Computer Power +ve & -ve requirements of AT & ATX plugs

Discussion in 'Hobby Electronics' started by Terry Collins, May 4, 2005.

  1. Okay, I have the pinouts of both the AT & ATX plugs (and others) of a PC
    power supplies. My understanding of these is that most power is consumed
    on the +ve side of the power supply.

    What I am trying to find out is what proportion of power is consumed by
    the -ve side in your average wintel compatible PC.

    Or to put it another way, if I take a 100Amphr battery and supply the
    +12v & +5v power lines, what size AmpHr battery would I need to supply
    the -12v & -5v? (ignoring battery discharge considerations).

    Yes, I know old laptops are only a few hundred dollars, but it is the
    satisfaction of doing this that is the interesting part.

    In scrapping what was labelled as a a Osbourne 386SX desktop, it turns
    out to have a Cyrix 586GXM-AV motherboard (microformat) with a 266GP
    CPU, so I am considering re-casing the mobo for various
    luggable/portable datalogging uses. I do not want to do the full battery
    to inverter to PC supply route, so the obvious repacement is two 12 volt
    batteries and regulators.

    TIA
    __
    ex-bongo for direct.
     
    Terry Collins, May 4, 2005
    #1
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  2. Terry Collins

    Q! Guest

    "Terry Collins" <> wrote in message
    news:42785808$0$94178$...
    > Okay, I have the pinouts of both the AT & ATX plugs (and others) of a PC
    > power supplies. My understanding of these is that most power is consumed
    > on the +ve side of the power supply.
    >
    > What I am trying to find out is what proportion of power is consumed by
    > the -ve side in your average wintel compatible PC.
    >
    > Or to put it another way, if I take a 100Amphr battery and supply the +12v
    > & +5v power lines, what size AmpHr battery would I need to supply the -12v
    > & -5v? (ignoring battery discharge considerations).


    Do you have ANY understanding of electronics???????

    Current flows from the negative terminal, via the circuitry, back to the
    positive terminal. In theory, whatever current leaves the negative terminal
    should reach the positive (some slight loss due to heat, etc). The battery
    you use to supply the +ve should also supply the -ve rail (in fact, usually
    the -ve is common and the +ve goes via the regulator to provide the desired
    voltage)..

    Perhaps, given your question, you should leave this to someone with some
    basic understanding of how it all works?
     
    Q!, May 4, 2005
    #2
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  3. Q! wrote:

    > Current flows from the negative terminal, via the circuitry, back to the
    > positive terminal.


    Then what is the ground terminal for? {:)
     
    Terry Collins, May 4, 2005
    #3
  4. Terry Collins

    KLR Guest

    On Wed, 04 May 2005 15:02:05 +1000, Terry Collins
    <> wrote:

    >Okay, I have the pinouts of both the AT & ATX plugs (and others) of a PC
    >power supplies. My understanding of these is that most power is consumed
    >on the +ve side of the power supply.
    >
    >What I am trying to find out is what proportion of power is consumed by
    >the -ve side in your average wintel compatible PC.
    >
    >Or to put it another way, if I take a 100Amphr battery and supply the
    >+12v & +5v power lines, what size AmpHr battery would I need to supply
    >the -12v & -5v? (ignoring battery discharge considerations).
    >


    in a typical modern PC - the -12 and -5 draw bugger all, and I would
    be surprised if they are even needed on modern motherboards. (except
    of course for the serial port needing the -12 v to swing from +12 to
    -12, if you are using the serial port that is). you would probably
    get away with a 1-2 AH battery in proportion to the 100ah battery on
    these other rails.



    The other exception being if you had an optional device attatched to
    the machine that specifically needed one or both of these voltages.
    Older motherboards may need these voltages too as in the late 70's
    processors like the 8080, 2708 Eproms etc did need these supplies, but
    again I doubt that they drew any significant current.

    >Yes, I know old laptops are only a few hundred dollars, but it is the
    >satisfaction of doing this that is the interesting part.
    >
    >In scrapping what was labelled as a a Osbourne 386SX desktop, it turns
    >out to have a Cyrix 586GXM-AV motherboard (microformat) with a 266GP
    >CPU, so I am considering re-casing the mobo for various
    >luggable/portable datalogging uses. I do not want to do the full battery
    >to inverter to PC supply route, so the obvious repacement is two 12 volt
    >batteries and regulators.
    >
    >TIA
    >__
    >ex-bongo for direct.
     
    KLR, May 5, 2005
    #4
  5. Terry Collins

    Q! Guest

    "Michael A. Terrell" <> wrote in message
    news:...
    > "Q!" wrote:
    >>
    >> Do you have ANY understanding of electronics???????
    >>
    >> Current flows from the negative terminal, via the circuitry, back to the
    >> positive terminal. In theory, whatever current leaves the negative
    >> terminal
    >> should reach the positive (some slight loss due to heat, etc). The
    >> battery
    >> you use to supply the +ve should also supply the -ve rail (in fact,
    >> usually
    >> the -ve is common and the +ve goes via the regulator to provide the
    >> desired
    >> voltage)..
    >>
    >> Perhaps, given your question, you should leave this to someone with some
    >> basic understanding of how it all works?

    >
    >
    > Do you have any understanding of multiple output power supplies with
    > a single common return? AT and ATX supplies have four or more separate
    > output voltages produced from different windings on the switching
    > transformer.
    >
    > A common AT/ATX power supply has:
    > + 12 Volt output
    > + 5 Volt output
    > - 12 Volt output
    > - 5 Volt output
    >
    > ATX power supplies also have a small, separate 5 volt supply for
    > standby mode. Each of these is connected to the common rail, which is
    > grounded to the power supply case and computer chassis. Some supplies
    > have a 3.3 volt supply for the CPU as well.
    >
    > Learn what you're talking about before you criticize the skills of
    > others.


    And, pray tell, what has that got to do with the fact that the current flows
    from -ve to +ve? All you are stating here is that there is more than just a
    5V and 12V supply rail, with all connected back to the -ve common ( "single
    common return")..

    That changes nothing about my reply, or the original question of "what
    capacity battery to connect to the -ve rail if (x) value is connected to the
    +ve rail".

    Bloody yanks...
     
    Q!, May 5, 2005
    #5
  6. Terry Collins

    Q! Guest

    "Terry Collins" <> wrote in message
    news:4278922d$0$94186$...
    > Q! wrote:
    >
    >> Current flows from the negative terminal, via the circuitry, back to the
    >> positive terminal.

    >
    > Then what is the ground terminal for? {:)


    Ground, which is NOT the same as the negative or common, although sometimes
    the negative is connected to the chassis of the equipment to provide a
    'common' connection. Ground is used on the AC side, not the DC side, and
    strangely enough connects to the GROUND! (i.e. bloody big copper rod driven
    into the ground outside with hefty green wire running to supply the
    electrical ground connection). In case of a fault it provides a path for
    the AC to ground, which helps the residual current device type circuit
    breakers to trip.. (they sense that the current in the active and neutral
    are equal - if some current is leaked to ground then the current in the
    active and neutral will be different and the breaker will trip,
    disconnecting the supply. If there is no path to ground then that doesn't
    happen!)

    Yes folks, that IS a very, very, very basic, simplified (and from the
    professional angle, ridiculous) explanation so don't start - but given the
    obvious level of OP knowledge (or lack thereof) I'm not entering into a full
    on theory lesson with pretty drawings and connect the dot type
    lessons........... If someone else wishes to do so, please go ahead!!
     
    Q!, May 5, 2005
    #6
  7. Q! wrote:

    >
    > And, pray tell, what has that got to do with the fact that the current flows
    > from -ve to +ve?


    Sigh, as they say "you can lead a horse to water but you can not make
    him drink".

    2nd try. If I have two 12v batteries in series and I connect a GROUND to
    the common lead, then I have +12V and -12V with respect to the common
    ground.
     
    Terry Collins, May 5, 2005
    #7
  8. KLR wrote:
    > in a typical modern PC - the -12 and -5 draw bugger all, and I would
    > be surprised if they are even needed on modern motherboards. (except
    > of course for the serial port needing the -12 v to swing from +12 to
    > -12, if you are using the serial port that is). you would probably
    > get away with a 1-2 AH battery in proportion to the 100ah battery on
    > these other rails.


    Thanks. It was what I suspected, but was looking for someone with
    practical knowledge to give some feedback.

    I will probably end up having the second battery as I am planning to use
    the serial inputs for sensors and keyboard inputs. It could also be used
    to run lighting, etc anyway.
     
    Terry Collins, May 5, 2005
    #8
  9. Q! wrote:

    > Yes folks, that IS a very, very, very basic, simplified (and from the
    > professional angle, ridiculous) explanation so don't start - but given the
    > obvious level of OP knowledge (or lack thereof)


    You're obviously an engineer.
    [aps to REAL engineers].
     
    Terry Collins, May 5, 2005
    #9
  10. Michael A. Terrell wrote:

    > I would use a DC to DC converter to supply the negative voltages at
    > the very least. Some ICs use multiple supply voltages and can be
    > damaged by a missing supply. Two batteries will not discharge at the
    > same rate. There are 12 DC in PC power supplies on the market, but a
    > small inverter and a standard power supply is a cheap and easy way to
    > go. Its cheaper than designing the separate switch regulators, and
    > regulates the 12 volt supplies as well.


    I will keep your points in mind if it becomes something permanent.
     
    Terry Collins, May 6, 2005
    #10
  11. "Terry Collins" <> wrote in message
    news:42796ee7$0$94186$...
    > Q! wrote:
    >
    >>
    >> And, pray tell, what has that got to do with the fact that the current
    >> flows from -ve to +ve?

    >
    > Sigh, as they say "you can lead a horse to water but you can not make him
    > drink".
    >
    > 2nd try. If I have two 12v batteries in series and I connect a GROUND to
    > the common lead, then I have +12V and -12V with respect to the common
    > ground.


    True, and 24v between the + of batt A and - of batt B!! Commonly used with
    Op-Amps, etc, which need a + and - supply...

    Did I really want to enter this debate???????????
     
    Martin, VK2UMJ, May 7, 2005
    #11
  12. Martin, VK2UMJ wrote:
    > "Terry Collins" <> wrote in message


    >>2nd try. If I have two 12v batteries in series and I connect a GROUND to
    >>the common lead, then I have +12V and -12V with respect to the common
    >>ground.

    >
    >
    > True, and 24v between the + of batt A and - of batt B!! Commonly used with
    > Op-Amps, etc, which need a + and - supply...
    >
    > Did I really want to enter this debate???????????


    Nope, I have no interest in debating whether the "computer motherboard
    manufacturers" are correctly using the term "ground" or not. Lets face
    it, with the dabble it could just as easily leak magic smoke before the
    debate warms up.
     
    Terry Collins, May 8, 2005
    #12
  13. Q! wrote:
    > "Terry Collins" <> wrote in message
    > news:4278922d$0$94186$...
    >
    >>Q! wrote:
    >>
    >>
    >>>Current flows from the negative terminal, via the circuitry, back to the
    >>>positive terminal.

    >>
    >>Then what is the ground terminal for? {:)

    >
    >
    > Ground, which is NOT the same as the negative or common, although sometimes
    > the negative is connected to the chassis of the equipment to provide a
    > 'common' connection. Ground is used on the AC side, not the DC side, and
    > strangely enough connects to the GROUND! (i.e. bloody big copper rod driven
    > into the ground outside with hefty green wire running to supply the
    > electrical ground connection). In case of a fault it provides a path for
    > the AC to ground, which helps the residual current device type circuit
    > breakers to trip.. (they sense that the current in the active and neutral
    > are equal - if some current is leaked to ground then the current in the
    > active and neutral will be different and the breaker will trip,
    > disconnecting the supply. If there is no path to ground then that doesn't
    > happen!)
    >
    > Yes folks, that IS a very, very, very basic, simplified (and from the
    > professional angle, ridiculous) explanation so don't start - but given the
    > obvious level of OP knowledge (or lack thereof) I'm not entering into a full
    > on theory lesson with pretty drawings and connect the dot type
    > lessons........... If someone else wishes to do so, please go ahead!!
    >
    >
    >

    That is the correct definition of GROUND. But I have noticed a lot of
    cheap stuff getting about calling negative/neutral the ground.

    In Aus though, the ground is neutral, the earth (ground) is hooked into
    the neutral bridge in the fuseboard, thus why f%$king up homemade
    GPO/etc installation jobs can cause interesting results. The powerpoles
    at 11kva have 3 active phases and then in many towns comes to 3 240v
    (3phase) lines plus a neutral return to the usually fairly large
    transformer.

    But since we are talking about 12vDC, ground would be a poor
    interpretation of negative.

    Q!, you are obviously aware of this as many others in the group are, but
    just a reminder to some why seem not to know.

    --
    *******************************
    Glen O'Riley
    Penrith, NSW, 2750
    ********************************
    ACREM - NSW Operations Coordinator
    NSW Rural Fire Service - Firefighter
    NSW State Emergency Services - Member
    Angel Flight - Earth Angel
    Storm Chaser
    ********************************
     
    Tsunami Australia, May 9, 2005
    #13
  14. Martin, VK2UMJ wrote:
    > "Terry Collins" <> wrote in message
    > news:42796ee7$0$94186$...
    >
    >>Q! wrote:
    >>
    >>
    >>>And, pray tell, what has that got to do with the fact that the current
    >>>flows from -ve to +ve?

    >>
    >>Sigh, as they say "you can lead a horse to water but you can not make him
    >>drink".
    >>
    >>2nd try. If I have two 12v batteries in series and I connect a GROUND to
    >>the common lead, then I have +12V and -12V with respect to the common
    >>ground.

    >
    >
    > True, and 24v between the + of batt A and - of batt B!! Commonly used with
    > Op-Amps, etc, which need a + and - supply...
    >
    > Did I really want to enter this debate???????????
    >
    >

    It appears to me that he only wanted to know the average power drain of
    a computer system to establish if he needs a 6ah or 18ah battery, the
    figures stated are just an example.

    I would be interested myself to find the average power drain/usage for
    such similar projects for use in country areas where the power is
    unreliable (up and down like a yoyo).

    --
    *******************************
    Glen O'Riley
    Penrith, NSW, 2750
    ********************************
    ACREM - NSW Operations Coordinator
    NSW Rural Fire Service - Firefighter
    NSW State Emergency Services - Member
    Angel Flight - Earth Angel
    Storm Chaser
    ********************************
     
    Tsunami Australia, May 9, 2005
    #14
  15. Terry Collins

    Franc Zabkar Guest

    On Thu, 05 May 2005 10:55:54 +1000, Terry Collins
    <> put finger to keyboard and composed:

    >KLR wrote:
    >> in a typical modern PC - the -12 and -5 draw bugger all, and I would
    >> be surprised if they are even needed on modern motherboards. (except
    >> of course for the serial port needing the -12 v to swing from +12 to
    >> -12, if you are using the serial port that is). you would probably
    >> get away with a 1-2 AH battery in proportion to the 100ah battery on
    >> these other rails.

    >
    >Thanks. It was what I suspected, but was looking for someone with
    >practical knowledge to give some feedback.
    >
    >I will probably end up having the second battery as I am planning to use
    >the serial inputs for sensors and keyboard inputs. It could also be used
    >to run lighting, etc anyway.


    The -5V rail is most likely not used. In any case, it is not bussed to
    the PCI slots.

    If you are lucky, the motherboard's serial ports may not require the
    -12V rail. I've seen several that use an RS232 IC (eg ADM213EARS) that
    generates the -12V rail from the +5V supply via an internal charge
    pump.

    Be aware that some motherboards have a BIOS or jumper option to
    generate an on-board "Power Good" signal. Make sure this is disabled,
    ie rely on the PSU's own PG pin instead.


    - Franc Zabkar
    --
    Please remove one 's' from my address when replying by email.
     
    Franc Zabkar, May 10, 2005
    #15
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