1. Computer problem? Tech Support Guy is completely free -- paid for by advertisers and donations. Click here to join today! If you're new to Tech Support Guy, we highly recommend that you visit our Guide for New Members.

Solved: Where does all that +12 volt power go?

Discussion in 'Hardware' started by Riverglen, Apr 6, 2015.

Thread Status:
Not open for further replies.
Advertisement
  1. Riverglen

    Riverglen Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    506
    First Name:
    Larry
    This is not really a problem, just a question for my own education. I recently helped a friend's son get his home build running. Along the way I discovered that a typical gaming capable machine needs a LOT of +12 volt power. I don't think I would have any trouble starting my car from the 12 volt rail of his power supply.

    So, what the heck does all that 12v power get used for? My hardware knowledge is very dated, but I'm used to thinking in terms of a couple of volts or so for most of the circuitry. The video boards seem to be the big user, but at the end of the day they're still mostly some dedicated processing and a bunch of memory, neither of which I would have expected to use +12v at all.

    So...?
     
  2. DaveBurnett

    DaveBurnett Account Closed

    Joined:
    Nov 11, 2002
    Messages:
    12,970
    Drive motors.
     
  3. Triple6

    Triple6 Moderator

    Joined:
    Dec 26, 2002
    Messages:
    52,935
    First Name:
    Rob
    CPU's and GPU's are the big users. But many other things do as well such as the motherboard and drives.
     
  4. DaveBurnett

    DaveBurnett Account Closed

    Joined:
    Nov 11, 2002
    Messages:
    12,970
    I was under the impression that it was just motors that used the 12v line and all the electronics was just 5volt or 3.3volt.
     
  5. Riverglen

    Riverglen Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    506
    First Name:
    Larry
    Well, the reason I got interested in the question is that it's hard for me to believe that any of the semiconductor devices, (CPU, memory, GPU, and misc logic) would use +12V at all. I think that old school IDE drives use +12V for the motors, and +5V for the logic. But it surely can't take much to turn a disk platter.

    I looked at the specs for a typical 620 watt power supply and came up with the following:

    +5V - 24A
    +3.3V - 24A
    +12V - 48A (!)
    -12V - 0.8A
    +5VSB - 2.5A (What the heck is "SB"?)

    Interestingly, if you add up all of those claimed capacities, you end up with substantially more watts than the claimed 620 watts for the entire supply. Must be that you can't run all of the outputs at their claimed limits at once

    I can see where cramming a lot of semiconductors into a machine can add up quickly, but it still doesn't seem to account for the 12V demand.

    Also, if you attribute the power consumption to the disk drives, consider that you can get external drives that will run off what can be drawn from an USB port. So, +12V, 48A......
     
  6. DaveBurnett

    DaveBurnett Account Closed

    Joined:
    Nov 11, 2002
    Messages:
    12,970
    The 12v IS just for the motors. That 48amp is needed just in case all drives try to start at the same time. Once running the power needed drops enormously.
    You may well have seen lights flicker when a fridge motor starts up and many industrial motors have a special start up switch that is turned to a different setting once running. Electric motors have virtually zero internal resistance when starting so the initial current is quite high
     
  7. Triple6

    Triple6 Moderator

    Joined:
    Dec 26, 2002
    Messages:
    52,935
    First Name:
    Rob
    The CPU, GPU, and motherboard manufacturer's seem to have it wrong then since they seem to believe +12 voltage is required.

    And hence why anyone who wants to upgrade their video card always gets questioned on their power supply and how much current is their +12 volt rails. For example an Nvidia Titan X requires 250 watts of power; that's 21 amps on the +12 volt rail, the card then converts it to whatever voltages individual components need.

    On the other hand hard drives and fans use very little power in comparison. Take a WD Blue Edition, at peak it uses 2.5 Amps, that's 30 watts at maximum. Fans are even less, a Corsair 120 mm fan uses 0.13 Amps or 1.56 Watts.

    There are many new systems that have power supplies that only output +12 volts and nothing else, the motherboard does the conversion for everything else. Almost all the Lenovo ThinkCentre systems we bring in use this design.

    Anyways, here's a Wiki article on computer power supplies: http://en.wikipedia.org/wiki/Power_supply_unit_(computer)

    SB is Stand By.

    And you're right that it adds up to more than the advertised limit and you are right as to why as well, not all of it can be drawn at the same time. For example you may be able to draw up to 24 amps on either the 3.3 or 5 volt lines but not 48 amps at the same time, but when both are being used the max for them combined might only be 30 amps.

    Just to clarify, neither the CPU itself nor the GPU on the graphics card themselves uses 12 volts directly. The voltage is stepped down to much lower voltages but the voltage supplied to the voltage regulators is 12 volts.
     
  8. Riverglen

    Riverglen Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    506
    First Name:
    Larry
    Rob,

    Thanks for your comments and the link you referenced. I find it very counter intuitive to think that if you want a lot of power at a couple of volts, that you would get it by regulating down from 12, but I suppose if you also need 5V, maybe it makes sense.

    I've been rooting around trying to find power requirement specifications for things like internal hard drives, SSD drives, and even fans. Nobody bothers to tell you what the power requirements are for anything. Couldn't even prove that a case fan runs at 12V, although I believe that it does. Things are so standardized that the assumption seems to be, "just plug everything into whatever socket it will fit into". Good luck it you want to actually plan ahead to be sure your power source will have the moxie to run everything.
     
  9. Triple6

    Triple6 Moderator

    Joined:
    Dec 26, 2002
    Messages:
    52,935
    First Name:
    Rob
    I went to Western Digital's and Seagate's sites and they both list power consumption for hard drives. Samsung has power requirements listed for their SSD's and probably a few others do as well, the Samsung SSD I looked at had rating of 250 milli-Watts. Realistically SSD's, fans, and even a few hard drives have extremely low power usage and are nearly irrelevant if you pick a quality power supply with sufficient wattage, which these days means a 400 watt for a basic system, and starting above 500 watts for one with a discrete graphics card.

    There's also several power supply calculators online to help get an idea as well: https://www.google.ca/search?q=powe...l5.4479j0j4&sourceid=chrome&es_sm=93&ie=UTF-8
     
  10. Riverglen

    Riverglen Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    506
    First Name:
    Larry
    Good info. I just took a quick look at a few supplies and hard drives on vendor sites like NewEgg and the specs weren't very complete. The power consumption figures you found don't surprise me in the least. If you really needed to worry about motor inrush currents on the order of tens of amps, youd have to wire up your machine with car battery cables :)

    Aside from getting an answer to my original question, it was interesting to learn that specs for power supplies are a little weird. You really need to be able to separately add up your power requirements from the individual outputs, and to do that you do need to know what's needed by each of the components. Or, more likely, just use some informed thumb rules and over buy a little. Supplies don't seem all that expensive anyway.

    Thanks again for taking the time to educate me a little.
     
  11. Triple6

    Triple6 Moderator

    Joined:
    Dec 26, 2002
    Messages:
    52,935
    First Name:
    Rob
    Avoid cheap power supplies, they use inferior components, have more power on the 3.3 and 5 volts rail then they need and less on the 12 volt, and they test and achieve their ratings under the wrong conditions which makes their wattage ratings completely useless. If you can pick up a cheap no-name power supply and compare its weight to a quality unit like a high end Seasonic or Corsair you'll notice right away there's a huge weight difference between them. You definitely get what you pay for with power supplies.

    Realistically you don't need to worry about the power consumption of most individual parts, set a good baseline for a basic system using an online calculator or just start at about 400 watts, and if you have a video card take a look at it's requirements and use their system recommendation and that should pretty much cover you.

    Any motor you find in a PC is a very low power motor with very small draw.
     
  12. Riverglen

    Riverglen Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    506
    First Name:
    Larry
    Well, I have no expectation of building a machine myself. I got asked to help a novice builder get his machine running (successful) and as a side issue got interested in the +12V power question.

    Fortunately the builder had a more than adequate quality supply (Corsair, as it turned out). Basic problem was that he neglected to connect all of the cables he needed, most importantly the power cable to the mother board.
     
  13. crjdriver

    crjdriver Moderator

    Joined:
    Jan 2, 2001
    Messages:
    39,867
    Actually you would ie it would not even come close to cranking your car. Most quality car batteries can deliver between 500 and 700 CCA [cold cranking amps] this is @32F. 700amp @12V equals 8400 watts. That is a little more than the average 650W pw supply in a computer. Which probably only supplies 400~500W on the 12V rail.

    Modern mb use the 12V to deliver pw to the cpu. This first goes through VRs [voltage regulator] This lowers the 12V to the 1.25 to 1.4V required by the cpu. Since modern cpus and video cards use a LOT of pw, it is more efficient to use the 12V rail than the 5V, or 3.3V rail. The 12V rail will deliver more watts at lower amps than say the 5V rail. Remember ohms law. VxA=W or volts times amps equals watts. If you use a higher voltage, you need less amps [to supply a given amount of watts]
     
  14. jack-o-bytes

    jack-o-bytes

    Joined:
    Jan 27, 2009
    Messages:
    2,607
    The other thing about having the 12v to cpu voltage step down means that it can be regulated.

    For example a cpu wont always run at it's highest voltage. The motherboard is able to regulate the voltage.

    Like Crjdriver said. Having more volts means less amps.

    A cpu needing 120w @ 12v only needs 10 amps. This doesn't need huge chunky power cable.
    The same CPU using 120w but this time the PSU putting out the voltage it needs of say 1.2v would need 100A to achieve the same amount of wattage.

    100A would need huge cables but 100a running down very short PCB tracks of copper wouldn't need to be huge.
     
  15. Riverglen

    Riverglen Thread Starter

    Joined:
    Aug 28, 2006
    Messages:
    506
    First Name:
    Larry
    Well, the remark about starting my car was a bit of a tounge-in-cheek exaggeration.

    I think that if I were designing computer power supplies, I wouldn't be inclined to regulate all the way down to a couple of volts from +12, unless something more substantial than a few fan motors, etc needed +12. Why deliver 50 amps at 12v to the mother board when nothing ON the motherboard needs more than a couple of volts, other than a few fans? And, my first instinct would be to do the serious voltage regulating in the PSU, vs on the mother board. But doubtless there are issues that I haven't thought about.

    And, simply adding up enough watts, isn't the important issue. What you really need to know is how much current you need at each of the several voltages you need to provide. That's why it was interesting to me to learn that the sum of the claimed current capacities for the various voltage outputs substantially exceeds the claimed wattage rating of the supplies I looked at the specs for. In one case, the supply was rated at 620 watts overall, but the summation of all the volts * amps of the several individual outputs came to almost 800 watts! Makes me wonder where the overall wattage rating of the supply comes from. For all I know it might be based on the AC voltage * current input from the wall. How much of that winds up heating the room?
     
  16. Sponsor

As Seen On
As Seen On...

Welcome to Tech Support Guy!

Are you looking for the solution to your computer problem? Join our site today to ask your question. This site is completely free -- paid for by advertisers and donations.

If you're not already familiar with forums, watch our Welcome Guide to get started.

Join over 733,556 other people just like you!

Loading...
Thread Status:
Not open for further replies.

Short URL to this thread: https://techguy.org/1146122

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice