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Old 06-15-2006, 09:29 AM   #1
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Default AC to DC Power Conversion

This is a bit of an electrical engineering question - I was wondering if there is a formula for calculating how many AC amps an AC to DC power supply will draw? Usually, they are rated in DC amps, but it's hard to tell how much power you need to supply that with on the AC side.

I was thinking about it, and it seemed to me that it's definitely not a 1:1 match because of all sorts of losses in the process of converting the AC to DC and stepping it down and so forth. Also, I would imagine that the RMS of the AC power comes into play as opposed to peak voltage and so forth.

Or am I wrong?
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Old 06-15-2006, 09:40 AM   #2
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Don't forget that the ac power in is typically at 110-120 volts. If the dc output is at say 12 volts, thats a roughly 10-1 transformer ratio. Its more likely to be a 16 volt ac output from a nominal 110 volts in, so around 6 to 1. In other words, if its transformer based, for every amp you put in at 110 volts, you get around 6 amps dc out.
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Old 06-15-2006, 01:01 PM   #3
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The transformers (by this I mean the actual electronic components) that 'step-up' or 'step-down' AC voltages have ratio's as mentioned. If you drop [email protected] down to 10V on a 10:1 transformer, you will have [email protected]
If the voltage is being reduced, the current will increase.

Of course this is theoretical and all transformers are not 100% efficient. On top of this you also have the circuits to create the DC voltage from the AC.

Computer PSU's I believe are about 75% efficient.

If someone out there has a 12V PSU for R/C racing where they can give us the ratings off the back we'll see what the losses are like.

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Old 06-15-2006, 02:36 PM   #4
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Yes. That's a key thing I was missing in my original thinking. I forgot about the fact that the current would increase when the transformer steps down the voltage.

The reason I am wondering about all of this is that I have recently been seeing "Personal" generators popping up all over the place. They are small 2 stroke generators that put out 6.8A @ 120V AC, or 8.3A @ 12V DC.

I am confused though why the DC power output is so low, but ignoring that, I am trying to get a ballpark idea of what kind of AC amperage a 12v, 23A DC supply will draw.

Using the formulas above, it seems like you would expect something like:

23A * (12v / 120v) / Efficiency

Does that sound right to you guys?

Say we got 60% efficiency from the system, we would be looking at about 3.8 amps AC. You should still have room to run a few extra AC goodies on there in fact, which would be great.
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Old 06-15-2006, 02:44 PM   #5
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Conversion is always lossy because you're powering the conversion and regulation circuitry and of course the internal resistance of the wiring and components themselves.

Watts = Volts x Amps.

What is it you're trying to calculate?
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Old 06-15-2006, 02:59 PM   #6
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Here's what I'm trying to calculate:

On the 12V side:

Watts = Volts x Amps

Watts = 13.8v x 23A = 317.4w



When I connect that power supply to AC, how much loss is there in the process of generating those 317w of DC?

For example, am I actually using 400W of AC to generate 317W of DC including the loss? or is it more/less than that?
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Old 06-15-2006, 04:06 PM   #7
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I hate to take the massively simplistic approach... If you have the power supply, there should be a tag on it somewhere that will say what the max imput amp draw will be. If you don't have that, but know what manufacturer makes that supply, I'd look them up and ask. Most should have that info somewhere nearby. If all that is not possible, worst case add 5 - 10% to the required amp supply and go for it. If it were me, I'd want to add in a little more to that margin incase I need to run other appliances off the generator at the same time... like a blender, or a kegerator...
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Old 06-15-2006, 05:11 PM   #8
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Just a thought.......

I'm assuming you want to charge batteries. Will you be close to your car? If so, skip the generator and power supply and simply hook the charger to your car battery.
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Old 06-15-2006, 06:27 PM   #9
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Quote:
Originally Posted by Manning
Just a thought.......

I'm assuming you want to charge batteries. Will you be close to your car? If so, skip the generator and power supply and simply hook the charger to your car battery.
If anyone intends to use 12V from a battery, your better off buying a leisure battery, as these are better suited to our needs.

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Old 06-15-2006, 06:33 PM   #10
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Unfortunately, our parking area is nowhere near our track, so we have to look at alternate ways to power our charging equipment as well as our transponder system, PA, etc.

We're looking for the most economical way build up a system that can handle all that.
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Old 06-15-2006, 06:42 PM   #11
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Quote:
Originally Posted by BigDaddyT
Here's what I'm trying to calculate:

On the 12V side:

Watts = Volts x Amps

Watts = 13.8v x 23A = 317.4w

When I connect that power supply to AC, how much loss is there in the process of generating those 317w of DC?

For example, am I actually using 400W of AC to generate 317W of DC including the loss? or is it more/less than that?
I would go by what the power supply's wattage is listed as. If the power supply does say 400W, then at max, it would draw 3Amps.

If you want to really know what its drawing, grab a Kill A Watt by P3.
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Old 06-16-2006, 03:51 AM   #12
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Calculate the maximum wattage the PS can give out (Watt=Volt*Amp). Or find that info printed on the PS.
Add 20% to maximum watts.
Divide by 110 and you'll have maximum input amperes.
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