This is a repost from what I said over here: http://forums.bit-tech.net/editpost.php?do=editpost&p=543597 So what do you guys think of this?
From what I've heard, those are incredibly unstable and the power they output fluctuates. This esternal PSU would let me not only drive the LCD's contrast, but power the other item that will be integrated into the external stand. LCD contrast, dual red cathode kit I'm using forbacklight, lit buttons, fan controller, USB hub, and oneor two other things I think of.
Depends really on the DC-DC converter. An easier and more reliable converter would be one that can do 12v to -10v DC. That gives the DC-DC converter enough overhead (converting from a high-low voltage is always easier). You can find some *really* stable units from most larger parts suppliers - specs include ripple and current draw, so you can judge which one will be more stable. However, expect to pay through the nose for them. The power supply you have would work fine. The problem with just sticking a resistor out there is twofold: 1. You increase current draw... coupled with what you want to draw from that power line in the first place, and you may exceed what the powersupply can put out. 2. That extra voltage is going to be dumped in the form of heat. So you may need a heat-sink, or some beefy resistors (power resistor type) to handle the temperature. In addition, you may want to over-engineer the resistor specs, as most resistors vary widely with temperature (cheap carbon resistors are not recommended). The best way is always to use 'power regulation'. This method provides the most stable voltage and current profiles for all draw types.
i wouldn't think that the contrast on an LCD would draw all that much current that this PSU couldnt handle? Or could anyone provide me with instrucions or a link for building something better and cheaper? I don't know allthat much about about building circuits or the parts and whatnot.
Why not just use a tiny transformer and regulator (7810) to get 10V. Then connect the 10V output to the ground of your LCD. The 0V output from your small power supply will then be at -10V with respect to ground.
if the transformer is completely seperated from the rest of the circuit... The best would be to use a 7910 (if they exist (7909 could work))... they are the same as the 78xx exept it's made for negative voltages... but if you use a seperate transformer i guess the 78xx is ok...
going on from what smilodon said, a 7909 would do you nicely. Add in a 7805, and you have everything you need, all from a 12V line. Alternately, if you're planning to integrate RS232 communications in, you can always use the V- line from your MAX233 (or whichever driver you're using) as the contrast - this works for most LCD panels (certainly all the ones I've used). The PSU you've posted basically looks like a pretty standard computer PSU, only without the casing, and possibly without some of the soft start circuitry.
Ive got a bunch of those DC-DC converters from allelectronics. They are perfectly fine. The only thing about them is that you have to have caps on the input and all 3 outputs. Then you put the correct load resistance (resistors/pot combination) on the negative output you want. If you dont put all the caps in and use the suggested load resistance, the thing goes nuts. I have had no problems (that Ive noticed) with these $0.50 beauties. The contrast voltage takes almost no current whatsoever. Its more of a voltage reference than anything. But to my knowledge, most negative LCDs want a bit more than just -10V though. As far as the listed power supply you have there, you should be just fine (using a pot). You will only be drawing uAmps, so 700mAmps should be plenty. And it will be worth it if you plan to use this for powering other fun toys. Just dont use any voltage tricks, like the 10V voltage reference tied to ground backwards. If you really want -10V regulated from the -12V source, get a LM7910.
