Discussion in 'Power Electronics' started by DigitalFubar, Jul 14, 2013.

1. DigitalFubar

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Jul 14, 2013
Hello every one i am new here and i have a question..

ok here goes...

I want to take 2 laptop power supplies and wire them together so that each psu take part of the load draw from my laptop..

what i would like to know is what i need to do this.. I built another version and although it worked it was far from load balancing..

so I have the power supplies... and i would like to know what would i need exactly (and a good place to get what i need ) and how it should be wired up..

thank you for your reply's and this looks like a great community..

EDIT: Although ive never tried i wouldn't be apposed to building my own psu for my laptop if i could be pushed/thrown in the right direction.. basically im looking for a little more power( i over clock and push limits) and recently
the people that make my computer decided it would be wise to use a power supply that was under powered..(im needing about 500-600 watts @ 19.5-20A )
So i would reverse engineer mine and use the same lay out but the right components to make it a bit more power full..

hope that makes some sort of sense to you guys here lol

Last edited: Jul 14, 2013
2. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
1) measure their voltages (let's say that they're 19.55V and 19.75V

2) Determine the maximum current (let's say they're 3.2A and 2.4A)

3) Place a resistor in series with each one that will drop the voltage to 0.5V less than the lowest measured voltage at the rated maximum current

OK so drop voltage to 19.05V

R1 = 0.50/3.2 = 0.15 ohms (3W)
R2 = 0.7/2.4 = 0.29 ohms (3W)

Connect the grounds together, and connect the resistors in series with the +ve output.

As long as the voltages remain fairly stable, the power supplies will share the load fairly equally. Note that the resistors need to be at least 3W (maybe go for 5W since you'll need to insulate them) and they'll get fairly warm.

Also. the voltage to your laptop will be around 0.5V lower than normal. This probably won't matter much.

If the power supplies are running without a load, one power supply can feed into the other in this arrangement. It will *probably* be safe. Placing diodes in series with the outputs will fix this, but at the cost of lower voltage to the laptop and more dissipation.

3. DigitalFubar

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Jul 14, 2013
ok thank you... now 1 last thing.. the reason im wanting to do this is to give my laptop more power in the long run for benching.. pushing my laptop to the max.. right now im shutting down at 440watts... with this set up will i get past that thresh hold?
i.e. if im shutting down at 440watts and the psu id chip is sending feed back to the laptop telling it to shut down.. would this load balanced set up keep me at say 200-250 watts per psu effectively by passing the tdp of the psu?

Last edited: Jul 15, 2013
4. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Possibly, but you'll have to calculate the power for the resistors yourself. (I*I*R and double it).

You might want to reduce the voltage drop at high currents and especially at low voltages.

440W for a laptop?

5. DigitalFubar

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Jul 14, 2013
yea 440 watts for a laptop.. Alienware m18x-R2 with dual gtx780ms 3920xm cpu and then theres the other stuff

but for main components the 780s pull over 120 watts apiece, the xm cpu will pull over that also.. .. so over clocked im putting my 330 watt psu at 440watts which is the maximum thresh hold for it...
so that tells me that im 50 to 100watts short of being able to run every thing at full.. maybe more.. 500-600watts would be ideal.. then i would know every thing would be powered properly..
that would include the screen and every thing else in the laptop/

ok so for the voltage drop... whats the best way to minimize that? (I*I*R) if im reading it wrong please tell me.. I am smart and learn very quickly but I am far from trained.. most of my knowledge has come from needing and building stuff that no one produces..

my current psu reads.. and i need at least double it

Last edited: Jul 15, 2013
6. KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Have you checked the capabilitiy of the switching supplies inside the laptop?

The 19.5V supply into the laptop is converted to lower voltages (e.g. 3.3V and lower) to run the components inside the laptop. The circuits that do this conversion will have maximum current limits as well. If you exceed them, the circuits may overheat and shut down, or shut down immediately.

7. DigitalFubar

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Jul 14, 2013
no I have not checked.. I need to..

8. BobK

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Jan 5, 2010
What is shutting down? Is it the power supply or the laptop itself? It is it the laptop, supplying more power is not likely to fix it.

Bob

9. KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Nothing is shutting down. I just suggested that he check the capabilities of the switching supplies inside the laptop, since he seems to be drawing a lot of power from them.

10. BobK

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Jan 5, 2010
Kris,

Then one of use is misunderstanding this statement.

Bob

11. KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Sorry Bob, you're probably right.
I'm actually having a trouble understanding ANY of the OP's explanations.
I'll "but out" now.

12. BobK

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Jan 5, 2010
No need to butt out. I agree that the OP is not very clear on what is happening.

Bob

13. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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OK, I just took the time to find out what an "Alienware m18x-R2" is.

My advice is that if it's shutting down, send it back to Dell while its still under warranty. It is most unlikely that a laptop like this is going to come with an under-specced power supply. If it does, then in my country Dell would need to make it work or refund my money.

14. BobK

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Jan 5, 2010
Everyone seems to be missing what the OP is doing.

Steve, did you see this part?

Bob

15. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Oh dear, maybe he's pushing a limit too far.

Unfortunately we (and I suspect, he) still don't know what limit has been pushed too far.