Power LCD from USB

neon_eddy · 06-01-2007, 06:23 PM

This isn't so much a CarPC question, as it as a laptop question, although I'm an active member here so help me out.

I have an older Screen, I'll have to build a case for it... but I would like to turn it into a secondary display for my laptop. I have a 17" MacBookPro.

I hate wires, I use BT when I can, so I'm thinking of trying to use the USB power source to power the lcd, I figured it out.. and it's like 750mA (9W@12v).

the maximum that USB allows is 500mA, that's I've read?.. so if we can squeeze just a smidgen more out of it, great.

Second question, I need to step-up the voltage, anyone done that? Or know the best way to do it from 5v?

I'll work the system as a 12v input for now, and maybe with any luck I can turn it into nice secondary truly mobile display.

psyrex · 06-01-2007, 06:38 PM

It's more about watts than the current.
9W. That translates to 750mA on 12V as you correctly figured out. On a 5v rail, however, 9W equals 1.8A. Throw in an overhead for a conversion, and say you need a little over 2A on 5V to convert to 750mA on 12V. That 2A is alot. You might want to look into a different way to power it. For example, there is a carnetix power supply sold in the mp3car store that produces both 19V and 12V power. You can use that power both your laptop and the screen.

RedGTiVR6 · 06-01-2007, 06:52 PM

electricity is not a toothpaste tube.

You cannot just "squeeze" out extra power.

What you're wanting to do is not possible.

There's a reason electrical devices are rated.

Not only that, but if the circuity of the USB port on your LCD isn't designed to power the screen, it doesn't matter how much power you pull down that USB cable. If the screen isn't looking at the USB port for power, it won't matter.

You can put a bottle of ice cold water behind a wall. If the guy that's about to die of thirst doesn't know it's there, it's of no use to him.

neon_eddy · 06-01-2007, 07:13 PM

Well, here was another thought... take power from more than 1 USB port on m laptop... so then I'm only pulling say 1amp per side,

getting power from USB is my attempt and getting a full secondary screen with no external power supply. So I don't intend on just feeding more amperage to a usb connection, I was going to use the power from that to convert to 12v to power the screen... but it seems that might not be possible...

I guess I'm looking for a clean and simple way to power one of these lcds, as a secondary monitor using only normal cables, and no real hacking of my laptop.

but thanks for the dose of reality, any more ideas?

RedGTiVR6 · 06-01-2007, 07:59 PM

if the LCD can run off of un regulated 12v, just tap into a 12v source in the dash.

TruckinMP3 · 06-01-2007, 08:04 PM

Or get a Point of Load regulator (POL)

RedGTiVR6 · 06-01-2007, 08:12 PM

if you can ensure your car is going to run over 13v at all times...else the screen shuts off.

Can you ensure that with the car off?

neon_eddy · 06-01-2007, 09:02 PM

sorry... I think somewhere people have gotten confused, this is NOT FOR A CAR, this is for my laptop, to be used where ever... like at a a coffee shop, to pull out a second screen.

So maybe this helps clear some things up, I love the feedback I'm getting.. I've honestly all but given up.... on this idea however... at least powering it from a USB connection.

hmm, could you use USB in series? to build 10v? ...

grayvader · 06-01-2007, 10:48 PM

No.
Sorry to say, but what you want just can't be done with USB power.
Even if you could use a charge pump or something to up your 5V out to 12, you'd trip the laptops internal fuse -- even if you used all the USB ports.

I hate to say it, but give up on USB as a solution.

(and just so you know where I'm coming from, I design motherboards for a living)

neon_eddy · 06-01-2007, 10:51 PM

ok... good to know... is there any other option other than an external 12v PSU?

grayvader · 06-01-2007, 10:55 PM

hacking inside your laptop to find 12V inside it?