PC Power Supply

Discussion in 'General Hardware' started by redsierra, Oct 20, 2011.

  1. redsierra

    redsierra Newbie

    Hi all,
    Can anyone explain the difference and why we have -5, +5, -12, +12 volts in power supplies.
    tks for any help given.
  2. Trinity

    Trinity Little Kiki Staff Member

  3. empire23

    empire23 BRB. Attacking Russia

    Traditionally, 12 volts was for motor based equipment that liked a higher voltage, such as HDDs, Floppy Disk drives, Fans and so forth. Simply because buck/boost converters were horribly inefficient back in the olden days. Since these motors were DC and there are not such things as ramp up starts in small scale electronic motor equipment, keeping a higher voltage for these made sense, you'd have less of a current inrush as the motors started and stopped.

    5V has been traditionally been used for logic and control circuitry. Why? Because is the closest to the required voltage for most logic circuits. Again, we zoom back to the olden days where most logic circuits used linear regulators. The more voltage such regulators had to drop, the more heat they would produce and the more power they'd waste. So dropping to the required voltage from 5v was much more sane than dropping from 12 volts.

    The negative supply stems also from an old need of certain electronics that couldn't take high levels of voltage but at the same time required high levels of swing. Swing is the distance between the voltage rails. For example 12v to ground has a swing of 12 volts, +- 12 volts has a swing of 24 volts. This was quite useful when virtual ground splitters weren't a popular and when PCs totally relied on their PSUs for most their power regulation and step up/down needs.

    These days, most equipment in a PC have their own miniature power supplies and control systems on them, ranging from LDO Regulators and filters, to multiphase MOSFET and feedback regulated switching power circuitry used by high end graphics cards controlled by chips made by the likes of Volterra. Thus the reduced importance of multiple voltage delivery and a high emphasis on the supply of the 12 volt rail these days.

    I don't post very often so but if you have any questions, just ask :D
  4. redsierra

    redsierra Newbie

    Re poer suply voltages

    thank you all for the reply. Much appreciated.

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