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Voltage drop

Discussion in 'Electronic Basics' started by [email protected], Jan 30, 2008.

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  1. Guest

    Hi all,

    I had a circuit of output = -3V (for a lcd supply). However, when i
    connect the lcd up, the voltage to that pin drops to -0.7V. Initially
    I thought it may be due to the insufficient current, hence i changed
    the DC convertor in the circuit to a higher power. The voltage still
    drops to -0.7V.

    Any reasons what caused the drop in voltage?

    PS: the drop is actually increase. (-3.0 to -0.7)

    Tks
    ywz
     
  2. Replace the display with a variable resistor, momentarily,
    and see what resistance loads the supply to the same
    voltage. This will tell you about what the resistance of
    the supply is. Perhaps the LCD display has a short or near
    short in it.
     
  3. Pj

    Pj Guest

    I've seen things like this happen with poor ground connections. Check
    your grounds to be sure they're making good contact.
     
  4. Guest

    Hi,
    Checked... all connected.
    Erm, Sorry, but i don't quite understand this part.
    The LCD is able to display black boxes when powered directly to a
    power supply.

    I tried replacing the whole power circuit with a simple potential
    divider circuit. Again the voltage drops upon connection.

    Vcc (8V) -- Connect to Vcc of LCD
    |< 5K resistor
    |
    O -- Connect to GND of LCD
    |< 3K resistor
    |
    GND -- Connect to Vo (-ve voltage supply of LCD)

    Any ideas?

    Tks
    ywz
     
  5. <snap>

    Any ideas?

    Is is a supply pin? Or perhaps a contrast pin?

    In case of supply, maybe you missread, it should be +3V and you're running
    into some internal protection diode?

    If it's contrast, maybe your LCD has contrast hardwired on the LCD board?

    If none of the above, I don't know.

    Goodluck!
     
  6. What is the part number of the LCD you are using? Can you provide a link to
    the datasheet?
    Which pin numbers are you connecting power and ground to? In a later post,
    it looks like you are connecting a 5k resistor between Vcc and ground of LCD
    and a 3k between ground and Vo/-ve. However, it doesn't look like you have
    the ground pind of the LCD connected to ground (unless that is what the O
    is). That looks like it could be causing you some problems.
    What is supplying power and what is current capability?

    Richard
     
  7. Guest

    yup the " O " is the connection to GND. This is to achieve the -3V.
    In the original power supply circuit, I used a negative voltage op amp
    (TL084) to invert the 5V to -3V. The dc supply to provide the +- Vcc
    is by tying 2 dc convertor, 0.6A.

    Another connection i tried for the negative voltage is:

    http://www.reconnsworld.com/power/voltinvert.gif

    Btw, the LCD is controlled using a PIC18F4331. The Vcc (5V) and Gnd of
    both circuits are tied together and connected to the LCD pins
    respectively. This is because the LCD only has a single gnd rail for
    its Vo, Vcc and Data lines.

    Some background: The LCD is tested workable with the latter connection
    (http://www.reconnsworld.com/power/voltinvert.gif
    ) with a PIC16C54C.

    ywz
     
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