How to get -5v.

Discussion in 'The Projects Forum' started by Austinpatton16, Apr 2, 2016.

  1. Austinpatton16

    Thread Starter New Member

    Apr 2, 2016
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    0
    I need to build a power supply for my colecovision, it requires 12v, 5v, and -5v. I plan on powering it off of a 12v powerbrick and use a step down converter to get 5v. My question is how do i get a -5v?
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    5,003
    745
    You need an isolated dc to dc converter.

    converter
     
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  3. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Right, think of it like adding a 5V battery. Ground of the first circuit is connected to +5 coming from the new 5V source. The opposite pole is now -5V relative to ground of the first circuit. The key word is isolated. Neither pole of the new source is tied to the first circuit's power supply.
     
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  4. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    How much current is needed for the -5 Volts.
    If it is a few mAmps an ICL7660 might be enough.

    It might also be possible to convert a second 5 Volts down converter to -5 Volts, depending on the feedback construction.
    If it has a transformer it might be possible.

    Bertus
     
  5. SLK001

    Well-Known Member

    Nov 29, 2011
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    Use an ICL7660 converter IC. Converts +5v to -5V.
     
  6. Austinpatton16

    Thread Starter New Member

    Apr 2, 2016
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    0
    I just need .1A for the -5v line.
     
  7. SLK001

    Well-Known Member

    Nov 29, 2011
    825
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    Your going to need a transformer with more than one secondary, or a CT secondary for that much current, or another brick converter to provide the -5V.
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    Many DC/DC buck converters can be run in an inverting boost configuration. +12V in, -5V out is a net voltage differential of 17 V, and most of these parts are rated for at least 24 V. National Semiconductor Simple Switcher datasheets have the app circuit. You have given zero information about what you are using to go from +12 V to +5 V, but the odds are pretty good that another piece can go from +12 V to - 5 V.

    Or, for only 100 mA, use a bipolar 555 square wave oscillator running on the +12V, and couple the output through an electrolytic capacitor to a half-wave rectifier. Output will be around -9V, easy to regulate down to -5 V.

    ak
     
    Last edited: Apr 2, 2016
  9. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    The switched capacitor voltage inverters (ICL7660, LTC1144, 555) can not provide the current you are looking for. AK make a good suggestion about the simple switchers. A simple and cheap way to generate a negative voltage.
     
  10. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
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    That's probably pushing it for a 7660, you could build a charge-pump inverter on the output of a bipolar 555 - it can source/sink 200mA (I think the CMOS type can only do half that). Because of losses you won't get the full -5V from 5V - I'd go for a burst mode controller running off the 12V supply. You just need a simple voltage sensor to pull down the 555 reset pin at the required voltage level.
     
  11. SLK001

    Well-Known Member

    Nov 29, 2011
    825
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    The ICL7660A has a max current output of only 100μA.
     
  12. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    The LT1054 will give 0.1A out.
     
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