[Lcdproc] Backlight resistor question

[Dave Liquorice]

On Fri, 20 Apr 2007 09:59:50 -0400, Ethan Dicks wrote:

> 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.

Used as a pure switch (ie driven hard on or hard off) a transistor would 
only be disipating a couple of watts when on and none when off. All you have 
is the voltage drop across the junction multiplied by the current. I'm 
rather rusty on this is the CE connection of a silicon transistor one or two 
junctions? Even at two junctions you only have 1.2 * 2 = 2.4 W. When off no 
current is flowing so no disipation.  B-)

Simple on/off just requires a transistor with suitable collector current 
rating. You might have fun with the logic and need two transistors one 
signal transistor to invert the control signal before feeding on to the 
power switcher. I'm not up on this enough to know if using an NPN or PNP 
power transistor would work without first inverting the control signal.

Cheers
Dave.