I have a portable 12v car battery charger, jumper, maintainer, and inverter with integrated USB ports, 12v car adapter slot, jumper clamps, emergency LED light, and a 3 prong 110v/60hz wall outlet. It is charged by a 15vDC input using a common cylindrical DC plug. I have a male connection that fits and is capable of supporting the current drawn by the unit. I however do not have an AC/DC conversion box for 15v.

I have USB ports but I have heard that you CANNOT chain USBs in series. I also have several desktop PSUs with 20/24 pin ATX, 6 pin PCIe, 4 pin IDE, 2 pin CPU, and SATA connections.

My question is, using the available connections, can +15v DC be achieved? If USB can be used in series, how? If not, can I use the PSUs?

I have 4 units to work with, one being 20 pin and 3 being 24 pin. I think Ihave 2 possible options using the PSUs. Wire 5v in series to get 15v. Would 3 PSUs be needed? I dont fully understand how the circuits connect internally. Would this simply be a matter of: GND|+5v -> GND|+10v -> GND|15v?

Alternatively I could use 9 sources of 3.3v in series to make 29.7v. I could then halve the voltage to make 14.85v. I am still unsure of the internal workings of a PSU, furthermore i do not know how to divide the voltage of a DV circuit. (probably my next google search)

I can also acquire if needed 19.5v and 9v adapters

Please forgive me for any errors. I would be happy to correct any issues in this post. I mean only to improve upon available content and enrich the community.

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    \$\begingroup\$ To connect two power supplies in series, they must be isolated from each other. The power rails provided by an ATX PSU are not isolated. \$\endgroup\$
    – Hearth
    Dec 30, 2020 at 14:22
  • \$\begingroup\$ @Hearth If i understand you correctly, your saying that since the ground is a shared connection for the entire unit, a series connection would all use the same ground and effectively result in a parallel connection. \$\endgroup\$ Dec 30, 2020 at 15:36
  • 2
    \$\begingroup\$ No, a series connection would use all the same ground and result in a short circuit. \$\endgroup\$
    – Hearth
    Dec 30, 2020 at 15:36
  • \$\begingroup\$ @Hearth I may have failed to clearly state: I have 4 separate PSUs. Would the use of separate units result in the necessary isolation to achieve a series connection? and would the psu be able to handle the increased voltage? EDIT: So the use of the same ground by 2 voltage sources connects the 2 sources together aswell, that being the problem correct? \$\endgroup\$ Dec 30, 2020 at 15:39
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    \$\begingroup\$ The 0V output of an ATX PSU is, I believe, referenced to mains earth, so they still aren't isolated enough for this purpose. \$\endgroup\$
    – Hearth
    Dec 30, 2020 at 15:41

1 Answer 1


First off, you won't need to combine any of the power supplies OR any of the voltage rails, UNLESS you have a seriously funky charger there. I'm willing to bet that it came with a cable that had a car charging connector on both ends, meaning it was designed to charge from a car's 12V charging port. All three of mine have, and all of them charged just fine from 12V. While this may not allow you to "top" it off, it will work just fine.

That means you'll just have to convert one of your power supplies to stand alone operation. Personally I just buy one of those boards that you plug the 20/24 pin connector into and it does all the work.

But if you decide you want to do it yourself, here is quick list of what you'll need to do to get it working.

** You'll be using the yellow wire(s) to supply 12V to your charger.
** You'll need to connect the green wire to ground to enable the PS(maybe with a switch)
** You'll also need to supply a load to the 5V rail(red wire), as most ATX power supplies will refuse to work if they think they are not plugged in to a motherboard. I used 3 2 ohm 5W "sandbar" resistors in series because I had them in my junk drawer. From what I've seen, 5-10 ohms works, just make sure that the resistors can dissipate the power safely. I wired my resistors to the air input on the power supply.

That's it. At this point fire it up and test your 12V output. Then it's just a matter of wiring your coaxial connector up and plugging it in.


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