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To power a laptop computer from 24V bank


By force9BOAT, Section Newbies
Posted on Thu Dec 21, 2006 at 12:38:06 AM MST
24V -> 15V  (75 watts)

Hello,

I want to power my laptop computer off my 24V battery bank without going from 24Vdc to 110Vac to 15Vdc.  Instead I want to go directly from 24Vdc to 15Vdc.  If my understanding is correct I only need to add a resister to the line to give me a voltage drop to 15V, is the correct?

My laptop draws 75 watts.  If my understanding is correct I need a 1.8 ohm resister rated for 75 watts.  Is that correct?  

V = iR
9V drop = 5A * 1.8ohm

Does this yield 5A at 15V?  5V at 15V is what the laptop AC adaptor output is rated.

I ask this question because I keep getting confused about the current value 3A at 25V = 5A at 15V so which current value do you use in the V=iR equation to determine the desired R value.

Also, in the resister causing the voltage drop where does the lost energy go?  If 5A at 24V becomes 5A at 15V there appears to be a loss of 45 watts.  I presume my understanding is wrong or the resister would melt.

What is an example of a resister rated for 75 watts (or more)?

I see the OtherPower.com web site says Radio Shack has a product to do this but I'd like to understand the process better.

Thank you,
Rob
To power a laptop computer from 24V bank | 14 comments (14 topical)

Re: To power a laptop computer from 24V bank (3.00 / 0) (#1)
by stephent on Wed Dec 20, 2006 at 07:34:24 PM MST

You will get a good understanding of Ohm's Law when the lappy goes into sleep mode and the voltage isn't dropped as much.. or when the currant draw isn't at 5 amps (which is high for a laptop?).
see what happens when currant draw is at 2 1/2 amps or even 1 amp.
But it's your electronics...


Re: To power a laptop computer from 24V bank (3.00 / 0) (#2)
by force9BOAT on Wed Dec 20, 2006 at 08:06:20 PM MST

I see your point, as current goes down the voltage drop goes down too so actuall voltage goes up.

So how is this done correctly?

[ Parent ]



Re: To power a laptop computer from 24V bank (3.00 / 0) (#3)
by icicle on Wed Dec 20, 2006 at 08:36:33 PM MST

I know you don't want to here it.
But I would just by the inverter be much easier and mor simple
If in doubt, Read the directions!!

[ Parent ]


Re: To power a laptop computer from 24V bank (3.00 / 0) (#4)
by dinges on Wed Dec 20, 2006 at 09:01:00 PM MST

Your resistor trick won't work. At high powerdraw of the notebook, voltage would drop correctly to 15V; at low powerdraw (battery save mode; sleep; HDD turning off; etc.) voltage could be anything between 15V and 24V. Not what you want.

The options, as I see them:

  1. run the notebook off one 12Vdc battery (usually works, even if laptop battery won't charge)
  2. get a 24->15V DC-DC switchmode-converter (availability?)
  3. get a 14->12V DC-DC switchmode-converter (easily available)
  4. build a switchmode DC-DC converter (hard, esp. for someone who hardly knows Ohm's law, no offence)
  5. build a linear voltage converter for 15Vdc. Easy, but very lossy; the excess voltage/power is wasted as heat.
  6. use an inverter: 24Vdc->220Vac->15Vdc.
Many roads lead to Rome...


[ Parent ]


Re: To power a laptop computer from 24V bank (3.00 / 0) (#5)
by dinges on Wed Dec 20, 2006 at 09:02:47 PM MST

correction to 3): a 24V->12Vdc converter

[ Parent ]


Re: To power a laptop computer from 24V bank (3.00 / 0) (#6)
by TAH on Wed Dec 20, 2006 at 09:29:25 PM MST

How about just using a 15 v voltage regulator?

[ Parent ]


Re: To power a laptop computer from 24V bank (3.00 / 0) (#8)
by dinges on Wed Dec 20, 2006 at 09:35:19 PM MST

A 15V regulator as you mean would fall under heading 5).

Most simple voltage regulators I know/use, like the 7805 (5V), 7812 (12V) and 7815 (15V) are meant for maximum of 1A. There exist 3A versions of some of these regulators, but the original poster needs something that could handle at least 5A. None exist that I know off.

He could try parallelling enough of them too, with a small resistor in series of each volt. regulator to equally distribute current.

But, it's also a lossy route, as he would have to dissipate (24-15)=9V at max. 5A = 45W. That's a lot of power that you waste as heat. I estimate that most of the time his laptop draws about 2.5-3A though, not 5A.

[ Parent ]



Re: To power a laptop computer from 24V bank (3.00 / 0) (#13)
by Seaspray0 on Wed Jan 03, 2007 at 02:40:59 PM MST

It can be done with a voltage regulator.  Sometimes you can buy a regulator with the voltage and power rating you want from local electronic parts stores (ones that makes radio shack look bad as if that wasn't hard to do).  Unfortunately, you don't get these kind of stores unless you're near a big city.  You can also order them online.  The regulator will be the cheapest and simplest way to go.  Most of the regulators I see are about the size of a dime, have 3 leads and a tab to screw to a heatsink.  If you can't find one, there are ways to build your own out of a good power transistor and a few minor components.  The simplest regulator uses a power transistor, a resistor, and a zener diode.

[ Parent ]


Re: To power a laptop computer from 24V bank (3.00 / 0) (#14)
by ghurd on Wed Jan 03, 2007 at 05:27:37 PM MST

This falls under Peter's #5, and is called a TO-220 (or so), 7815 regulator.  
Very Lossy for that kind of use.  Semi~complex for 5A and long term reliability.
G-
Ghurd.info
[ Parent ]


Re: To power a laptop computer from 24V bank (3.00 / 0) (#7)
by stephent on Wed Dec 20, 2006 at 09:33:20 PM MST

a 15v regulator with wrap around pass transistor will work fine--but the "waste" watts goes same place as resistor---heat.
but it's simple to do. and cheap.
and probably just what the commercial thing is made of.


Re: To power a laptop computer from 24V bank (3.00 / 0) (#9)
by crashk6 on Wed Dec 20, 2006 at 11:32:44 PM MST

Forget the resistor approach, the answer to your question of where the power goes, HEAT! Heat = wasted power.

Try this instead
http://www.mitedu.freeserve.co.uk/Circuits/Power/1230psu.htm

The design is straightforward and can be adapted to your need rather simply.
Substitute the 7812 with a 7815. Half the number of power pass transistors. Feed it with your 24vdc source instead of a mains transformer, leave the bridge rectifier on the input side however. The value of R7 in the diagram may need adjustment. Change the output  fuse rating from 30 down to around 8 amps which should pass enough for any modern laptop computer but still safeguard things in the event something interesting happens.

A little further research on your part should get you there.
--
crashK6



Re: To power a laptop computer from 24V bank (3.00 / 0) (#10)
by maker of toys on Thu Dec 21, 2006 at 02:53:56 AM MST

don't buy the radioshack part. . . . you'll very likely pay too much and get disappointing results.

instead, look at:
http://www.artesyn.com/media/pdfs/bxb50s_bto.pdf

(which will do most of what you want, but as a single unit doesn't have the oomph to charge the laptop battery while you're using it. . . . this is a 50w 24V-15V DC_DC converter for about $125, first place I checked.)

or maybe try a Lambda-EMI PH100F24-15 (100 W, about $210 the first place I checked)
http://www.lambdapower.com/

most devices of this sort have some provision for paralleling their outputs, so if you found a good deal on a pile of, say, identical 30W units, you could probably lash up 3 to get 90W. . . .

remember that the units I called out above are expecting fan cooling and heat sinks to achieve rated output.

hope that helps-

-Dan


It's not a case of 'Save the planet,' it's a case of 'Save the humans.'



Re: To power a laptop computer from 24V bank (3.00 / 0) (#11)
by ghurd on Thu Dec 21, 2006 at 07:37:05 AM MST

I have had Very good luck with PowerStream products.  Cheap too!
Peter's option #2, 18 to 28V in, 15V out.
PowerStream DC/DC Converter #PST-D24/15-100, with 138W for 10 minutes, 95W for 3 hours, 70W continuous.  $40.75.
A larger version, about double, for $60.25.
http://www.powerstream.com/

G-
Ghurd.info



Re: To power a laptop computer from 24V bank (3.00 / 0) (#12)
by force9BOAT on Thu Dec 21, 2006 at 10:30:51 AM MST

Thank you for the reference to PowerStream.com.  I'll order from them.  

Thank you all for you many good replies.

Rob


[ Parent ]



To power a laptop computer from 24V bank | 14 comments (14 topical)
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