+5v and -5v to logic chip?

Discussion in 'General Electronics Chat' started by Guinness, Apr 21, 2012.

  1. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
    1
    Hi,

    cant remember name of chip as its at work, but im trying to activate a chip that requires +5v on the vcc and -5v on the inputs. I have 2 power supplies, but not sure how to wire it up as there is obviously only one ground pin on the chip. The inputs on the board go through a resistor to a processor chip that I have no control over, so have to try to put -5v without a external ground pin to work with.

    Any way I could wire up an external resistor or something with the input pins to get the -5v or any other idea's?

    Thanks.
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    You do like this. It is important that ground (GND) is properly connected. This should be the first you connect to the board
    [​IMG]
     
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  3. Guinness

    Thread Starter Member

    Dec 31, 2009
    81
    1
    was thinking of doing that, was not sure how the 5v of the second power supply would effect the chip on its ground pin?

    I wasn't sure if it would be ok as it would be connected to the 0v of the first power supply, also was not sure if the first power supply would be ok with +5v connected to its 0v?
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    As long as the two (or at least one) of the supplies is floating (and batteries qualify), then you are okay. If you have a bench supply (anything plugged into the wall), then make sure that the outputs of (at least one of) the supplies is not connected to the chassis/earth ground plug (if you have one). If you have supplies for which this is not easy to determine, then do the following experiment. Power up both supplies and measure the voltage between the positive terminal of one and the negative terminal of the other. If one of the supplies is floating, then there should be no voltage showing (or only a very tiny voltage due to noise) and you can connect the two together to get +/-voltages relative to the common connection. If you are seeing any appreciable voltage, then neither supply is floating and are somehow being referenced to some other point.

    Be aware that the above description does assume that the circuit you are connecting to is not connected to ground in any way. If it is, then you need to be more careful to either ensure that both power supplies are floating or that the ground reference of the one that isn't is connected to the ground reference of the circuit. The same applies to any function generator or oscilloscope you connect to the circuit. Most oscilloscopes, even $10k ones, do NOT have differential inputs and the shell of the BNC (or whatever) connector is tied to the power supply neutral.
     
    Last edited: Apr 21, 2012
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