What resistor should I use for the backlight?

Backlights in LCD displays utilize a variety of technologies, although by the far most popular are LED's. Most electronic practitioners realize that a current-limiting resistor is usually required for an LED. Some, but not all, LCD's have these resistors built into the display, making the choice of the backlight resistor on the LCD board something of trial and error affair. Below are some tables summarizing our testing on the two blue (white backlight) LCD's that Modern Device sells, and recommendations for the backlight resistors for each particular display.

Backlight Resistor for the Modern Device 2 x 16 blue LCD character display

The datasheet lists the "backlight input" at 3.3 volts and 60 mA. This specification is somewhat puzzling but our testing indicates that the LCD display must have an internal resistor in series with the LED's.

Resistor (ohm):	Backlight current in mA:
27 ohm	13 mA
15 ohm	14 mA
10 ohm	14.8 mA
5 ohm	16 mA
0 ohm	15.7 mA

Recommendation: Use only a jumper in RBL for the 2x16 blue displays.

Backlight Resistor for the Modern Device 4 x 20 blue LCD character display

The datasheet lists the "forward LED current" at 30 mA at 4 volts. We recommend a 15 ohm resistor for this display with a smaller resistor yielding a brighter display but reduced LED lifetime.

Resistor (ohm):	mA:
33 ohm	22 mA
27 ohm	24 mA
15 ohm	30 mA
10 ohm	33 mA
5 ohm	38 mA
0 ohm	42.5 mA

Recommendation: Use a 15 ohm resistor in RBL for the 4x20 blue displays.