Easiest way to derive 5v from 12v DC

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ex-parrot's picture
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Easiest way to derive 5v from 12v DC

Hey everybody, glad to see Applefritter's back up and running Smile

I'm trying to figure out the easiest and/or most efficient way of deriving a 5 volt supply from the 12 volt supply present in a car. Normally I'd just use a simple 7805 circuit, but that's not going to cut it considering I want to power a Pentium 75 motherboard and HDD.

I suppose a switchmode PSU of some kind would be the best solution here, but that adds greatly to the complexity of getting it working.

I have also considered a simple potential divider using two resistors, but that might be tricky as the resistance of the load is relatively unknown here.

Any ideas, EE majors etc Wink ?

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i know this may sound craxy (

i know this may sound craxy (and it may not work). but try a high power car adapter. this will five you 5V and a hiugh current. mayb this will work, and it fits in a car easily

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I've seen DC-DC power supplie

I've seen DC-DC power supplies for PCs, specifically desinged for use in cars. Of course, they're ATX, so you'd need to use a little more recent of a motherboard, but that would be the simplest solution.

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ATX to AT converter... I am u

ATX to AT converter... I am uploading a very rough one in my pictures.

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You will also need a 12V regu

You will also need a 12V regulator also as a automotive power systems generally runs at 13.8 VDC. Automotive power systems are unstable and there is ALOT of line noise so you will need to filter the power also.

One I idea I had was taking a a ATX power supply. Find where the step down transformer is and place the input there. The couple out power supplies I look did not look feasible but I did not spend any time on it. So that might be worth looking into.

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Re: You will also need a 12V regu

redrouteone wrote:
You will also need a 12V regulator also as a automotive power systems generally runs at 13.8 VDC. Automotive power systems are unstable and there is ALOT of line noise so you will need to filter the power also.

One I idea I had was taking a a ATX power supply. Find where the step down transformer is and place the input there. The couple out power supplies I look did not look feasible but I did not spend any time on it. So that might be worth looking into.

That's an interesting idea. I'm also finding that the motherboard doesn't want to update its CMOS settings without the negative voltages connected, so I might be able to use this to generate the negative voltage as well.

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Vehicle PSU?

Surely a dedicated vehicle PSU would do what you are wanting:

http://www.mini-itx.com/store/?c=10#p1807

EDIT: I see URLs still don't auto link...

TOM

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in car pc, here i come!

yes! that's exactly what i was searching for ... about half a year ago!
ex-parrots post reminded me of that celeron box collecting dust under my desk. i wanted to put it in the car and use it as multimedia station, but gave up on that because i wasn't able to build a suitable psu.

finally, car pc, here i come!!!

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The main problem I have with

The main problem I have with those vehicle PSUs is the expense Smile

Anyhow.... the Pentium 75 I had in mind didn't want to run linux, boot FreeDOS or install BeOS, so I threw it off the balcony. Good fun.

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my main problem

my main problem is time. the celeron is still happily collecting dust. that may also be the only thing that stupid box is capable of. i had such a great idea for my car's interior, but as i'm working from 8am to mostly 8pm, monday to friday, and on weekend i'm most often drunk, there's actually no progress on this topic. i have already setup a copy of xp pro with an os x style dock and original tiger / panther icons, but that's it so far...

hm, time for some holiday...

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Re: Easiest way to derive 5v from 12v DC

Ex-parrot wrote:

I'm trying to figure out the easiest and/or most efficient way of deriving a 5 volt supply from the 12 volt supply present in a car.

When it comes to power supplies, "easiest" and "most efficient" are generally mutually exclusive terms. The easiest is to use a 3 pin linear regulator. These can get pretty hot though and waste a lot of power. The good news is, most have a very wide input voltage range.

If you plan on pulling 1 or more amps, I'd look into a switchmode regulator.

Also, car alternators make lots of nasty electrical noise so use extra filtering on your inputs.

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