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

ydoubleuz@gmail.com wrote:
> 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)

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.




--
Regards,

John Popelish

On Jan 29, 9:59=A0pm, "ydoubl...@gmail.com" <ydoubl...@gmail.com> wrote:
> Hi all,
>
> I had a circuit of output =3D -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

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

Hi,

> I've seen things like this happen with poor ground connections. =A0Check
> your grounds to be sure they're making good contact.

Checked... all connected.

>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.

Erm, Sorry, but i don't quite understand this part.

>Perhaps the LCD display has a short or near short in it.

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

<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!

<ydoubleuz@gmail.com> wrote in message
news:0cefc172-b716-4df9-897c-2b250acafe31@i3g2000hsf.googlegroups.com...
> 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

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

> What is the part number of the LCD you are using? Can you provide a link to
> the datasheet?

http://www.farnell.com/datasheets/47114.pdf

> 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.

yup the " O " is the connection to GND. This is to achieve the -3V.

> What is supplying power and what is current capability?

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