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Discussion Starter #1
Hello all. I haven't been posting lately. Too busy with whatever it is I'm too busy with. Anyway, I have a question concerning electricity. What is the formula for converting Voltamps into amps?
 

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Not really.

When you ask about a power supply and you are given a spec of 10 amps, that means it will put out 10 amps up to rated maximum voltage.

Volt amps is a sneaky thing, it is indeed Volts times Amps BUT at (usually) one voltage. What typically happens is that most packs rated in volt amps do NOT put out full current at max voltage.. they typically have a drop in maximum voltage at higher currents.

Get a MRC 6200 which is rated in volt amps. Measure it and see that you cannot put out max amps at max voltage.

I usually run screaming from any DC power supply rated in volt amps.

Regards, Greg
 

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That too, but the question was on voltamps. There's lots of equations out there.

The point is that people will see a 60 VA power pack, then it says it goes to 20 volts... they think that it also implies that at 20 volts you get 3 amps, not necessarily so.

That's the point.

Regards, Greg
 

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Posted By Treeman on 04/27/2008 9:02 PM

Amps X Volts = Power in Watts
While on the surface it may seem that way, but in fact it isn't, first off you have to remember you're dealing with an AC supply. Volt/Amp ratings reference the power supply and the designed maximum power flow. Where as the Watt rating references the power consuming device and as a rule of thumb the Watt rating (i.e. time-averaged power flow, RMS (root-mean-square)) equals out to about 65% of the Volt/Amp rating. Although, it really varies all over the place depending on just exactly what the electrical characteristics of the power consuming device are.
 

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By the way, it is TRUE that amps times volts is watts in DC (steady state, not pulsed).

But again, the question was about volt-amp ratings. To further cement this, next time you are in Office Depot, or a computer supply store, check out the backup power supplies... their output is 110v ac, and the largest rating on the box will be volt-amps... examine the box carefully, you may find the wattage rating, and it's different (and usually less).

My best advice is do not buy a power supply that has a volt-amp rating ONLY.

Regards, Greg
 

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Discussion Starter #8
Very good information from everyone. I think Greg hit the nail on the head when he said to look only for power packs and power supplies that are rated in amps and voltage output. It is nice though, to be able to calculate backward and see what a voltamp rating really means.
 

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The problem is that you cannot calculate back. Typically VA is calculated at some point where the product of Volts and Amps is maximum. Even if they tell you this precise "point" it tells you nothing of max voltage at max load.

It's more of a marketing ploy unfortunately, worse than the diagonal measurement of a tv screen, that you CAN calculate backwards from (assuming that you know the aspect ratio of the screen).

Regards, Greg
 

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Posted By Greg Elmassian on 04/28/2008 8:25 PM

«snip...» It's more of a marketing ploy unfortunately, worse than the diagonal measurement of a tv screen, that you CAN calculate backwards from (assuming that you know the aspect ratio of the screen). «snip...»
Isn't that the truth? Just about as bad as nailing Jello to the wall. /DesktopModules/NTForums/themes/mls/emoticons/hehe.gif:D
 

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and gasoline with 99/100ths of a cent added?

and vacuums with "peak" horsepower ratings, just like "peak power" in stereo systems.

anything for a buck...

Regards, Greg
 

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Actually, JELLO can be nailed to a wall. Just let the Jello sit around for a few weeks in the fridge. By then it is usually rubbery enough to stay put with that nail. /DesktopModules/NTForums/themes/mls/emoticons/laugh.gif
 
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