Power Supply Mod

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Hi

I was wondering how easy it would be to set up a desktop pc to run from a 12v car battery.

If a desktop power supply is 12v, 5v & 3.3v could it be as sinple as getting some chunky resistors in line to drop the voltage?

To utilise ohms law and work out the size of the resistor you would need to know exactly what your power consumption was going to be so i guess that is out of the question as it must vary depending what you are doing at the time.

Anyway has anyone had any experience with this or am i way off track?

Robbie

OK another quick thought, if you actually started the car the voltage would jump to 13.5.... Doh

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The way I would do it is by bypassing the Step Down Transformer, Bridge Rectifier, and Capacitors that make it into 12 volts anyway, but you are dealing with a slew of different voltages, mostly 12+, 12-, 5+, and 5-, so you have to tackle it carefully.

A computer power supply takes a 110-120 VAC (or wherever you are in the world, probably 220 VAC), and converting it into a much smaller DC voltage. AC, and DC, as most of us know, are not the same. In some models for power supplies, it would work something like this:

1) step down transformer converts wall AC into small AC voltage (12VAC, or sometimes higher, see below)

2) bridge rectifier converts AC sine wave to only positive voltage.

3) Capacitors are put in parallel with + and DC ground to "smooth" the sine wave peaks, and ultimately get close to a flat line.

4) a Zener diode is often used to cap the voltage to 12VDC, elimintating the small ripples left by the capacitor curves, leaving you with a flat, 12VDC signal.

5) this is then run through a series of voltage dividers (resistance network) to attain desired 5 volt, and whatever other voltages it needs to to its job.

Figure out how to bypass steps 1-3 that the power supply is doing, and you wont have any trouble. The Zener diode will take care (somewhat) of regulating the voltage to the needed rating, or, a 12 volt regulator IC (more likely). I'd also recommend a 1 farad capacitor (like they sell for amplifiers)for the power leads to smooth that voltage when you start the car. But be warned, if your power supply has a high wattage rating, and you plan to make use of all that, you're going to have to run a really thick power wire (id say 10 guage minimum) to make this work. Your battery drain will also be a lot, but this is definitely something that is doable.

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Voltage regulators ahoy!

Nah, it is possible to run a PC from a battery like that, though I wouldn't recommend it. If you did choose that route I'd advise you to try for a prebuilt DC-DC PSU, making your own can cause some nice big headaches, not to mention that car batteries aren't the cleanest things to work with and you'd have to charge it to use your PC. Oh, and car batteries tend to output less than 12v a lot of the time, though in a car you don't notice because car appliances are supposed to work according to the conditions found in a car,

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Thanks for you fast response, so a few things to think about

ill get a multimeter on an old power supply and just have a look at the step down transformer voltage, you never know 12 volt?

stability of the battery could maybe be sorted with a second battery and some split charging switch or something similar

Dig out a really old motherboard to test the thing on

Would a better solution be to use an inverter to go back to 240v first?

I guess the best solution would be to get hold of a laptop, but i never was one for easy solutions... Thanks again

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Oh, and car batteries tend to output less than 12v a lot of the time

Hmm....maybe in an engine off condition. But when the engine is running, the alternator keeps it at a nice 14.4 volts.

Edit: Fixed your quote box, Moonlit.

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Yeah, while the engine's running you get anything up to ~15v IIRC, but when it's off (especially with and older battery) you might only get ~11v. This should still be enough if you get your circuit sorted right it shouldn't make that much difference.

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This may sound to easy, and isn't as cool as making your own. Why not just use a DC -> AC inverter, plug in using you already existing power supply.

There should be one available here.

http://www.partsexpress.com/webpage.cfm?we...ectGroup_ID=326

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Inverters are horribly inefficient and very wasteful with power sadly. DC-DC is much nice... depending on the requirements of an inverter, probably cheaper then an inverter too :p

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• 4 weeks later...

A friend of mine has a windmill and solar panals chargeing several marine batteries and then runs an inverter, he runs his server room and several comps in rest of house off this he said it because his electric bill is to high I think its just because he can :) If I could I would. lol ;)

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• 1 month later...
Inverters are horribly inefficient and very wasteful with power sadly. DC-DC is much nice... depending on the requirements of an inverter, probably cheaper then an inverter too :p

In addition they do not produce a clean sin wave they use a step so that the power is not clean.

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LOL resistors yeah dude if you were to do that you would def need power converters. if you want to run a computer off of a 12 volt battery go buy a power converter

if you want....get a mini itx board. most of the cases for those boards use like a 12 volt laptop psu that plugs into case then the case changes voltages.

(if u do that be sure to put a huge 12 volt power converter on your comp)

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there is a tonn of comercial products that do this if you are willing to go out and buy something, but if you want to do this as a project use resistors hevily will work. probably need 8billion.

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