how to get negative V from +v source

Discussion in 'The Projects Forum' started by electronis whiz, Jul 29, 2010.

  1. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
    i am trying to desing a replacement switch mode psu for a computer. i am just trying to see if my idea is posible. how do you get - voltage from a positive source. EG in a computer -5 and -12 volts.
    i'v taken apart many old psus and i noticed it looked like both -5&12 v are derived by a negitive voltage regulator from the +12 or +5 v.
    is this right. if not then how is this done.
  2. SgtWookie


    Jul 17, 2007
    Sure, it's possible, using an inductive inverter. -5v and -12v are legacy voltages; really only around to support RS232, which is going away in favor of USB, Firewire and other more modern technologies.

    Here's a great site for you to read up on SMPS's:

    For a PC supply, you could start with an isolating converter to provide isolation from the mains - and then derive your other voltages from that point.
  3. tom66

    Senior Member

    May 9, 2009
    -12V is generated from the primary SMPS transformer. The transformer usually has four or five taps, +12V, +5V, COM, -12V. The +3.3V rail is often derived from a magamp regulator; if this is not the case then there is also a +3.3V tap. -5V from the -12V rail is easy using a 7905 regulator.

    -12V is only for RS232 and for a few legacy devices. PCI cards take -12V as well, but PCI Express drops this.
  4. Wendy


    Mar 24, 2008
    A simple voltage doubler will also do this, though they have significant losses and tend to be noisy. With a decent source a voltage doubler will create almost any polarity and voltage.
  5. electronis whiz

    Thread Starter Well-Known Member

    Jul 29, 2010
    i want to make it more eficiant than a switchmode psu. would this be more eficent using regulators than transformers. should i use a delay line to get the +5 v power good siginal.
  6. tom66

    Senior Member

    May 9, 2009
    Your average SMPS power supply is very efficient; on average 80% under full load. If you're messing with mains voltages be careful. A typical computer SMPS power supply has voltages of 350V peak in it, even in the US, because a voltage doubler is used when you select '115V'.

    The +5V power good signal should be provided by a comparator. When the +5V line exceeds say +4.75V power good should be pulled high.
  7. russ_hensel

    AAC Fanatic!

    Jan 11, 2009
    Look up charge pump -- good for doubling or inverting voltages. Usually for low currents in range of ma to 10's of ma.
  8. marshallf3

    Well-Known Member

    Jul 26, 2010