 276-1661) ....anybody disagree?

Posted By: omega
Date Posted: August 13, 2002 at 10:43 PM

Quote: Originally posted by day952 on August 08, 2002

Correction, how many amps a device draws based on the wattage it puts out?

Keep in mind that while P=I*V, the wattage something PUTS OUT may be less than the wattage (and therefore current) it DRAWS IN. What is missing from the equation in that case is EFFICIENCY...

Simply using P=IV to determine current draw from wattage OUTPUT assumes 100% efficiency which is not possible in such a case as an amplifier. Most class A/B amps are between 50-60% efficient, while a Class D will be about 70%+ efficient at most power levels. (At full power output Class D amps drop in efficiency to around 60% or so much like an A/B amp, just a little better in most cases).

For example, if your amp puts out 120W and is only 50% efficient it means its power supply draws 240W to output that power. In this case, at 12V, the current DRAW would be 20A, not 10A as one would mistakedly calculate without considering efficiency.