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Thread: Modify ATX PSU for incar use - possible?

  1. #1
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    Modify ATX PSU for incar use - possible?

    Hi everyone
    My biggest limiting factor when it comes to setting up a computer in my car is the power supply. I dont like the idea of spending AU$145

    http://www.auspcmarket.com.au/show_p...ategory_id]=96

    on a DC-DC power supply of only 80W when I have some perfectly god ATX power supplies just lying around and reasonable electronics knowlege.

    So...

    I was thinking of just taking a regular ATX power supply, ripping out (or bypassing) the primary transformer and rectifying bridge and just feed the 12V (approx) straight into the unregulated input of the power supply.

    Surely people have thought of this before - so what am I missing that would stop this from working ?

    I realise I wont have any auto shutdown controls etc, but all I want is a very basic power supply (only for a celeron 433).

    Comments / flames etc.

  2. #2
    FLAC evandude's Avatar
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    umm, the voltage coming out of the rectifying bridge is going to be about the RMS voltage of AC, which is around 170 volts. if you feed 12 volts into a power supply that is expecting 170 volts, it is not going to work.

    think about it, that'd be like putting 1 volt into a DC-DC that is designed to have a 12 volt input, and expecting it to work. Sure, it would be possible with some form of design, but a standard power supply is NOT going to be built to accept some insanely large input range, since it would never have to in normal use, and it would be far more expensive to make it that versatile.

    as for bypassing the main transformer... well sure, it'd work if there were a transformer with a 12v-ish output that was running the PSU. BUT the whole point of a SWITCHING power supply is that it doesn't use a giant transformer, rather it switches down the high rectified voltage directly. think about it, a 500 watt power supply would need a hella big transformer if that were the case.

    don't mean to dash your hopes here, but given the number of people with electronics skills on this board, I'm sure someone would have done it by now if it were possible.
    But don't take it from me! here's a quote from a real, live newbie:
    Quote Originally Posted by Viscouse
    I am learning buttloads just by searching on this forum. I've learned 2 big things so far: 1-it's been done before, and 2-if it hasn't, there is a way to do it.
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    You could just search... there are a few ideas floating around. I have some links that I am too lazy to find right now, but basically by the time you get it all done, even assuming your time is $1/hour, you would be better off buying one yourself. If anything, creating a DC-DC from scratch would be easier.
    I have found you an argument; I am not obliged to find you an understanding.

  4. #4
    FLAC evandude's Avatar
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    Quote Originally Posted by rushnrockt
    If anything, creating a DC-DC from scratch would be easier.
    given the amount of modifications you'd have to make to convert a 120VAC power supply to run on 12v, you pretty much WOULD be making it from scratch anyway.

    If you are really cheap, just buy a crappy inverter off ebay for like 40-50 bucks. It isn't efficient with regards to either power, size, or electrical or audible noise, but it's the only real route to take if you need a lot of power but don't want to spend a lot of money.

    "you gotta pay to play"
    But don't take it from me! here's a quote from a real, live newbie:
    Quote Originally Posted by Viscouse
    I am learning buttloads just by searching on this forum. I've learned 2 big things so far: 1-it's been done before, and 2-if it hasn't, there is a way to do it.
    eegeek.net

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    yeah well I was considering creating a DC-DC from scratch, just getting the inductors of appropriate specs would be a bit of a niggle. So I thought I could just rip them out of a dead ATX power supply, then I thought, well why not just use an ATX power supply.

    Bear in mind that this is a 240V input PSU, what would be the approx voltage on the secondary side of the input transformer.
    Actually I just reread your post again evandude - that does make sense that they only use a smallish transformer. That was basically the info I was after, whether the secondary side of the main transformer output a reasonably small voltage so that I could just "cut and paste" into the circuit.

    Ok, well how about this crazy idea - what about constructing a boost converter to bring the ~12V DC up to whatever the secondary side of the transformaer is normally at

    Probably not worth it, because all the power going to the computer ultimately has to come from that one boost circuit - probably a bit too much for it. (not to mention inefficient)

    It'll probably just be easier to construct my own, or just use the plans Maestro has come up with

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    Quote Originally Posted by evandude
    given the amount of modifications you'd have to make to convert a 120VAC power supply to run on 12v, you pretty much WOULD be making it from scratch anyway.
    Yep, but as far as I can see, better chance of it actually working.
    I have found you an argument; I am not obliged to find you an understanding.

  7. #7
    It ain't easy being a green moderator meddler's Avatar
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    it has been done in the past, but the information was quite vague on exactly how to complete it. I think one of the old members on here actually did it. It might pay to have a really good look at the older posts in the power supply sub forum. The links I had that show the modifications that you have to do don't work anymore. I'm sure they will still be out there somewhere
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  8. #8
    FLAC evandude's Avatar
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    Quote Originally Posted by Cooties
    Ok, well how about this crazy idea - what about constructing a boost converter to bring the ~12V DC up to whatever the secondary side of the transformaer is normally at
    Yeah, at 240VAC that's 340VDC... somehow I don't think building a 12v-340v DC-DC converter would be easier than just building a complete ATX DC-DC from scratch. Hell, if you just buy an inverter and put a rectifying bridge on it that's exactly what you'd have, but at that point you can just leave the ATX alone and run it straight off the AC inverter.

    by the time you got done with the modifications and such to get it working on 12VDC (if it even ever worked) you would probably end up with a supply that was bigger, more expensive, less functional, and less efficient than the original supply running on an inverter. I hate to recommend an inverter, but it's really your only choice if you're truly that cheap
    But don't take it from me! here's a quote from a real, live newbie:
    Quote Originally Posted by Viscouse
    I am learning buttloads just by searching on this forum. I've learned 2 big things so far: 1-it's been done before, and 2-if it hasn't, there is a way to do it.
    eegeek.net

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    Quote Originally Posted by evandude
    Yeah, at 240VAC that's 340VDC... somehow I don't think building a 12v-340v DC-DC converter would be easier than just building a complete ATX DC-DC from scratch.
    Doesnt the 240VAC go through a transformer first before being rectified to DC? And I assume this transformer steps the voltage down as well, so are you sure it will be as high as 340VDC (yes I know 240VAC is a rms value and hence the 340VDC) ?

    By the way on a side note, this super cheap power supply that I pulled apart had input capacitors that were only rated for 200V, thats pretty poor.

    Maybe I wont be totally convinced until I see smoke
    (hopefully not smoke coming out my ears)

  10. #10
    FLAC evandude's Avatar
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    Perhaps they are stepping down 240VAC to 120 VAC, which would just allow for a 200v input cap.

    regardless, it's still not a small value by any means... the fact that they used 200v input capacitors should be good indication that it's not something you really want to mess with...
    But don't take it from me! here's a quote from a real, live newbie:
    Quote Originally Posted by Viscouse
    I am learning buttloads just by searching on this forum. I've learned 2 big things so far: 1-it's been done before, and 2-if it hasn't, there is a way to do it.
    eegeek.net

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