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Electronics Advice needed on PSU voltages

Discussion in 'Modding' started by Marmotta, 2 Jan 2008.

  1. Marmotta

    Marmotta Member

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    Hi there! I'm fairly familiar with general hardware modding, but am building my own bespoke system for the first time and need some advice on the voltages in my PSU.

    Basically, I want to build a mini-itx system within an Xbox 360 case using a 120W Pico PSU and in the interests of authenticity, wanted to keep the 360's power brick. The Pico PSU requires a 12V supply, which the 360 brick can obviously supply, but appears to have a peak load on the +12V channel of 10A and a max load of 7A, whereas the 360 PSU supplies 16.5A to the 12V line.

    As I really am not at all knowledgeable on this sort of stuff, I wanted to know if you guys would know if it would work - and also whether the 360 PSU would require a constant 5V supply of power.

    If not, I also noticed that it was possible to use PC PSUs with a 360, so I assume the opposite would also be the case. After all, the only reason it wasn't possible with the original Xbox (which I did most of my modding work on) was because it used 3.3V for standby. If I take this route, what wire would you suggest using? (that's easily obtainable in the UK).

    Edit: Aaargh, just browsing the 360 motherboard pinouts and realised that of course it can't be used directly because the only inputs on the 360 are 12V and 5V, so I'm now back donw to the PicoPSU option.

    Thanks for any replies in advance:D
     
    Last edited: 2 Jan 2008
  2. Cinnander

    Cinnander New Member

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    As long as your 12V supply to the pico PSU can provide more power than it will be able to draw (i.e. if it was running at full capacity), it will be OK.
    As it states it is 96% efficient, you can assume 4% of whatever goes in is wasted as heat. If it's running at full capacity of 120W (output), then 4% more than 120W (124.8W) will be required as input, from your 12V supply, which gives you 120W of power and 4.8W of heat.
    As you have 124.8W from 12V this means 10.4A drawn at max. power, (12V*10.4A = 124.8W). As the '360 PSU can supply 12v@16.5A, this is OK.

    With power supplies you don't need to worry about excess current ability. If they can supply 12V at 100A, but you have some device that only draws 0.02A, then it will only draw this. Over voltage on the other hand is bad :D
     
    Last edited: 3 Jan 2008
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  3. Marmotta

    Marmotta Member

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    Thanks ever so much for the detailed response. The 360 PSU should easily provide enough power consistently to the PicoPSU, as most of them are 203W.

    The next thing is, will I need to have 5V going to the 360 power supply and is it OK if that is provided from the molex connection on the Pico?
     
  4. Cinnander

    Cinnander New Member

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    If you join the grounds (0V) of the XBox and Pico together, then both of the 5V lines will have the same reference point (i.e. "0 volts" will mean the same thing to both). It will then be OK to use 5V from there yes - this is for standby?
    If you don't join the grounds it might work but then again maybe it wont depending on the structure of the picopsu.
     
  5. Marmotta

    Marmotta Member

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    Yeah, the Xbox needs the 5V for standby. From what I can tell, the pinout from the Xbox PSU are the following, where Pin 7 Turns Pins 4, 5 and 6 on when it is tied to ground.

    1 GND
    2 GND
    3 GND
    4 +12 VDC
    5 +12 VDC
    6 +12 VDC
    7 Power Enable
    8 +5 VDC (Standby)
    9 Shield (GND)
    10 Shield (GND)
     
  6. Marmotta

    Marmotta Member

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    Another thing - the 5Vsb rating of the PicoPSU are 1.5A load and 2A max, but the 360 power supply I've just checked at work shows 5Vsb of 1A. Technically, is the 5V going to be coming from the PicoPSU or the 360 PSU and should it cause problems?
     
  7. Cinnander

    Cinnander New Member

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    It will only become a problem if your system tries to use more standby current than the power supply can provide, in this case the PicoPSU because I think, though I'm not entirely sure, the picopsu uses the 12V supply for *everything* (the site said something likes "requires only a 12vdc supply"), so you're limited by the PicoPsu's 1.5-2A rating on standby current (as this will be drawn from the 12v supply [via a voltage regulator to get 5v, which is probably where the 2A limit comes from for temperature reasons] which as we discussed can supply over 15A).
     
    Last edited: 5 Jan 2008
  8. Marmotta

    Marmotta Member

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    OK, thanks:thumb:
     
  9. Marmotta

    Marmotta Member

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    [​IMG]

    It's been quite a while, but I've only just got round to starting my mod! Anyway, as you can see from the reading above, the Xbox PSU outputs 5V on standy, so I can't use the PicoPSU on its own, as it only takes 12V. Would it be possible to circumvent that by having a 20pin extension lead between the PicoPSU and the motherboard and then conact the 360 PSU directly to the 5VDC Standby Voltage? Would there be any other points that would require connecting in this manner? (ref - http://pinouts.ru/Power/atxpower_pinout.shtml)
     
  10. Marmotta

    Marmotta Member

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    I'm attempting to build a PC-in-a-360 mod, but need some help with the power supply, as it's causing me a few headaches. I want the use the 360 supply in order to keep the mod looking as tidy as possible. Power shouldn't be an issue, as I'm using a Mini-ITX motherboard with onboard graphics and sound and a 45W CPU, which should require max 110-120W altogether.

    I'm using a PicoPSU power supply, which I hope should take the 12V supplied by the 360 PSU and spread it over the ATX connections required. It requires 12V-input, so there's no need to convert the supply.

    [​IMG]

    The next picture is the connection for the 360 PSU. I've circled the connections I'm using - yellow and black are the 12V & Ground connections for the PicoPSU, purple is 5Vsb and green is /PS_ON. I've also included a picture of the ATX connection pinouts just to give a bigger picture of what is conencting to the motherboard.

    [​IMG]

    [​IMG]

    I've taken a 20-pin ATX extension cable and spliced (not just connected as the pic shows) the 5Vsb and /PS_ON lines to the 360 PSU connection - below is a WIP picture of this:

    [​IMG]



    Now for the problematic parts...

    When I connect the Xbox 360 PSU up the the PicoPSU and motherboard, the PSU outputs 5V, as it always does in standby, so the motherboard LED lights up as is to be expected...

    [​IMG]

    ...however, the 360 Power Supply turns on automatically and displays a green LED, which means that when /PS_ON is connected on the motherboard in order to switch the power supply on (i.e. to alter its current state) it turns the power supply back to standby and an orange LED. A second connection on the motherboard doesn't resolve this state. The CPU fan spins very briefly, but no other sign up booting comes from the PC. The PicoPSU power LED stays on throughout the length of this procedure

    PicoPSU LED:
    [​IMG]

    Motherboard LED:
    [​IMG]

    Xbox PSU:
    [​IMG]

    Any ideas how I can change this, so that when connected to a power source, the 360 PSU will be on standby, so that when a connection is made over /PS_ON, it will go to on rather than standby?


    Another issue is a reading of the 360 PSU connector when powered on, shows that the 12V rail is currently 17V, which would mean that the 12V & 5V are combining somewhere, but I don't know how. Anyone able to shed some light or share thoughts on this as well?

    [​IMG]
     

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