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  • Calculating Amperage

    Hello,

    I need a little help... I'm testing out my ATX system to see how much wattage / amperage it is drawing so I can make my final decision on which power supply to buy. I believe I am doing this correctly but the numbers seem a little weird to me. I took an old ATX connector and stripped off two small sections about 2 inches apart on each of the wires which i wanted to measure current on (-3.3vdc, +3.3vdc, +5vdc, -5vdc, -12vdc, and +12vdc).

    Now on to my mesurements I started my computer and started up a dvd to try to get a pretty decent system load going. I then proceded with my multimeter to test each of the wires by switching it to the 10A symbol for testing amperage/current and cliped the alegator clips on each of the stripped parts of the wires individually as follows

    powersupply-----------Y -----------Y------------computer
    +clip -clip

    Now comes the fishy part....

    +3.3 = .07 A = .23W
    -3.3 = 0 A = .00W
    +5 = .05 A = .25W
    -5 = .00W
    +12 = .01 A = .12W
    -12 = .00 A = .00W

    Now i know that this is not correct but can someone help me correct my problem.

    Thanks
    Tyler
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  • #2
    Hm, seems like you hooked it up right. Remeber it shoudl always be hooked yup in series, not in parallel.

    For an atx system that isn't mini-itx, your best bet is either a Opus 150W or one of Mastero's 250w power supplies (also on this site too.)
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    • #3
      Originally posted by marshallh
      Hm, seems like you hooked it up right. Remeber it shoudl always be hooked yup in series, not in parallel.

      For an atx system that isn't mini-itx, your best bet is either a Opus 150W or one of Mastero's 250w power supplies (also on this site too.)
      nope, he hooked it up wrong.

      the multimeter has to be in the circuit, not alongside it
      so you have to cut the wire and connect each othe the probes to each of the wires

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      • #4
        So I guess you need to hook it up liek this:

        +VDC----------|+| ammeter leads |-|----------computer

        you need the cut the wire and put the leads in, connecting the circuit.
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        • #5
          I should not have to cut the wire to test this should I? I mean cut the wire between the two ends of the multimeter
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          • #6
            well, it might be the only way to do it. But it would be better to simply post your compoents and let us have a rough guess at how much your PC would draw, rather than going through the complexities of measureing amperage...
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            • #7
              Sorry i was posting at the same time ... if i don't cut the wire i was just reading that i could just multiply the number of amps by 2 right because there are two paths for the electricity to follow.

              Is this correct
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              • #8
                No, that wouldn't work because the wattage readings would still be way off. You should at least be in the neighborhood of 30-50 watts.
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                • #9
                  Ok that seemed to give me some better numbers... one more question to caculate the total being pulled off the rails do i add up each of the +5v wires for instance there are 4 +5vdc wires which are red in a ATX connector would i multiply the amperage by 4 to get the total amperage or is calculating one wire sufficient?

                  My new numbers are still weird?

                  Orange 3.3V = .28amp = .92w
                  Red +5.0V = .28amp = 1.4w
                  Yellow +12V = .25amp = 3w
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                  • #10
                    By the way im probably going with opus however, i would still like to calculate it myself
                    Thanks
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                    • #11
                      As far as I know, just one wire is sufficient, I'm pretty sure they're hooked up in the same place anyway, they're just for redundancy. If anybody knows more, correct me.
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                      • #12
                        Originally posted by marshallh
                        As far as I know, just one wire is sufficient, I'm pretty sure they're hooked up in the same place anyway, they're just for redundancy. If anybody knows more, correct me.


                        gotta cut them all, and tie them all together.

                        multiplying by 2 or 4 will not work because the multimeter has a higher resistance than the piece of wire that is completing the second path - that is, more current will flow throught he second or third or fourth wire than the multimeter

                        either cut them all and measure or just tell us what you have


                        alternatively you could measure current going into the PSU itself.

                        strangely enough, my computer under no load only draws about 20 watts, and it's a P4 1.8ghz

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                        • #13
                          6.5A +5.5v
                          4.06A +3.3v
                          .25A +12V

                          Thanks for all your help .. i've got it figured out now.

                          My system specs are
                          Mini-ATX ECS Motherboard
                          2100+ Sempron CPU
                          256MB DDR 2100 Ram
                          2.5" 40GB Laptop Drive
                          Slim DVD-ROM TECH USB adapted
                          USB Sound Blaster MP3+
                          USB GPS Ritiklan
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                          • #14
                            well clearly the CPU is powered off the 5 volt line.

                            looks like you're right at about 50 watts

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                            • #15
                              Ya thats what i figured and it makes sense since my MB does not have a P4 style connector on it.
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