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Powering a PC from a Car Without an Inverter? 80

Posted by Cliff from the hardware-hacks dept.
(H)elix1 asks: "Without using an inverter, what is the good cheap way to power an ATX motherboard from a DC source? I have a mini-itx based computer I'm looking to use in the car and house. I'm looking for something like a laptop uses, with an AC-to-DC brick that feeds a DC-DC internal connector. Linitx.com has one, but it is a bit pricey after shipping to this side of the pond, googling turned up even more expensive industrial PSU's. Am I missing a simple hack? I have a soldering iron and am not afraid to use it..."
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Powering a PC from a Car Without an Inverter? More

Powering a PC from a Car Without an Inverter?

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  • Maybe I'm missing something here... (Score:3, Informative)

    by gklinger ( 571901 ) on Wednesday February 26, 2003 @05:03AM (#5385272)
    Have you considering using a DC power supply? [amtrade.com]

    • You are missing something here... (Score:1, Informative)

      by Anonymous Coward
      Have you considering using a DC power supply? [amtrade.com]
      Input Source:
      -42v ~ -56vDC

      Now all he has to do is figure out where to get a 12VDC to 48VDC converter.

      This kind of DC power supply is intended for telecom sites that have 48VDC power sources. Phone company central offices have huge banks of batteries to supply the 48 volts. That's why the phones work during a power blackout.

    • Ummm... I think the part you're missing is this stat from the linked page:

      Input Source:
      -42v ~ -56vDC

  • Don't skip the inverter. (Score:3, Informative)

    by Bastian ( 66383 ) on Wednesday February 26, 2003 @05:03AM (#5385274)
    The DC power supplied by your car only in the region of 12v. It's riddled with power spikes that will be damaging to the motherboard's electronics. This is the main reason why you're supposed to use a power inverter on anything that wasn't specifically designed to be powered by a car's DC outlet.

    A DC adapter can't effectively smooth off the spikes. On the other hand, switching the current to AC using an inverter and then converting it back to DC with an AC-DC adapter will supply power that isn't dirty.

    You can get a decent 300 watt power inverter at a truckstop for 30 or 40 bucks if you look around. It would certainly be a lot cheaper than having to replace a fried motherboard.
    • Absolutely true. Although I stick to the computer/physics end of electrical engineering I've seen the tolerances for the 12V bus and there is absolutely no way a motherboard will survive the spikes and voltage swings that automotive componenets are designed to withstand. Of course you can always try to design your own conditioner, but that will take more than just soldering knowledge. I would think for efficiency sake 300 watt inverter maybe a bit much, but a personal descision.
    • But that only applies if you have the board in your car. Would it be save to have a single battery without any other stuff, that can create spikes connected to the board?
    • IANAEE but shouldn't some configuration of capacitors and or resistors be able to smooth out spikes and sags on a DC line? I recall my father building a "Capacitance UPS" for his Timex Sinclair in the early '80s when we moved to a house with flaky power.
      • An appropriatly sized capacitor could probably take care of the sags, but large electrolytic capacitors (probably the type you're thinking of) generally suck at filtering out spikes and transients.

        But none of that really matters. Computers generally operate off of several different voltages, many of which supply a non-trivial amount of current. You would need to design a supply which would produce each of the required voltages from the 12V supply. If you are capable of designing and building the supply from scratch, you probably won't have much trouble dealing with the nastyness of an automotive 12V supply.
        • Well I know of at least one board that outputs PC ATX power from a 12V DC input. I use it here in my VIA EPIA 800. I think it tops out at 75W or something, but that's enough for a slimmed-down system with a throttled-down CPU and laptop drive.

    • Re:Don't skip the inverter.--BAH! (Score:5, Insightful)

      by His name cannot be s ( 16831 ) on Wednesday February 26, 2003 @09:16AM (#5385828) Journal
      What?

      Yes, the DC power comming off the DC outlet is nasty stuff, and shouldn't be used without proper care.

      But WTF are you talking about moving to AC and back to DC. Hardly the elegant solution.

      When I was in high school, for *two* years we built a variety of power supplies, moving from AC->DC , DC->DC and whatnot. Not one cost more than $10. And they still wouldn't.

      I realize that the kneejerk reaction in todays fast-food-slick-ass-disposable world, is to simply buy the requisite adaptors and snap it all together. Unfortunately, that costs you alot in the power transfer. Most car AC inverters can only put out less than 200 watts (tho' the guy above says 300w--apparently I don't visit enough truck stops ;) ) (Mine says 140). Don't try to push them to that, as they'll kick the breaker on it often. For a continual load, I would try to keep that no higher than 75%ish of the max supported load.

      So, at 200w x .75 you are now looking at about 150w. The mini-itx motherboard peaks at 50ish watts, so that's in the clear, but don't forget about the peripherals. Yada yada yada...

      But I digress.

      You say you are not agaist picking up a soldering iron. FANTASTIC. Go Googling(tm). Find some DC power supply schematics. Look for one that has large capacitors. Found it? good. Put more capacitors in parallel. that'll suck up the spikes.

      Heck, take a trip to the public library. Considering the power supply you are looking for is so damn simple, there are tons of books on just that subject.

      It's not like electricity is a magicical force without reason. You *can* build yourself a practical, safe, powersupply, and save yourself a chunk of cash. You also get supreme bragging rights with your geek friends. Now, you are no longer an parts assembler, you are a creator!

      muhaaaahahahahahaha

      • Cheap DC->AC inverters are often just that - CHEAP. Might work ok for radios and things that don't require necessarily consistent power but I wouldn't want to plug really power sensitive devices into a $30-$40 300 watt inverter. I'm sure there's very good reason that places that aren't truck stops sell a 300 watt inverter for the area of $150 or more.

      • Re:Don't skip the inverter.--BAH! (Score:5, Insightful)

        by cybermace5 ( 446439 ) <g.ryan@macetech.com> on Wednesday February 26, 2003 @10:06AM (#5386037) Homepage Journal
        Look.

        A switching power supply is vastly more efficient than a linear power supply.

        In order to get 5 volts from ~12 volts, spending no more than $10, you use a voltage regulator chip. However, I honestly don't think you'll find a voltage regulator IC that can handle the 10 or so amps required for a low-end motherboard: the Radio Shack variety don't even handle an amp. It will also be horribly inefficient: 42% efficient in fact (has to dump those 7 extra volts somewhere).

        Add to this the fact that you can't use passive components to regulate 12 volts to 12 volts. You'll want to keep the voltage from going up to the 13.6 volts found when the car is running, but any regulation circuitry takes the voltage below 12. Typically you need a supply several volts above your desired regulated level.

        The circuits you built in high school aren't going to be helpful here.

        On the other hand, a switching power supply uses a high voltage, high frequency transformer to minimize loss and dump energy to the load, not to a regulator. An inverter and then a small switching power supply may not be as ideal as DC-AC-DC supply, but will be more efficient than linear voltage regulation and provide more protection for the computer. And big capacitors is not necessarily a good sign: just means the designer had to use them to get smooth power.

        Power inverters aren't expensive either...if you know where to look. I once bought four 100 watt inverters on eBay, for $5 each. I had an AC outlet mounted in my car console years ago, before I crashed that one. :(
        • And you can't make a switching power supply that runs from a dc-source why?!?

          Just because it has an oscillator (that is fed with somewhat clean dc, btw.) in the circuit does not make it an ac-dc supply. in ac-dc switching supplys you actually have to rectify the incoming ac and filter it before you can feed it to the switching section.

          Why not just use the dc-input to run the internal oscillator and be off a lot easier. Couple of filters in front and some overvoltage protection and everything is fine.. Minimal power-loss too.
          • That's what I meant when I said this wasn't the ideal situation, compared to a switching DC-DC converter. Of course that's the obvious most efficient solution, and the one that others have linked to at high prices.

            The fact remains that you aren't going to build a very good switching DC-DC converter for as low as you could buy an inverter. Show me a $20 car-to-PC-motherboard switching supply schematic, and I'll believe you.
        • The funny thing is that just yesterday I was going over this very idea with an EE that designs switching power supplies for a living.

          I've been looking to power my laptop in the car (or on a plane), and my laptop charger is rated at 20V output@2A. So the basic trick is to boost the ~12V output up to 20V.

          Here's the advice I was given to build my own (slightly modified) based on my output needs.

          The key is to find a National Semiconductor LM2587T-ADJ [national.com]. The "T" signifies through hole as opposed to "S" in surface mount. [...] The LM2588T-ADJ, which is a 7-pin device, would be OK too (has extra features we would not need). The total package could fit on a 3"x3" board space with heat sinks. You could go to National's website and check out this device. If you sign up for their "Webench" simulation tool, you can see your circuit on line. If you do, enter 11Vmin and 16Vmax as input voltage. The thermo simulation tool indicated that there could be hot spots on the IC as high as 100 degC at max load -- heat sink [required] for sure. You can probably come up with everything else.
          "You can probably come up with everything else" means that the parts are available, not necessarily that the rest of the circuit is obvious. Naturally, you will need to put some capacitor(s) across the input to protect the circuit, and whatever other circuit features you think are necessary.
          • Those require an external coil and some passive components depending on the design, and they do simplify switching supply design. However, you don't want to slap one of these into a stripboard from Radio Shack, assuming the hole pattern would fit. So you either have to etch or mill your own circuit board, or have one made (cheapest I've seen is $25 for one-offs). The poster above was talking about power supplies they built in high school, which were most likely rectifiers and LM78xx series regulators, nowhere near computer supply requirements. Five amps isn't that close either, except when your computer is a laptop.

            I would probably do something like that for the fun of it, and to make it exactly to my requirements...plus I have experience and friends with the right equipment. However, it's not going to save any money for the average person.
      • That's not neccessarily a good idea either. The inrush current from the DC outlet in the car will cause a spark. It may also be enough to pop the breaker. Look at putting some sort of low ohm high power resistor in series. Many car outlets are rated for 120 Watts maximum. Make sure the wiring can stand "continuous" high power. I would run a separate set of wires from my car battery (fused at the battery) to a heavy duty outlet in the car. Also for safety you'll want to put a diode in series with the incoming line so that you don't have as much chance of shocking yourself from the capacitor discharge when you unplug the converter. For clamping the incoming spikes find yourself a good 14V Zener diode and shunt the incoming line with it.

        Think safety when you build these things. Unfortunately, many times easy safety tricks cost efficiency losses. Electricity can be dangerous and you don't want to hurt anybody because of your stupid mistakes. Great White didn't think about the pyrotechnics in a small building and they lost a guitarist.

      • Now, you are no longer an parts assembler, you are a creator!

        Only if you roll your own capacitors.

      • There is a downfall for putting larger and larger capacitors on the input to the 12 volt power supply.

        Consider what those capacitors do. They absorb power from spikes you say. Well, The more capcitors you put on the line the more instantaneous current you will be pulling from the cars alternator. A car's alternator has diodes that convert the alternating current to DC. If you put too high a load across those diodes they will fail.

        That load can be instantaneous current brought about by spikes being sucked down to 13.2 volts or whatever the battery is happy with at the temperature and current draw it is working at.

        There are also theories that the charging of a lead acid battery works best if you have various spikes and sags in the incoming power. Constant pure DC tends to lower the number of cycles of charge and discharge that a battery can perform.

        You are smoothing the voltage for the whole car when you smooth it for the input to your power supply with lots of capacitors unless you put a COIL in series with the input to the power supply BEFORE the capacitors. A coil used in combination with capacitors will do a better job without loading the diodes with crazy current spikes. These coils or coil will have to be made of fairly thick wire as you want them to stop the reactance,(spikes and sags), without generating their own DC voltage drop due to high resistance.

        As a capacitor uses Farads a coil uses Henries. There are probably equations for figuring out what henry of coil you need for such an application. I do not remember them but I am not the one planning on frying my alternator, :^) .

        As another aside.

        There are 400 and higher wattage inverters available. I picked up a 300 watt inverter from the local Sams club for about $30.00 USD. A few months after I purchased it I saw Sams come out with a 400 watt inverter for not much more. Technology marches on, :^) . The nice thing about inverters are that the good ones use active switching to get you the voltage and current you need rather than brute force caps and coils. A few good size caps and coils and you will have paid for that inverter and still not have a warranty.

        Good Luck,

        dzimmerm
    • You assert that a MB cannot handle the spikes and such from the car's 12VDC, so you say, use an AC inverter instead. So why is it that this inverter can handle the spikes? Why wouldn't the same spikes destory the AC inverter? Whatever they use you could also use for a DC-DC converter. In all cases, you need to filter the power supply properly and there is no guarantee that the AC inverters (much less cheap ones) will protect anything from spikes.

  • Thoughts (Score:2, Informative)

    by Futurepower(R) ( 558542 )
    The DC directly from the car's battery terminals is almost completely clean. This is because of the low internal resistance of a charged car battery. There are IC regulators that output the other voltages, using a +12 Volt input.
    • If you hook up to the battery terminals, you are also hooking up directly to the alternator, which is NOT clean. It would only be safe if the car were off.

      Disclaimer - I am a fuckwit regarding most things electrical.

      • Re:Thoughts (Score:2, Funny)

        by PD ( 9577 )
        Not to mention the hazard of hooking up to a source that can supply 12 volts at 400 or more amps. You've got to have a fuse on that thing, but suppose you have a short upstream of the fuse. It's possible, and in that event you'll burn your car down, explode your battery, and shower yourself with hydrochloric acid. If you drive a Geo Metro, nothing to lose. Go for it.
        • That's a little dramatic. The car may burn. The battery probably won't explode. It'll sag down to a very low voltage and then die.

          Remember violent explosions only happen on TV for effect. The real world is usually much less dramatic.

          But you're right about the Metro thing. Go for it.

          • Re:Thoughts (Score:1)

            by t ( 8386 )
            That is definately not what happens. A car battery has so much juice available that any wire that is shorting the terminals instantly turns into a line of fire with possiblity a plasma arc. The explosion happens if your battery is leaking hydrogen or there is gas leaking and accumulating under the hood (both rather unlikely). And even if did happen, you'd probably only get a nifty fireball then nothing.

            I knew a guy once who spent thousands on a fancy hydralic system for his car sans the fuses. One day the power cables all ignited and the entire interior of his car caught fire. Pretty funny.

          • Batteries do explode. Not necessarily in a huge fireball, but if a conductive enough path is established between the terminals to allow a great enough amount more current to flow than should, the battery will overheat so quickly that the water in the electrolyte will turn to steam and burst the battery's case.

            If you're lucky the hood will be closed at the time.

          • Remember violent explosions only happen on TV for effect. The real world is usually much less dramatic.

            Just for the record, I had a car battery explode a few feet away from me (some dumbass swapped the positive and negative terminals from a charger and plugged it in). Despite the fact that everything within a good ten feet of the battery was pock-marked with damage from the eletrolyte, I somehow avioded any chemical burns. We later found bits and pieces of the battery 30+ feet away.

            If you short-circuit a car battery, you are going to have two problems to deal with. The first problem is the heat produced by the massive current supplied from the battery. This will occur at the short circuit, along the wires carrting the current, and at the battery. Secondly, the massive current will likely produce a large amount of hydrogen gas within the battery. If you are unlucky, this gas will be ignited by the heat or arc produced by the short circuit, causing the battery to explode.
        • For sure, you need a fuse.

    • Re:Thoughts (Score:5, Insightful)

      by adolf ( 21054 ) <flodadolf@gmail.com> on Wednesday February 26, 2003 @06:50AM (#5385501) Journal
      Clearly. There's nothing wrong with using the battery as a source of current.

      But there's a lot more to the system than just a battery, and things turn way fucking different when the car is running. The alternator will generally float the battery at around 14.4V, and the system is not generally able to cope with dynamic loads very quickly - particularly at idle.

      And before anyone starts saying that a PC is not a dynamic load, allow me to state that it doesn't matter: There's a plethora of other things in a modern car that present anything but a constant load, and they also connect to the battery.

      Try this: Start your car, at night. Switch on the headlights. Observe their brightness. Switch on the rear defroster. Observe how the lights dim, however briefly they do it. Now switch the heater blower to high. Observe how the lights dim again.

      This might be stiff enough voltage for quality audio reproduction, but even that is the subject of much debate.

      Aftermarket folks have a tendancy to use fairly large capacitors, coincidentally along with a length of wire (read: resistor) to help with these problems, and it certainly wouldn't hurt to do the same with a PC. But is it enough? Analog audio equipment isn't easily damaged by low voltage (things just sound a bit worse, generally), but a modern computer would seem to portray a somewhat different set of needs.

      Having seen PCs with quality switching power supplies (read: so tolerant of variable voltage that they rely on it to operate) go into spastic fits from slight brownouts, I submit that a "12 volt" automotive power system is a completely inappropriate means of providing juice for commodity PCs, without a lot of help.

      And even once this problem is solved, one must face the issue of how to deal with the issue of doing a clean shutdown when the starter runs, which indisputably causes huge voltage swings that you just don't want a live PC to experience.

      It's easy enough kill power to the PC when the key is in the "start" position, but suppose it's a cold morning, you really should've gotten that tuneup 50k miles ago, and things don't fire up immediately.

      Crank, crank, crank, whine, whine, click. Cuss, swear, release the key (returning it to "on"), and the PC boots up. After waiting a bit for the starter to cool, turn key back to start, and the PC shuts off immediately. Crank some more, wait some more, PC boots up again. Observe the action of your car stereo while starting your car for an easy hands-on demonstration.

      Can anyone really trust EXT3, ReiserFS, softupdates, or journaled NTFS to be able to cope with this random abuse? A simple startup delay (think R, C, and a relay) won't fix it, either.

      • Can anyone really trust EXT3, ReiserFS, softupdates, or journaled NTFS to be able to cope with this random abuse? A simple startup delay (think R, C, and a relay) won't fix it, either.

        Yes, because unless you're explicitly uploading new material, you're booting your filesystems in *read only* mode.

      • Having seen PCs with quality switching power supplies (read: so tolerant of variable voltage that they rely on it to operate) go into spastic fits from slight brownouts, I submit that a "12 volt" automotive power system is a completely inappropriate means of providing juice for commodity PCs, without a lot of help.

        In this case, I am somewhat buffered from some of the fluctuations - I'm using gel cells to power the computer and having the car trickle charge the cells. Learned my lesson about tapping the battery the hard way early on. (whimper)

        And even once this problem is solved, one must face the issue of how to deal with the issue of doing a clean shutdown when the starter runs, which indisputably causes huge voltage swings that you just don't want a live PC to experience.

        The shutdown is less of a problem. I'm booting from DVD and caching/swapping in RAM - no hard drive, read only. It takes a bit to spin up, but the shutdown can be instant without fsck'ing things. The current draw should be low enough to never need to actually power down, however. All I need to power is a 2.8W cpu and about 3W-6W for the mainboard and RAM. I might be able to jump-start the car with the gel-pack array I've got in there now...

        It is an issue, however... Got to tackle that one before I can upload to the car.
      • The above post seems technically correct to me. Three problems: Cranking voltage and voltage variation caused by the alternator and mistakes during car maintenance. So, some intermediary regulator would be required.
      • A few things:

        When you are first starting the car, you'll probably only get as far as POST if it doesn't start immediately... not worry to need about FSCK'ing a filesystem since it isn't near being mounted yet.

        A capacitor will handle the downswings. Also, if you put a setup to detect when the ignition is off and then turn off your machine, you might be able to get a cap or other power-holding systems capable of keeping it up for long enough to dismount and do a clean shutdown.

        Again, power upspikes are the real problem, when the alternator starts. I might say to use a higher-voltage cap (to handle the additional alternator voltage when it is running) - with something to drop it down to 12V clean, and then another cap.

        The problem comes in that for many consumers higher voltage good-capacitance caps are not always easy or cheap to come by, but at least this could allow one to have clean, more-direct power, without loss/etc associated with an DC->AC->DC conversion.
        • Well, you missed the point, but that's OK.

          Realize that if you're operating the car correctly (RTFM lately?), you'll pause long enough with the key in the "On" position to observe the function of the dash lights, before starting the car. Motherboards that POST and get on with loading the boot sector in less than a second are not uncommon, as of late.

          And Linux, on such specialized, limited machine would boot -fast-. Which means that chances are good that the thing will be at a mid-point in the boot process by the time the box gets killed by the starter.

          And don't tell me that you've never switched your car on and listened to the radio/talked to a friend/finished a phone call for a moment before starting it.

          But that's not the point, either.

          The point is about building a reliable system. I -never- want to be faced with debugging my car's embedded PC during a vacation, just because I got distracted by an attractive girl and missed the window of opportunity for starting the car.

          That said:

          Even if a cap would solve any of these issues I raised (and I don't think it will, though it will tend to lessen them), the problem is hardly that they're difficult and expensive to come by:

          These 100,000 microfarad, 15VDC caps [meci.com] can easily be wired in parallel - and we all know that multiple parallel caps are generally better for current peaks than larger, monolothic caps. At $39.50 per high-current Farad, they're hardly expensive. And the same place has stocked them for many years, making them rather easy to get.

          • A little long in replying but: I really appreciate the link, though it's often recommended to use 1.5-2x the operating voltage for the cap's maximum. A 12V battery (up to what, 15V with powerful alternator) would run more safely with a 24V cap - know where to find any of these @ 4-500000uf?

  • Try .. (Score:5, Informative)

    by .milfox ( 75510 ) on Wednesday February 26, 2003 @05:42AM (#5385364)
    http://205.147.44.194/store/commerce.cgi?product=S olarPC

    There's a 12V DC/12V DC/ATX inverter there. :P
    • 12v DC-DC power supply
      60 watt 12 v DC-DC power supply for EPIA boards. Proven mature technology. Works with EPIA, EPIA V and EPIA M motherboards.
      $ 34.99

      Looks pretty good - though if it is so 'mature' why do we not see more of them?
      • dc to dc conversion is a mature technology.

        The real reason you don't see more of them is that most people don't care about running a computer motherboard from their car.
        • I major in electronics - I do realise this - I realise that people in general do not want to run computers from their car - Though it would be a great thing to keep the kids happy

          The only driver use would be for accessing multimap.com or similar - but for that you would need wireless access and to stop the car (most people have problems touch typing - I would dread to think what typing one handed whilst driving would look like)

          • Re:Try .. (Score:1)

            by t ( 8386 )
            If you do a full analysis, you'll find that the easiest and most reliable solution is to simply use a laptop. That's why this market will never take off. Especially as pdas and such are even easier than the laptop, and perform a lot of the tasks that you would've needed a laptop for in a car.

  • In car MP3? (Score:5, Interesting)

    by steve.m ( 80410 ) on Wednesday February 26, 2003 @06:18AM (#5385438) Journal
    It depends what you want to do - If you don't need the -5v and -12v lines (mainly for serial??) it's fairly straightforward. A friend of mine built a switch mode supply to run an MP3 player from in his van.

    It works fine, he gets a steady +12v and +5v out of it running or parked up , and this is enough to power a P90 motherboard on a plastic tray under the passenger seat, with a sound card standing in the ISA slot, the HD suspended by elastic bands in a box and a 4 line 40 char dot matrix display up where the rear view mirror should be, diplaying all the usual MP3 data. Control is by a custom keypad he also built.

    The worst bit is it's all done in PASCAL on top of DOS6.2. yuk.

  • A few sources, and a site for more info (Score:5, Informative)

    by TwoStep ( 36482 ) on Wednesday February 26, 2003 @08:30AM (#5385679) Homepage
    DC-DC Power supplies:
    The best one is Opus Solutions [opussolutions.com]
    Another good one is Keypower [keypower.com]

    You can find a lot more info at MP3Car.com [mp3car.com], and especially at the forums [mp3car.com] there.

    Twostep
  • What not get a laptop that will take a 12v input?
    you can use L200c voltage regulators [google.com] to make sure you get 12v from the batery, they cost 1$. Radio shack, Tandy or whatever should sell them.
    • Most of them are 15-21V input, even if the battery inside is nominally 12V. Even the ones that are 12V really do mean 12V, will stop charging at 11.5V and blow up at 14V (not uncommon in a car to see swings 8-15V, spikes to zero and x00V).

      Andrew's going to sucker-punch me for mentioning this, but there's an Australian company about a month away from releasing a large-sandwich-sized PC which can conceptually have it's own internal 12V LA battery (really LA, or SLAC) for UPS-like effects and so you can wire your car battery into that spot in the circuit. A 12V screen might be another story but if you're reinventing the empeg that doesn't matter. The PC sucks about 15W flat tack, less for the slower totally-fanless versions.

      The company is only interested in selling wholesale so bookmark this link [cyberknights.com.au] (which doesn't exist yet) and I'll whack a page up there with details when they're available (look at end March, mid April).

      BTW, the next Konqueror allows you to bookmark a link without visiting it. Message to IE: feature by feature, we're gunna eat'cher. (-:
    • The max current on that is about 1A. It will be dissipating a couple watts. The heatsink will be hot. Max short circuit current is 3.6A. Too small. Probably not easy to parallel either. They tend to fight with eachother when they are in parallel.

    • What not get a laptop that will take a 12v input?

      I have most of the hardware and code complete. My bride would kill me if I did it... (Thought about it, however) The box has to handle Minnesota winters too - which makes for some heat dumping/water proofing issues. That was one of the reasons for building my own chassis.

  • Yes, It Can Be Done (Score:2, Informative)

    by Anonymous Coward
    Moderators - please mod this up as this is a serious reply.

    Your task can be done, but a certain degree of care needs to be applied or you will end up with a fried motherboard. An ATX psu generates five different output voltages and these must be considered on an individual basis:



    +12 volts - This is the most difficult so we will consider it first. Your options will either be a switched-mode regulator with a wide input range (say 11v-15v) or a parallel regulator which uses a low value resistor in series and a power transistor wired across the load in parallel. Excess voltage in this circuit will be disappated as heat which means that it can get damn hot !!



    +5 volts - Use any regulator circuit you can find but switched mode will be the most efficient.



    +3.3 volts - Wire as for +5v but change values. An alternative source is an old ATX supply which may have the 3.3v regulator as a seperate board inside the case



    -5 volts - The load on this circuit will be low so any switched mode circuit will probably do the job. You need a switched mode in this application because of the voltage inversion.


    -12 volts - again a switched mode circuit must be used but the load on this might be a little higher.



    With all circuits you MUST have some form of surge protection or you WILL kill both the supply and the PC. I don't think you can get varistors as low as 12v and 5v so your only option will be to wire the biggest zener diode you can find across the outputs. I suggest using 5.3v for the 5v lines and 12.5v for the 12v outputs. The 3.3 volt circuit needs special attention as this supplies the CPU, use the closest zener you can get to the output voltage. You should also seriously consider isolating the whole thing from the car electrics until the engine is started. A Google search for 'switched mode regulator' should help you find the circuits you need.



    Hope this information helps.



    Edward Almos

    • As is noted above its possable but I have not noticed anyone mentioning that your AT/ATX power supplies actually convert the incomming AC to DC then is switched to high frequency AC (hence the term switchmode), it is possable in theory to modify your power supply to work off your car.

      1st obtain a power supply with a schematic. Difficult but not impossable.

      You will then need to remove the AC-DC coverter, this is not to hard, and boost the input filter caps to help remove the noise. Then the transformer will need to be rewound to deal with the lower input voltage and increased current which will be required on the input, also the switching transitor will need to be beefed up and a new heatsink added to deal with the extra heat.

      Once you have gotten it to this point make sure you test it hard before you ever attach the motherboard.

      Before the nay-sayers jump in it can be done and has been done, but unless you have an extensive junkbox and a heap of time it is not practical due to the cost, unless of course you live in New Zealand where a decent inverter cost a small fortune.

      Of course if you can find a power supply that will allow you to rip out all of the high voltage section and directly input your car's DC you have all of the required output voltages, this is normally easier if you have a truck or bus that runs on 24 volts.
  • Anything that you make/buy is going to essentially be performing the same task as an inverter and the normal PSU, which create a DC-DC converter when connected in series. There's no way around the fact that you need a DC-DC converter, and I HIGHLY doubt you'll find a DC-DC converter with all the proper output voltages thats cheaper than an inverter and a normal PSU.

    Depending on your car, the 12v system MAY be reasonably clean, but it needs SOME filtering. Almost all 12v systems carry alternator whine. Also, during starting, there can be some nasty transient voltages. ALSO the voltage level can vary from as much as 15v in some vehicles to as little as 11v on a weak battery. ALSO most computers require NEGATIVE voltages. That's the real PITA. So forget about just hooking it up with a couple of regulators or something.

    If you ABSOLUTELY CAN NOT use an inverter, then you're best bet is to go with an embedded mobo, rather than an ATX. Linux Devices has a pretty comprehensive list of single board computers [linuxdevices.com], many of which would be much easier to run off 12v than any normal board. Of course you're not going to get as much computer for your money, but that's the way it goes.

    Your cheapest option by far is to just use an inverter.

  • Laptop's are the perfect solution for in car use. Their power supplies are already designed for DC in, they have a battery to smooth out any sags, they are small so they are easier to find mounting points for. If you don't need a working LCD you can find ones with a fairly powerfull cpu for not a lot of money on eBay.

    • Laptop's are the perfect solution for in car use. Their power supplies are already designed for DC in, they have a battery to smooth out any sags, they are small so they are easier to find mounting points for. If you don't need a working LCD you can find ones with a fairly powerfull cpu for not a lot of money on eBay.

      This is a good point, and worthy of consideration. However, the remaining issue is one of regulation.

      When you take the power off the car's electrical system, step 1: Capacitors. Step 2. Inductor. Step 3. Capacitors. Step 4. Regulator.

      Make sure that the capacitive stages include a large (~30,000uF) capacitor in parallel with some smaller ones (0.1uF). The net effect isn't just 30,000.1uF; the 0.1uF will have less inductance to bypassing RF noise to ground. (This is why, for example, an old XT motherboard will have 0.1uF in parallel with the power supply leads to each chip, rather than just 0.1uFxN uF filter caps, where N is the number of chips.)

      With the notebook, your power requirements will probably be less, and at a lower voltage than the 12V (13.8V actual) voltage of the vehicle's supply. Linear regulators might be able to do the job. Design the regulator to survive >20V, since you don't want back EMF coming from inductive loads (wiper motors especially) or even a failed alternator to lunch the computer or regulator.

      Even a modest computer supply requiring 200W is a non-trivial load on an automotive electrical system. That's ~15A, more when you consider that the computer power supply is probably rated for an output power of 200W, and it's not 100% efficient. Most rear-window defrosters are fused at 30A, which means that your desktop PC's supply capacity is half that of the second biggest single continuous load in the car's electrical systems. (Biggest is headlights.)

      A notebook is designed from the ground up to be energy efficient, and so even if your regulator ends up being a little home-brew linear setup, it's still probably drawing less current than the alternative.

  • Mini-itx Power supply. (Score:3, Insightful)

    by 1101z ( 11793 ) on Wednesday February 26, 2003 @02:11PM (#5388053) Homepage
    Try this one www.mini-box.com [mini-box.com]
    This power supply is designed for mini-itx has power brick, runs on 12v DC. It's small and depending on your itx motherboard it can plug right in with no cables.
  • This site [dashpc.com] is all about car-mounted PCs. I'm pretty sure it's the same guy that sells this DC/DC power supply [dashwerks.com].
  • I just bought this: ATX compatible power supply [attbi.com] from a company called MpegBox, it was designed for a car mp3 player based on the mini ITX style board. works pretty well and I have no complaints.

  • How about using a DC->AC inverter and plugging a UPS into that. It will help smooth out spikes and lulls and will give the added benifit of a gracefull shutdown in the event that the car is off for too long.
  • There was a guy that did something similar although he apparently did use an inverter. There might be something here you could use:

    http://slashdot.org/article.pl?sid=01/10/25/208237 & [slashdot.org]

  • You can buy a nice ATX power supply from this company. http://www.a-powertech.com/

    http://www.a-powertech.com/dc2dc-atx.htm

    really nice unit ( althought its normal atx size not micro)

  • Designing DC electonics to run reliably in a car is an ugly business.

    A PC in particular has some interesting challenges, You've got multiple output voltages, different polarities, and absolute cr*p for an input.

    The charging circuit in your car delivers roughly 13.8VDC with the engine running. This is enough over the desired +12V to run a series pass type regulator with no problem. The trouble is you are not guaranteed to get that voltage. With the engine off or with something drawing heavily, you may have less than 12 volts available. Expect to see about 11.5 volts with the engine stopped.
    Even worse is while cranking the engine, the voltage will probably dip into the +8-10 volt range.

    This dictates a DC-DC step-up converter to ensure you never run below the regulators minimum input voltage. You could buy/build/borrow one.... but why? It would be far cheaper to run an inverter and a standard PS.

    If your heart is set on doing this, You might want to hack up an old ps... redesign the input switcher to work on +12. That would probably require winding a new toroidal transformer and a smallish PCB to hold the new switcher. The bonus here is you get to reuse the regulator circuitry and case. I've done this with excellent results, but I've been building power supplies for 30+ years. There's two huge time wasters implied here: figuring out the supply's original schematic, the other being getting the right magnetics. I ended up winding a new transformer.

    whatever you decide to do, you'll probably want to stay within the ATX specs, one source is here [http].

    Good luck :)

  • Do yourself a favor and just look on your own before asking on Slashdot. Spend a day or two, or even a few hours, browsing electronics sites. Someone somewhere has surely already solved that problem. I've seen it... I just can't remember where I found it. But it's out there, so look for it. And whatever you do don't blindly trust what you read online. You'll probably need to work on it a bit to get it to work. If there isn't a mistake in it, or if something isn't clear, you will find that Murphy likes power supply projects :)
  • I ran into this same question, in terms of design, when I was working out the details of the 'TechMobile,' [bluefeathertech.com] as my wife dubbed it. Originally, I was going to buy a DC/DC converter supply for the onboard computer (yes, ones with 12V input exist), but I changed my mind after discussing it with a fellow engineer.

    He pointed out something that I'd not thought of. Sure, a DC/DC will run the computer easily enough. But consider all the time I spend at electronics/ham radio swap meets, and the fact that there are times when having power tools available on-the-road can be mighty handy.

    For about the same price that a DC/DC box would have cost, I got a nice heavy-duty Cobra inverter from an Ebay seller. I've got power to spare for both the onboard PC, and for testing stuff I might want to buy right at a swap meet. Much better value for the $$, IMO.

  • IANAEE, but what about a second battery and a battery isolator? many RV's, diesel trucks have a deep cycle battery for miscellaneous electrical function (dome lights etc.)that gets charged by the alternator but doesn't contribute to the normal electrical requirements of the vehicle. That way, if your system can be put into a 'sleep mode', you can conceivably leave your computer on while the car is off overnight. if you're going to make your own 12VDC>12vdc converter, there's probably also plans that provide for a UPS-like notify circuit so that the 'puter will do a *shutdown -r now* ( or something to that effect) should the source voltage drops below N. Just my $.02

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