[Lcdproc] Backlight resistor question

[Ethan Dicks]

Hi, all,

It's not an LCDproc software question, but I was hoping with the LCD
and electronic expertise here, someone might have a suggestion.  I
just hooked up a Densitron LM4790 4x40 LCD to my lcd2usb module, and
wanted to see about using its LED backlight.  The lcd2usb has a small
circuit onboard that permits the ATMEGA8 to drive a rather ordinary
LED backlight via software control.  The Densitron 4790 is anything
but ordinary.  First off, this thing is *huge*.  The individual pixels
are 1.0mm x 1.7mm.  The chars are nearly 15mm tall, so this thing has
a viewable area about 64mm x 240mm - that's about 2.5" x 9.5" for the
US folks.  The backlight is 220 LED junctions (30% denser than one per
character position) with a rated current draw of 10mA per diode
(normal) for a stunning total of 2200mA.

( http://www.densitron.com/Displays/Displays.aspx?nCategoryID=1 ...
click on the link for the 4790 to pop up a registration window to
download the datasheet).

So... the question is, what can I use to switch over 2A @ 5V?  I could
tap into the lcd2usb backlight transistor to switch something beefier
between the backlight and its power source, but except for a great
honkin' power transistor in a TO-3 or perhaps a TO-220 package w/heat
sink, I'm not sure what's a good choice.  I don't have any Powered USB
ports on my present desktop box, but since they _can_ supply +5V @ 6A
for an external device, at least that's an option for the future (
http://www.usbpluspower.org/ )

In the short term, though, I'm just going to run an independent power
wire from the desktop PSU through something, but I could use some
specific suggestions on what to rig up for backlight control.  To just
get a backlight at all, I can follow Densitron's suggestion of a 0.47
Ohm 4W current-limiting resistor and a hard-wired (or perhaps
mechanically switched) +5V supply.  Digikey has a suitable sand-block
resistor for about $0.36.  I just would like a suggestion or two about
transistor selection.  Stuff of this magnitude is a bit out of my
experience.

It's pretty cool to watch this thing go, though.  If only I can come
up with a suitable housing for it - my first thought is a 30 cm-wide
(12") sheet of lexan heated and bent over to a 45 degree angle.  It's
certainly not going to fit in a drive bay.

Thanks for any input.

-ethan