HELP need -8.5V from -9V

[mitchjs]

Hi, i fixing a piece of gear that had a dc-dc convertor that put out -8.5v
it was used for the VEE on a LCD display

i cant find one that does -8.5v, so i got one that does -9v

9v is too much, the contrast is off, i need between -8.2v and -8.7v

im sure i cant use a regulator since im at 9v and need only to drop 1/2v

how about a zener diode?

how would i connect it?

i got GND and -9v

i dont know the current draw the the LCD on the VEE line
its not much, my meter shows 3ma
thats not much, is that right?

anyway.. help please

mitch

 

[mitchjs]

is duh, a resistor voltage divider?

50ohms and 1kohms

??

mitch

 

[Pyroandrew]

the potential divider will not work I'm affraid, it will not be able to source enough current.

I would suggest you use a low drop out variable regulator, you can get them with drop outs of only a hundred millivolts or so. LM337 might be a good start, don't know if it is a low drop out reg though.

The zener idea would work make sure you build in good enough protection though, you don't want to blow it up!

 

[instruite]

As you said your load (LCD) consumes 3mA and you wana reduce voltage by 0.5V
Simple way I think is to put a resistor in series of load and source
to select the series resistor
VR = 0.5V
IR = 3mA
R = VR/IR = 0.5/3mA
R = 166.67 ohm you can use a nearby value resistor like 150 ohm or 175 ohm
Let me know if there is any problem with above method

 

[Styx]

sod a reggy/zener far to lossy,
just put a series DIODE and drop the 0.6V through that, alot less power loss and pretty much any-old diode can be used (as long as it can handle the current)

so 9V - 0.6V = 8.4V

 

[mitchjs]

duh a diode in series

sometimes a simple solution blinds...

mitch

 

[panic mode]

how about LM337 so you can set it to anything you want?

 

[Jay.slovak]

how about LM337 so you can set it to anything you want?

LM337 has a minimal dropout of 2.5V, so that won't work.