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PostPosted: Thu Dec 13, 2012 5:57 pm 
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hi
i've got two STN LCDs, the first is requiring Vee=-20V while the second wants Vee=-40V.
Negative voltage is needed for contrast.

So, i need a negative voltage generator of -20V, -40V, on a current of 5..6mA.

Any idea for the proper circuit ?


Last edited by legacy on Thu Dec 13, 2012 8:03 pm, edited 1 time in total.

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PostPosted: Thu Dec 13, 2012 7:04 pm 
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What voltages do you have to start with, and how good does the regulation need to be? (I imagine that for that, the regulation does not need to be good at all.) What I've done sometimes is to make an oscillator with decent output drive, even if that means a relaxation oscillator using a 5V schmitt-trigger inverter followed by the other five inverter sections in the hex inverter IC in parallel to provide more drive (74AC is strong, and 74ABT is even stronger), then follow those with a voltage multiplier with capacitors and Schottky diodes. It is compact and the efficiency is not bad. You probably won't even need to regulate it for your application.

Unfortunately switched-capacitor voltage-multiplier ICs won't work for the high voltage you need, and switching regulators are hardly in the hobbyist's reach because good board layout is so extremely critical to good operation, and you can't do it on prototyping board of any kind.

I do plan to put some power supply designs in my circuit potpourri page of the 6502 primer, including this kind of thing. I also have a small module here that I made to start with anywhere from 7 to 17 volts (allowing a wide range of battery, wall-wart, and vehicle power) and put out +5V and ±12V that I wanted to supply to hobbyists but unfortunately it will be too labor-intensive for me to assemble it to sell without losing my shirt, and I know there won't be the sales volume to justify the expense of setting up for automated assembly.

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PostPosted: Thu Dec 13, 2012 7:49 pm 
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I found this at digikey.com

http://www.digikey.com/product-detail/e ... ND/1927144

5v in, -24,-48,-72vdc outputs - cost is $14.45

Daryl

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PostPosted: Thu Dec 13, 2012 8:07 pm 
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@GARTHWILSON
the LCD power is 3.3V, the board power is 5V, so Vin may be 5V =D

@8bit
NMT0572SC module ? Wow, it is perfect for my goal, thank you a lot =D


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PostPosted: Thu Dec 13, 2012 8:46 pm 
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8BIT wrote:
I found this at digikey.com

http://www.digikey.com/product-detail/e ... ND/1927144

5v in, -24,-48,-72vdc outputs - cost is $14.45

Daryl

Good find, Daryl. I think that device would work in this application. The regulation is pretty good as well.

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PostPosted: Thu Dec 13, 2012 8:59 pm 
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BigDumbDinosaur wrote:
8BIT wrote:
I found this at digikey.com

http://www.digikey.com/product-detail/e ... ND/1927144

5v in, -24,-48,-72vdc outputs - cost is $14.45

Daryl

Good find, Daryl. I think that device would work in this application. The regulation is pretty good as well.


Just be sure to follow the datasheet. There is a MINIMUM current draw requirement that you need to be aware of. If using just the -24v output, minimum is 4.2mA If using just the -48v output, minimum is 2.1mA

Cheers!

Daryl

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PostPosted: Thu Dec 13, 2012 9:04 pm 
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and fortunately my STN LCD eats 5mA as Iee =D
here it is my STN LCD hitachi sp14q006-t 's data sheet.

Also i have an other STN LCD (i do not remember its part number now) that wants -40V, with 6mA as Iee =P


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