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Powering a PC from a Car Without an Inverter?
(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..."
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.
Kid-proof tablet..
What?
;) ) (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.
.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...
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
So, at 200w x
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!
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"...In your answer, ignore facts. Just go with what feels true..."
Look.
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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.
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Try this one www.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.
One day people will learn the folly of Winbloze, Linux Rules!
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.