Knowledge Base

The LCD module supplied with the MMi99DK216 and early shipments of MMi200DK216 has no provision for backlighting. If you supply your own LCD module you may use one with backlighting.

There are 2 technologies commonly used for backlighting LCD modules: EL (Electro Luminescent) and LED (Light Emitting Diodes).

EL backlighting requires a special high voltage power supply. Please consult your supplier for details.

LED backlighting comes in two flavors. The older type uses an array of LED chips, usually green, mounted on a substrate, and is quite current-hungry. The newer type uses one single high intensity white LED in conjunction with special polarizers and other materials in the LCD proper, with a resulting blue light. Whereas the old type of display can be viewed without the backlighting turned on, the new type is unreadable without the light on.

New, blue light LED backlights using white LEDs

The LCDs used in the MS12, MS120, in later MMi200DK216’s and in MMi201cDK216, use the newer type of backlight. The current consumption of the backlight is about 20mA. On the MS120 the backlight is controlled by output 20, and defaults off. In the MS120 with recent Firmware it is also dimmable. On the MMi200DK216 and MMi201cDK216 the backlight (if fitted) is permanently on. On MMi202s recent firmware allows dimming of the backlight providing a special dimmable LCD is fitted.

The white LED technology used for the blue backlight has a lifetime of approximately 3000 hours. To extend the your product lifetime on an MS120 you should therefore program an automatic turn-off or dim when the LCD is not in use. Example code. For MMi202 you can select whether to use a backlit or non-backlit display. If you have a display with dimmable backlight you can program it to dim down. Dimming or turning off the backlight will extend its life. For programming examples see Example LCD backlight auto-off timers. Dimmable displays for the MMi202 are available from us.

Old, LED array backlights

The older LED-array type of backlighting requires a comparatively high current (100-400mA, depending on the module) at a fairly low voltage. This can be quite difficult to supply. LEDs require a controlled current, not a controlled voltage. That means there must always be a current controlling device between the supply voltage and the LED. Usually this will be a simple resistor, but the selection of the resistor is critical. Do the wrong thing and you will blow up the display instantly. We have never offered this type of backlight on standard products like the MMi because there is no safe, cost effective and practical way of handling a wide range of supply voltages with such a backlight.

Under no circumstances connect an LED backlight directly across the supply voltage, no matter what the voltage is.

Here is an example calculation:

The manufacturer’s data sheet for the LED backlighting specifies a typical operating current of 125mA, with a voltage drop across the LEDs of typically 4.1V. The supply voltage to the SPLat board is 24VDC. Calculate the required series resistance.

The LEDs take about 4.1V. That will leave 19.9V across the resistor. From Ohm’s law the resistor needs to be 19.9V/125mA=160Ohms. The nearest commonly available standard value is 150Ohms.

The heat dissipated in that resistor will be 19.9Vx19.9V/150Ohms=2.64Watts. That means you need to use at least a 5W resistor, and it’s going to get hot.