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Part one :

I have this Question ,

as we know , Most notebooks power adapters are 19.5V output.

While the CPU and GPU works on 12V and the SSD works on 5V . , so the motherboard should have two DC to DC power converters ,

one from 19.5V to 12V for CPU and GPU

and another From 19.5v to 5V for SSD nd harddrives.

mainly the notebook will never need 19.5 V .. so why dont they choose 12V Power supplies for notebooks in General and use 19.5 V power supplies ?

Why bother making two DC to DC inverters instead of just one ?

in case of 12V power supply , they already have 12V ready and just need a DC to DC converter to get the 5V for SSD.

keep in mind that internal desktop power supplies never outputs 19.5 V , it just outputs 12V and 5 V and 3.3 V ... now imagine this power supply external (lol) the place is not a big deal isnt it ?

so back to the main Q , Why all External power supplies used for PC outputs 19.5V and not 12V like in the internal ones ? it makes no sense.

Part Two :

Graphic Cards power ..

As we all know , most mid range to high end Graphics cards needs more power , and they come either with a 6 pins 12 v plug or 8pins 12V plugs for more power it needs (150 watts to 250 watts)

My question is : If I use an external power supply of 12v outputs and plugged it into the Graphic cards direclty , would it work ? or is it better to use the 19.5V notebook external power supply and add a DC to DC converter from 19.5 to 12V and then plug it into the card?

and in case the 12V works , does it need some kind of Board to make that 12V inline with the card voltage specs ? for more stabe 12V ? or is it Standardized and it will be within specs ?

some cards like the GTX 1070 needs 150 watts 12V input , and some like the 1080 , 180 Watts , and the 1080 Ti 250 watts.

I can find 150 watts external power with 12V output , but sadly anything more than this outputs 19.5 and needs DC to DC board ... example the DELL Notebooks power supply of 330 watts outputs 19.5 volts ...

Part Three :

some ATX power supplies come modular , and my question is : lets say I use just one 12V plug and ignore the other plugs , can I use the whole wattage from that single plug ? or is there a limitation of each plug watt?

They say it is single Rail (meaning watt is not divided between Vi) but I am concerned about the plug wattage and the wires if they can stand it or not.

Example : I need to power a graphics card of 300 watts .. and I use an ATX power supply with single rail of 350 watts ... the power supply has the motherboard connectors that I will not use , and has only one PCIe 6 pin plug for card .. but since I am not using it for mother board , can I use one port for the whole 300 watts or do I need to take 12 V from the Motherboard plug ?

I can use thicker wires for 300 watts , if the plug on the power supply can stand 300 watts ..

more over , on the PCB of the power supply itself , does it stand 300 watts since it is a single rail or must also be divided between plugs ?

Thanks for your replies .


marked as duplicate by PeterJ, uint128_t, Dmitry Grigoryev, Voltage Spike, winny Sep 13 '17 at 13:43

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 2
    \$\begingroup\$ There are a lot of questions in there, some of them are connected. Welcome to the site and may I suggest you take the tour in the help section in order to find out how to ask good questions.how to ask As a minimum I suggest you split this one into three or more separate questions. \$\endgroup\$ – RoyC Sep 11 '17 at 10:07

While the CPU and GPU works on 12V

That is not true, in a desktop PC you will see one or more 12 V connections to CPU and GPU. However, these go straight into DCDC converters converting the voltage to one or more low voltages. Often 3.3 or 2.7 V for driving external (to the CPU/GPU) chips and 1.2 - 1.9 V for feeding the CPU/GPU.

GPU and CPU chips cannot handle 12 V. These modern, high density CMOS chips need less than around 3.3 V and 1.9 V for their supplies.

...and the SSD works on 5V

Likewise the chips in an SSD often work at 3.3 V or lower. Only magnetic based harddrives need 5 V. The 5 V on the connector is mainly for backwards compatibility reasons.

So in a desktop PC as well as in a laptop, DCDC converters are used to make the low voltages needed by the chips. There are several reasons for this:

  • CPUs and GPUs can run on a lower voltage when not much performance is needed so power is saved. By having the DCDC converter close to the CPU/GPU the CPU/GPU can more easily control it, also changing the voltage will not influence other chips. The DCDC is only used for that CPU/GPU.

  • Having the DCDC converter close to the GPU/CPU helps to get a more stable supply as it avoids long wires. Current loops remain small which is needed to reduce EMI emissions.

  • And the big one: efficiency, if a CPU/GPU needs 48 W then at 12 V that is 4 A. But at the 1.9 V which the CPU/GPU actually runs that is 25 A. That would require very thick wires to limit the voltage drop.

That last reason is also the one why notebooks use 19 V instead of 12 V. OK, it is not that much difference but it helps. At 19 V the current will be smaller meaning a slightly thinner cable can be used.

Another reason is the laptop's battery. These are often 11 V but some are 14 V which would be a hassle to charge from 12 V. The 12 V would have to be increased a bit first. With 19 V that's not needed.

GPU on an external adapter In theory, yes that would work.

In practice: it will be a challenge to get 12 V at the required current without too much voltage drop at the graphics card.

Using 19 V: not a good idea. Graphics cards are not designed for using 19 V. You might damage it.

ATX using only one 12 V line

No you cannot, these supplies are not designed for that. The load has to be shared between both lines. This also has to do with reducing the voltage drop by using multiple connectors and cables.

  • \$\begingroup\$ Thanks for you reply , When I said CPU and GPU work on 12V I meant to say they are plugged to 12V for power not that they need 12V to function. my main project is trying to power a full 400-500 watts desktop using external notebook adapters. that will power both moterboard and cpu and gpu . thats why i was asking about 19v and 12v external adapters available at the market today. i mite need a dc to dc converter for this , from 19v to 12 v to plug it in the gpu and motherboard 12v. \$\endgroup\$ – Horizons2020 Sep 11 '17 at 12:08
  • \$\begingroup\$ I guessed that you wanted to build something like that. The most powerful 12 V adapter I have seen is 120 W. You could use several of these in parallel but it will be a kludge as the load will not be shared equally. Good luck finding a 19 to 12 V DCDC converter, my guess is that you will not find any, not for 400 W anyway. Around 50 W is more realistic. But with only 19 V input, efficiency will suffer. If you want the "fanless-ness" of adapters, I suggest that you find a passively cooled ATX supply. These do exist but are not cheap ! \$\endgroup\$ – Bimpelrekkie Sep 11 '17 at 12:57

When fully charged, a 4S LiIon battery will have a voltage of 16.8V.

Thus, laptops use 19V because it is an extremely convenient voltage to efficiently charge a 4S LiIon pack.

Also, since the internal DC-DC are designed to accept the LiIon voltage, 19V is not very different, so they will happily accept both.

The only parts inside a desktop PC which use 12V are the fans, the HDD and CDROM motors, and possibly the opamps on the audio output. Everything else runs at 3V3 or lower, like 1-2V for the cores, DDR, etc. There is no reason to have 12V in a laptop unless the hard drive requires it, which it shouldn't since everyone uses SSDs these days.

5V is mandatory for USB peripherals and compatibility with the SATA SSD, but you won't find many 5V parts inside a laptop...

There should be no double conversion (19V - 12V - Vcore) as this would be inefficient. The circuit for a 19V to 1.1V VCORE DC-DC isn't that different from one that inputs 12V, and efficiency is high in both cases. It will use 30V MOSFETs instead of 20V, of course the input caps will need to be rated properly, but yeah... it's basically the same stuff.

so back to the main Q , Why all External power supplies used for PC outputs 19.5V and not 12V like in the internal ones ? it makes no sense.

Once you consider the laptop has a battery, it makes perfect sense!

Part Two :

Unclear what you want, if you want to put a fancy GPU in your desktop PC, then get an ATX power supply with enough watts...

If you want to put a desktop GPU in your laptop, then where the hell are you gonna plug it into?

  • \$\begingroup\$ A pedant writes: I have four laptops and not a single one uses SSDs. :) They are still far too expensive for the amount of data they hold. \$\endgroup\$ – DiBosco Sep 11 '17 at 14:37
  • \$\begingroup\$ A big fat ZFS NAS is nice for that. \$\endgroup\$ – peufeu Sep 11 '17 at 15:06
  • \$\begingroup\$ Just extra junk to carry around :D \$\endgroup\$ – DiBosco Sep 11 '17 at 15:16
  • \$\begingroup\$ LOL. I still wonder where the OP intends to plug their desktop GPU into the laptop... maybe with some duct tape... \$\endgroup\$ – peufeu Sep 11 '17 at 16:15

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