+ - voltage from 9v batteries???

Discussion in 'General Electronics Chat' started by Guinness, Aug 7, 2012.

  1. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
    1
    Hi,
    Basically i need to use 2 x 9v batteries and a pot to get -9v when the pot is on one side, 0v when its in the middle and +9v when its on the other. Obviously increasing the voltage steadily as the pot moves, I know it wont go from -9 to +9, might loose a volt or 2, thats fine.
    Can use other components if needed.
    Just needs to be simple and cheap.
    Thank you.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,394
    1,606
    That should work... put the 2 9V batteries in series joining a + to a -.

    The joint is zero volts. Connect the pot to the + and - ends. Wiggle the pot and from the pot wiper to the joint you will read -9V to +9V (or whatever the batteries actually read).

    You don't "loose" any voltage this way either.
     
  3. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    Yeah, what Ernie said, just connect the 2 batteries together like so >> http://www.morse-code.com/90cc7a60.png

    then connect each end of the pot to each end of the batteries, and which ever way you turn the pot should give you -V or +V, somewhere in between when centered on the wiper...
     
  4. bretm

    Member

    Feb 6, 2012
    152
    24
    Be careful about short-circuiting the output. Let's say the pot is turned almost all the way to one end or the other, so that one leg is only 1 ohm. If there is a short, a couple of 9V batteries can temporarily drive enough current to burn out that segment of the pot. It might be good to add extra resistors on either end of the pot to protect it, but this will reduce the range.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,052
    3,244
    To reduce the zero voltage point setting sensitivity to relative battery voltage (if that's important) you can add a resistor from the pot output to ground. The lower the resistance, the less sensitive the zero point is to the battery voltages but, of course, that makes the voltage setting somewhat non-linear with respect to the pot rotation value.

    They also make pots with a center tap connection that you can connect to ground for the purpose of providing a stable zero voltage point independent of relative battery voltages.
     
  6. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
    1
    Thank you all, the circuit shown above is how I thought it should work, but for some reason in my head looking at it I was getting +9 either end. Was just me having a moment I guess.

    But good point about adding extra resistors either side of the pot, I will experiment as to values as dont mind loosing a volt either side but no more than that.

    Thanks again all.
     
  7. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
    1
    Having a stable 0v point would be useful as the signal is used for motor drives, so starting at 0v would mean the motor doesn't kick in as soon as power is applied. Would be nice as some are very big powerful industrial motors.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Using a pot across two 9v batteries can get you the whole range from -9v to 9v, but that will be at a pretty high impedance - you won't be able to drive much of a load with it.

    You've just mentioned driving motors; a simple pot just won't work. You should have told us what you were doing long before this.

    Now you need to give us some specifications on the motors; otherwise we have no way of knowing whether or not something is going to be adequate for your needs.

    A 9v battery is a pretty weak power source. Alkaline 9v batteries have a mAh rating around 500; that's 20 hours at 25mA, or enough to light up a few LEDs in series. If you try to load them down more than that, they will go dead very quickly.
     
  9. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
    1
    Sorry your right, I should have explained more. But the batteries wont be driving the motor. Its used as a signal source for industrial motor controllers, they draw very low current, so that wont be a problem. Also have to make a 4-20 mA signal source as some controllers require that, but that's another thing.
     
  10. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Radio Electronics (decades ago) ran a contest to readers to submit the best circuit for coming up with a design that can get a +/- supply from a 9V battery.

    The winner was a guy who slid the battery apart and soldered a wire on the midpoint of the six cells in the 9V battery for a ground point, giving a +/- 4.5V supply with no "loss".
     
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