# High Current DC Power Supply

I'm attempting to build a compact way to charge six laptops (of the same model) so that I may avoid using six individual power cables. I'm trying to make it cost-effective yet safe.

My initial design was to take a simple 24V 15A DC power supply off Amazon and branch the output connection to six individual boards that each drop the voltage to a specific value using an LM317T. However, all parts including an enclosure came out to be about $45. I need 5 of these power hubs. Additionally, the charger for this brand of laptop is rated at 19.5V 3A. Wouldn't that mean that all six laptops draw a combined load of 18A, or does the manufacturer design the charger with some sort of leeway? I am unable to find any DC power supplies that exceed 15A and are under$30. How hard would it be to build something that is capable of providing, let's say, 20A? Could I take a circuit designed for 5A and substitute parts that are rated for higher amperages?

Below, I've provided an image that illustrates the device I'd like to build:

• Once you break 10A most of the cost goes into the heatsink. – Ignacio Vazquez-Abrams Dec 31 '15 at 12:44
• Costly laptop + cheap charger seems like a recipe for disaster. You will end up spending more than what you are trying to save once you have fried a laptop. Try not to compromise on quality. – Whiskeyjack Dec 31 '15 at 13:59
• To drop 24 V to 19.5 V at 3 A with a linear regulator you need to dissipate 13.5 W of heat, which needs a reasonable heatsink, and you can't use an LM317T as those are only good to 1.5 A. It's possible that the laptops would charge OK from an 18 V supply, but I think you'll still struggle to either find or construct a suitable 18 A supply (or two 9 A supplies, etc) for under $30 new. – nekomatic Dec 31 '15 at 14:03 • This is a bad idea - most laptop chargers are a bit more sophisticated than brute-force these days. And you already have the laptop chargers, don't you? So those cost nothing, .vs. wasting money on a crude power supply that is probably far less efficient than the original supply. Buy a power strip - bim, bam, it's one cord into the wall, problem solved. – Ecnerwal Dec 31 '15 at 14:12 • Five Euro's and you're done, one cable coming out and powering up to 6 laptops. – jippie Dec 31 '15 at 14:15 ## 2 Answers This is unlikely to work. Most laptops nowadays have more than just power and ground going between them and the charger. There is often a communication channel too. The laptop and the charger communicate over some proprietary protocol so that the laptop can scale back and not give you full charging features if you try to use something other than the manufacturer's charger. Some of this is legitimate in that the manufacturer doesn't want to be on the hook for a warranty repair if the unit got damaged by a crappy charger. Much of it though is to force you to pay for their overpriced charger. Nonetheless, it is what it is. Some third parties have gotten the specs or reverse engineered them for certain laptop brands, and you can by their chargers for much less than the laptop manufacturer's. Instead of trying to do what you suggest, it would be better to get a bunch of such laptop chargers that are compatible with your brand of laptop. They can usually be had for low 10s of$, so a small fraction of the price of the laptop.

• Thank you for your answer. I also thought about that when thinking of a design. Since I already have the chargers for the laptops, I suppose a power strip will work. I'm just trying to reduce exposed wires. Do you think housing six chargers and a power strip in an aluminum casing would be bad? The block parts would be stacked on top of one another. Would there be too much contained heat? The reason why I'm really try to route all the chargers to one little box is because these laptops are around children and I really don't want an array of things laying around to trip on. – user5705019 Dec 31 '15 at 19:52

Statistically, there is an acceptable chance that 15A for 6 laptops rated for peak 3A will work, since not all will always draw peak, unless, possibly, there is a chance that all will come back at once with empty batteries that need charging. Do mind though, that dropping 5V at 3A is 15W, even if you regulator can handle 3A.

Better would be to use the same power supply and use a switching DC/DC solution to drop 24VDC to 19VDC (19V is okay for 19.5V Laptops, another 0.5V gain!).

If your DC/DC is 90% efficient (assumption! Check it!):
19V * 3A * 6 = 342W output power peak
Peak input power = output power / 0.9 = 342W / 0.9 = 380W

I(at 24V) = 380W / 24V =~ 15.83A

Which is much closer to the PSU limit.

If you get one of 48V and DC/DC units that can do that on their input, you get half that current.