How to get 5 Volts 3 Amps from a PC Power Supply?

Question

Hello,
I'd like to make my own lab power supply with the precise output of 5V for 3 Amps.
I got ahold of an old PC Power Supply and managed to manually turn it on by shorting the green wire to a COM black wire.
When I put my meter to the 5V cable, it indicates an amperage of only 1 Amp. Yet on the power supply it indicates a potential amperage of 10 Amps for 5V.
My question is how I can I get the 3 Amp output.
I intend to use it for a perchlorate production cell and connect the wires to anode and cathode.
Sorry for being so ignorant about that :-)

Answer 1

Here are plans to convert an old ATX power supply to a 'lab' power supply:
http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

Even though it is capable of 5V @ 10A you will only draw the current required by your production cell. But if you reallly must have exactly 3Amps, then here is a current regulator circuit using a constant current diode with a booster circuit.
http://www.centralsemi.com/pdf/Boosting%20the%20CLD.pdf

You will have to modify it for 3Amps, or build 3 of them (in parallel), all using the 5V ATX modified power supply as a source.

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Answer 2

As an electrical engineer, your question puzzled me. According to your description, your source provides 3 - 12 volts, but is incapable of supplying 5 amps. But, of course, that 5 amps must come from somewhere. So you want to feed a self-constructed device with your original 3 - 12 volts, but supplies the 5 amps from another source. If that source is your PC's power supply, why not simply use one of the extra output connectors for both your voltage and current requirements? Otherwise, you'd probably be better, and cheaper, off to simply build (or buy) an AC-input variable power supply rated for up to 5 amps. The only other scenario I could think of, where you would need portability, is such a rare situation that you'd pay more for the necessary parts for the convenience.

Answer 3

I = V/R This means the current equals voltage divided by the resistance.
To get 3 Amps you need a total resistance of 5/3 Ohms. So, if you cell's resistance is less than that, you add another resistor in series so that the total resistance is 5/3 Ohms. If your cell's resistance is >5/3 Ohms, then you connect a resistor in parallel to end up with a total resistance of 5/3 Ohms.
Either ask an electrical engineer, or read any introductory electrical circuit theory book if you don't understand how to combine resistors in series and in parallel.
Always use the Ampere meter in series with the resistor, never by itself.

Answer 4

You may construct (or buy) a power supply with 5 V output and able to give 5 A or more. But the real current (Amps) depends not only on the power supply, as Mr. Ohm suggested long ago. Never "measure" the current of a power supply.
http://www.grc.nasa.gov/WWW/K-12/Sample_Projects/Ohms_Law/ohmslaw.html
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Answer 5

You need to limit the current. That means that you need a special constant current circuit. PC power supplies on their own just supply as much current as they can.

Do not measure current the way that you did, by connecting an ammeter directly across the power supply. That is a *really* bad mistake. If you do not understand Ohm's law you need to get an electronic engineer to set up your electrical equipment. You are lucky that the PSU's current limiter kicked in when it detected a fault condition.