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Thread: Help with Clamp meter and 12v measuring amperage

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    Help with Clamp meter and 12v measuring amperage

    What I'm tring to do.
    Measure the current being pulled by my 9800gtx's (two of them) but only the current bing pulled down the pci-e connectors.

    Why? the Enermax Pro82+ 525w psu I'm using for my build only has two 6+2pin and one 6pin pci-e connectors and I need 4 pci-e 6 pin.
    I'm currently feeding the last pci-e connector with a molex to pci-e adaptor, however due the the cable layout of the Pro82+ this has made neat cableing a real pain.

    I'm pritty sure that this psu can feed four 6pin pci-e, seeing as 8pin pci-e are rated for twice the wattage of the 6pin version.
    So I'm currently considering modding the cables.

    However before I start hacking a working PSU I want to check the actual working amperage.

    A clamp meter should be just what I need to check the current flowing along the wires, however I've never used one before and my multimeter skills are very rusty.

    Would something cheap like this work? http://www.maplin.co.uk/Module.aspx?ModuleNo=222032
    What do I need to look for to measure 12v DC amperage?


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    Re: Help with Clamp meter and 12v measuring amperage

    No, cheap clamp meters will only measure alternating current. (In effect the clamp inductively couple to the current carrying conductor and makes a curret transformer) You must only clamp onto one conductor (otherwise the magnetic fields cancel each other out)

    You can measure DC durrent using a Hall Effect clip on ammeter, such as one of these

    http://www.thefind.co.uk/hardware/in...nt-clamp-meter

    again, you must only clamp round one conductor. Hall effect tend to be more accurate than transformer type.

    Of those in the link, this

    http://www.tooled-up.com/Product.asp...&source=affwin

    is the cheapest.

    However you should note that these are really designewd for use on high energy ccts. The lowest ranges are of the order of 40A max, with the max range up to 300Amps or higher.


    Personally, I would be looking at something like :

    http://uk.rs-online.com/web/search/s...uctNum=3886706
    Last edited by peterb; 01-03-2010 at 11:40 PM.
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    Re: Help with Clamp meter and 12v measuring amperage

    Thanks peterb, £60ish for the cheapest I could get a new psu for that.
    Although I'm still a little confused as to what exactly I need to look for in the clamp meter . . .

    I'm scoruing fleabay for cheap options
    This looks correct http://cgi.ebay.co.uk/UEI-DL49-DIGIT...item3caaa4b817
    http://www.ueitest.com/product-dl49.html
    The DL49 is ideal for HVAC/R technicians, electrician's and plant maintenance professionals. With the ability to measure AC and DC amps, along with-featured DMM capabilities, the DL49 makes testing generators, compressor pump motors, and power converters quick and easy. It's compact size and convenient case makes it easy to carry and use.

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    Re: Help with Clamp meter and 12v measuring amperage

    The £60 won't measure DC current. You need a hall effect device. The DL 49 is designed for high current applications (specifically HVAC) where one would encounter surges and high current.

    I would also shy away from second hand test equipment - yoiu don't know how it has been treated.

    You also need to clamp ROUND the conductor - not to it, so it would not be suitable for measuring the current drawn through the PCI-E connector.

    The only way do that would be to make a test (or extender) card - with a PCI plug on one end, which would plug into the PC, and a socket on the other, into which you would plug the card. The power track would be broken, so you could insert a convential multimeter in series with the power conductor, and take a direct measurement that way. (It would be more accurate too)
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    Re: Help with Clamp meter and 12v measuring amperage

    Ok your confuseing me now.

    What I'm trying to figure out is how much current is flowing along the pci-e cable, ie from the psu into the graphics card. and that only, I'm not worried about how much is being feed via the pci-e slot on the motherboard.
    Or anything else being feed by 12v, just the pci-e cables.

    As a pci-e cable is made up of 3x 12v wires and 3x earth wires and I can get to them as the sleeving stops eairly, so what I was thinking is a clamp meter around the 12v wires only leading from the psu to the pci-e plug.
    That should, with the correct meter, give me the current going to the graphics card by each card. yes?

    I could get to the back of the individual pins in the pic-e plug with a multi meter would that be any use? or am I right in thinking all I'd do would be to create a short curcit?

    EDIT: Basically my therory is that as 8pin pci-e power connectors are suposed to be able to supply far higher current than 6pin connectors (although how that works when the 2 extra pins are only earth I don't quite understand) so in theory it should be ok rewire two 8pin + on 6pin into four 6pin connectors.
    However before I do that I want to find out exactly how much current is being pulled by each 9800gtx from each connector as that should give me a better idea of the actual amperage involved and how feasible it is.

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    Re: Help with Clamp meter and 12v measuring amperage

    Two 9800GTX's in SLI will require about 230W absolute maximum, a figure you'll rarely or even never attain, so just do it, you'll be fine!

    You have 480W available on the 12V rails but there are three of them so just be careful how you distribute the load.

    Cheers

    Paul

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    Re: Help with Clamp meter and 12v measuring amperage

    You should be able to just get another 2x 6+2 cable, surely? Mine came with 2, but mine's the 625W version.

    I wouldn't have thought it'd struggle too much with it tbh.

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    Re: Help with Clamp meter and 12v measuring amperage

    Well there is another option I've been considering, my OCZ GameXtream 700w
    Now this has a max combined 12v of 50a and 18a on 4 rails, break up is motherboard=12v1, pci-e1=12v2, pci-e2=12v3, sata&molex=12v4
    However I know that while the GameXtream can supply 700w continious and the voltages are stable, however they are know for very bad ripple over 600w load. I should aproach that so I should be safe there.
    The bigger worry is on two pci-e 6pin connectors, to break them into four pci-e 6pin might be too risky. esp without knowing how much is pulled from the pci-e leads.

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    Re: Help with Clamp meter and 12v measuring amperage

    Quote Originally Posted by matty-hodgson View Post
    You should be able to just get another 2x 6+2 cable, surely? Mine came with 2, but mine's the 625W version.

    I wouldn't have thought it'd struggle too much with it tbh.
    You're missing the point I think.
    I have it currently powering my two 9800gtx in sli on a p55 motherboard with a i5 750 cpu.
    That's not the problem, the problem is the annoying way enermax bundled the cables, making it very hard to cable this thing neatly.

    opps I forgot the link in my first post
    http://forums.hexus.net/chassis-syst...ml#post1883320

    what I currently have is,
    Two 6+2 pci-e
    One 6pin pci-e
    Two molex feeding One 6pin pci-e adaptor.

    What I'm thinking of doing is rewireing the current Three pci-e connectors into Four 6pin pci-e connectors.

    The main reason I want to read the current is give give me a better idea on how to split and cross connect the cables up to make 4.

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    Re: Help with Clamp meter and 12v measuring amperage

    The cables are effectively in parallel, so measuring the current flowing through all three cables is the way to go. A hall effect clip on ammeter clipped round either the three earth or three supply cables will do the job (better to use the supply cables as there ay be other earth currents flowing back) - but a chaep AC only ammeter will not do the job. Alternatively, you can put a conventional ammeter in series with the power cables (get an extender cable and cut it would be one way) Remember that at 12 volts, a power requirement of 120 watts will be ten amps, so you will need an ammeter with a suitably high curret rating. If you are talking about a power requirement of 240 watts, then that will equate to 20 amps.

    However that is only 1/20th of the maximum range f the clip on ammeter you referred to in the earlier post (the DL49) and while that would probably work, you need to be aware of any limitations on the accuracy. It depends on how accurate you need the figures. If it is to the nearest amp - no problem - nearest 100mA - probaby OK, but I haven't studied the specs closely - nut nearest 100mA is probaby as close as you would get (with that device - and regardless of what the display says - don't confuse precision with accuracy!! ).
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