Get negative voltage from a battery

Discussion in 'General Electronics Chat' started by Dritech, Nov 22, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,
    Lets assume a 9V battery is used. I know methods that can be used to get -4.5V, ground and 4.5V, but is there a way to get -9V, ground and 9V?
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Depending on the current level you can do what the MAX232 chips do. They make ±supplies from a positive supply.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
    3,211
    619
    It's straightforward to get a negative voltage. What are your constraints?
     
  4. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,866
    988
    The 34063 will do this. An old chip but very versatile and very available.
     
    absf likes this.
  5. ramancini8

    Member

    Jul 18, 2012
    442
    118
    use two batteries
     
    absf, ErnieM and AnalogKid like this.
  6. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks for the replies.
    This will be a portable device so using two batteries will not be best option for my project.
    I will be requiring the negative voltage for an operational amplifier supply. Will be MAX232 be good for this application?
     
  7. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,135
    200
    You didn't mention how much current. www.ti.com has products called "rail splitters" which do exactly that.
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
    3,211
    619
    No. What is the current requirement?
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,515
    1,245
    Depends on the 19 things you are not telling us about the project, like the part numbers, the current requirements, the run time, etc. Charge pump ic's can act as a power inverter, making a negative voltage approximately equal to the positive voltage running the chip, but the output can't make very much current. A switching power control chip in a flyback topology can make a regulated negative output voltage at much higher currents, but makes more conducted and radiated noise that could corrupt your signals.

    This is an electrical engineering forum. If you provide some electrical engineering information in your question, you'll get some electrical engineering answers.

    ak
     
    absf likes this.
  10. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Actually I will be powering 5 op-amps which consumes around 2mA each.
    For the supply I will most probably be using a 7.4V LiPo battery.
     
  11. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Only if you need an RS-232 connection. I mention the chip because it was among the first to generate the ± power supply voltages required by RS-232 interfaces from a +5VDC source. IIRC the original chp produce ±9VDC at a couple of tens of milliamps.
     
  12. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,300
    879
    This chip will probably do what you need:

    http://www.linear.com/product/LTC1144
     
  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,386
    1,605
    Why do you need negative supplies when all you want to run are some op amps?

    Just use op amps that work off a single supply.
     
  14. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,786
    Can't you use single-supply opamps?

    The place to start is with your project requirements. If your project requirements demand max-min voltage rails that exceed the battery you will use, then that is one thing. If your project requirements don't demand this, then don't make things harder than they need to be.
     
  15. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi again,
    My input signals will vary from -50mV to 50mV. If I buy an op-amp with a reference pin for offsetting, I will still need negative supply, no?
     
  16. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,786
    Why?

    What are these input signals referred to?
     
  17. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    They are with reference to ground. I have the impression that if the input goes to negative, the op-amp supply has to have a negative supply. Am I wrong?
     
  18. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,786
    What "ground"? Ground (more properly called "common") is just a node that YOU choose to call 0 V. There is NOTHING that says that either terminal of your battery has to connect to this "ground".
     
  19. shiva007nand

    Member

    Sep 25, 2015
    38
    1
    Opamp needs positive as well as negative supply.
    eg. 8 pin DIP 741 IC
     
  20. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,786
    Many opamps are specifically designed to run from a single supply.

    Even the ones that aren't can be run from a single supply as long as the signals are biased properly. There is nothing magical about a "negative supply". The opamp has no way of knowing whether either of its supply voltages are "positive" or "negative" relative to "ground", just whether they are positive or negative relative to the signals at the rest of the opamp's pins.

    If you have a single 30 V supply and power a 741 opamp from it the amp will be perfectly happy. But your signals should be biased to about the midway point of the supply, or about 15 V, for good results. No need to create a "negative" supply at all.
     
Loading...