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Thread: Question about DC-DC vs ATX PSU Efficiency - First Post!

  1. #1
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    Question about DC-DC vs ATX PSU Efficiency - First Post!

    Ok, here goesÖ as this is my first post, I would like to first introduce myself and my project, and Ďgetting to the pointí has never been my forte, so bear with me.
    First of all my name is Ashley, I'm a guy (if you have problems with a guy being named Ashley then... you have problems), Iím 22, I drive a 2000 Trans-Am WS6 6-speed (yay!) and Iíve wanted for the past 5 years to put a computer in my car, but Iím only now getting to the point of having the money, skill and parts.
    I have a long history as a bit of a NERD, Iíve been programming since age 11 (C++ since 15) and built quite a few computers, networks, etc, so needless to say, I know a great deal about computers. I have a fair knowledge of electronics and limited experience with micro-controllers, but from what Iíve seen in the forum, nothing at all to brag about. If you really need to know anything more or just have some time to kill visit my car-club at http://www.nfspmotorsports.com sorry if it still has my recently totaled 94 Trans-Am listed as my car, Iíll fix that soon enough.
    The parts Iíve scraped together for my car project are an Asus A7N micro ATX mobo from an old Dell, with an Athlon 2800, 1G ram, an old Albatron nVidia 5700 256Mb AGP video card, and a 10G 2.5Ē 4600RPM laptop HD. Parts I still have to get include the Panasonic slot-load dvd burner and the 8Ē widescreen Lilliput touchscreen from mp3car. I havenít picked out a sound card yet (onboard is broken and disabled and sucks anywayÖ) a 500G external HD that Iíll probably run off a separate power supply, and most important of allÖ a Power Supply.
    SoÖ all that being said, hereís my question. What power supply do I get? Iíve considered the inverter idea, but that seems problematic, and on top of that, itís a pain to setup automatic turn on, turn off, battery monitoring, stay alive during crank, etc. So right now Iím leaning toward DC-DC. Iím thinking either an A2-MTX (or maybe 2 if need be?) or the DSATX or the OPUS 250 (though Iím trying not to break the bank here)
    The testing Iíve done so far is this: I have the computer running right now off the OEM 250W AC PSU. The only difference from the way the system is listed above is that I donít have the parts under the Ďstill to getí list (duh) but Iím substituting a regular tray-load DVD burner. Iíve done some testing of how much wattage the computer actually pulls by checking the AC load from the wall with an AC ĎClampí style Ammeter, and checking the voltage at that outlet VxA=W . Under these test conditions I get a standard idle at about 1.2A @ 120V so 144W. At a full load I canít get it to go over 172W though it will hit 166 during games or other heavy CPU loads. So basically my question is will an A2-MTX be enough? I know itís only rated for 160W but I donít know if itís under rated. I also have to consider that Iím measuring how much the AC PSU is pulling from the wall, not how much the components are actually pulling from the PSU. Just as an inverter is not 100% efficient so to an AC PSU is not. Iím just not sure how much so. If itís 95% Iím still cutting it close to 160 at full load but if itís more like 80% then I never go over 135W peek. I donít want to spend more than I need to, but I donít want to have a PSU that wonít cut it. I would also really like to survive cranking, or at least only have to go to stand by.
    So thatís my story, thanks for reading it, thanks for any help you can give, and sorry I took up 4 hours of your time while you read all this crap!

  2. #2
    Neither darque nor pervert DarquePervert's Avatar
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    Quote Originally Posted by nfsp_trans_am View Post
    What power supply do I get?
    Read this: FAQ: Will <insert PSU model> Power Supply work with my system?
    Have you looked in the FAQ yet?
    How about the Wiki?



    Under normal circumstances, a signature would go here.

  3. #3
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    Thanks for the link. I had already read it, unfortunately my processor wasn't listed there, it's a Socket A not a 754 but... thanks to some searching this morning I found this http://www.amd.com/us-en/assets/cont..._Rev04_ENG.pdf so according to it the processor draws 68.3 MAX so we'll say 70? right? So the M2-ATX should leave me at least 16W left on the 12v. Then M2-ATX it is. If need be I could always hook up another one for Hard-Drives, DVD, USB, etc. Though It'll be a couple more weeks till I put in my order, so if anyone has more input, it's always appreciated. Hopefully the hard-drive manufacturers will have some similar charts. Also, can the Lilliput handle the dirty power of the car?
    Ashley - http://www.nfspmotorsports.com
    Check out my car club.

  4. #4
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    You might want to research running more than one M2 at a time. There's been a thread about it going on for a while. It's not as simple as just chaining them together.

    You will also want to read up on the lilliput. It needs to have regulated power. They do come with a power filter on the cig lighter adapter, but I'd recomend powering it off of the PSU.
    Jan Bennett
    FS: VW MKIV Bezel for 8" Lilliput - 95% Finished

    Please post on the forums! Chances are, someone else has or will have the same questions as you!

  5. #5
    Low Bitrate MibMib's Avatar
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    Be careful -not all DELL mobos compatible with ATX standard- they have the same 20 pin socket , but pinouts is different!!! Check pinout of your mobo 20 pin socket - maybe will needs in special adapter.
    Also look at this thread Amd Athlon Xp 2200 i think it could help you to understand difference in powering pentium and amd systems.
    400Watt CarPC power supply for powerfull systems- thaks for Beta testers- it was perfect.

    Remember - AMD Athlon XP (socket A) powers from 5v rail in most times !

  6. #6
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    It's a standard ATX, I checked that, but thanks for the warning! I misspoke earlier, the mobo came from an HP, not a Dell. Unfortunately there's not a 4 pin plug and it's a Socket A AthlonXP so yeah, I'm guessing that it's running off the 5v rail. So basically I'm stuck having to buy a much more expensive power supply. The only bright side is that I should have more than enough 12v for the monitor, 3.5 HD, DVD Drive etc. Though I have almost nothing left on the 5v for USB unless I opt for the 320W OPUS. Then I'm spending $130 more! At that rate I may as well scrap the entire DC-DC Idea and fight with the inverter or (gasp) use my old P4 1.6 Mobo (ewww) sitting in my closet, or just buy an totally different Mobo and CPU altogether! This is very frustrating
    Ashley - http://www.nfspmotorsports.com
    Check out my car club.

  7. #7
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    Ok, forget the p4, just found out how power hungry they are. Here's what I'm looking at now: The OPUS 320 is $130 more than the M2, for $140 I could buy a Socket AM2 mobo with integrated everything, use one of the new 45W 3800+ Athlon 64bit CPUs rather than the 2800 AthlonXP 32 bit and have a Gig of DDR2 667 rather than the DDR 333. Then I should be able to fit well under the 160W max of an M2 leaving room to spare for my Monitor, an xtra HD and maybe some USB stuff. PLUS I'll still have the other guts to build a Media Server for my house. Ok, so now I'm leaning in this direction. Thoughts?
    Ashley - http://www.nfspmotorsports.com
    Check out my car club.

  8. #8
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    It's not just about total wattage, you need to check wattage on each individual rail.

    5v, 12v, and 3.3v.

    Each rail has a max wattage, you need to make sure you're not going to be pushing it.
    Jan Bennett
    FS: VW MKIV Bezel for 8" Lilliput - 95% Finished

    Please post on the forums! Chances are, someone else has or will have the same questions as you!

  9. #9
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    Quote Originally Posted by RedGTiVR6 View Post
    It's not just about total wattage, you need to check wattage on each individual rail.

    5v, 12v, and 3.3v.

    Each rail has a max wattage, you need to make sure you're not going to be pushing it.
    Right. What MibMib brought to my attention though is that the motherboard I'm using, for that matter all the AMD Socket A Motherboards Iíve found, draw from the 5v rail for the CPU, not the 12v like most others. Many of the DC-DC PSUs have the most wattage on the 12v rail (most CPUs draw from there.) I was so frustrated because to get a DC-DC PSU with enough on the 5v to keep up with the CPU would have to be an OPUS and even then it would be pushing it on the 5v still (but there would be almost nothing on the 12v, so it would be kindof a waste). If I go with the Socket AM2 board I mentioned in my last post I would be moving to a faster CPU that drew 45W instead of the 68W my Socket A CPU does, and it would draw those 45W off the 12v rail, not the 5v, so everything would be far more balanced.
    Ashley - http://www.nfspmotorsports.com
    Check out my car club.

  10. #10
    Fusion Brain Creator 2k1Toaster's Avatar
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    I'll give you the same advice I give others... ebay!

    The thing is, desktop and mobile chips (with the exceptions being the newest generations as a rule) are the same price. So a P4-Desktop and a P4-Mobile are the same price on eBay. Same with AMD-Desktop and AMD-Mobile chips. So sell your desktop chip on eBay and buy a mobile chip. Sell your desktop mobo on eBay and buy one that will take the mobile chip (usually same price if not a tad higher for mobile so you spend a bit here depending on form factor).

    Then you can get a DC-DC and do it correctly.

    Also I saw in your first post you ask if the M2-ATX is "underratted". Well just to clarify, no it is most definately not. If anything overrated. I would not run more than 80% load on the M2. I use it and it works flawlessly, but with a mobile chip.

    Also not sure what the "P4, yuck" thing was about, but a true nerd will not care of imaginary boundaries of Intel or AMD or even ati or nvidia. I use both, I own both, they both do what they are meant to do. A Celeron 300a can be a CarPC (look at user TruckinMP3's sig) so there is more power available on either chip manufacturer's products. So your P4 will more than suffice, and since it draws from the 12v rail you have more options there but I would still reccommend the trade in to something in the Centrino line or the AMD equivalent...
    Fusion Brain Version 6 Released!
    1.9in x 2.9in -- 47mm x 73mm
    30 Digital Outputs -- Directly drive a relay
    15 Analogue Inputs -- Read sensors like temperature, light, distance, acceleration, and more
    Buy now in the MP3Car.com Store

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