Driving an LCD backlight with FETs or transistors?

[blueroomelectronics]

Well as usual audioguru got me thinking about driving transistors to saturation. I've been using MOSFETs (sometimes incorrectly) and I have a problem that may or may not have a simple solution. I wanted to use a 2N7000 but it's only rated for 200ma continuous.  I'd like to switch as close to the full 5V as possible for use with a GLCD display. I will see how bright a 2N4401 with its 0.7v drop will run both a 128x64LCD or a standard 2x16 LCD backlight.

PS the PTC is getting uprated to 250ma (they open at twice their rating)
PPS the Unicorn can be USB powered or Inchworm+ powered (up to 1A)

http://www.blueroomelectronics.com/Unicorn.htm

 

[audioguru2]

Hi Blueroom,
I made some LED "flashlights" (they flash 5 times in half a second then pause to cool for half a second. The LED is blasted with 90mA for 30ms so it is very bright and its 9V battery lasts a long time).

I tried 2N7000 Mosfets and 2N4401 transistors and they both performed the same.

The 2N4401 has a typical saturation voltage of only 0.18V at 250mA with a base current of 20mA. A weak one is like a weak 2N7000 Mosfet that costs much more.

 

[blueroomelectronics]

I'll have to add 2N4401 to the junkdrawer. I've got plenty of 2N2222A. Can I use a 680 resistor to drive it.

 

[audioguru2]

Hi Blueroom,
A PIC has a max output current of 25mA so a base resistor value of (5V - 0.9V)/25mA= 164 ohms will allow the transistor to saturate the best. Use 180 ohms.

I used 680 ohms for the Mood Light because its current per LED never gets anywhere near 350mA.