LCD Modules: Reversing the polarity of LED backlights

What is Polarity?

Polarity as defined by the dictionary:

  1. the property or characteristic that produces unequal physical effects at different points in a body or system, as a magnet or storage battery.
  2. the positive or negative state in which a body reacts to a magnetic, electric, or other field.

However you state it, polarity is critical; even in LCDs. Last week I was teaching my grandson how to connect jumper cables when jump starting another car. Now some people say you should connect the black of both batteries first, and then connect the red side. Others say you should connect the red and black to the dead battery first and then to the live battery, while others don’t care and just call the auto club to help.

No matter which order you choose to connect the car batteries, one thing is for sure. You always connect the black to black and the red to red. The polarity is critical and you want to make sure not to cross the polarity of one battery with another.

Polarity of LED backlights?

The same holds true for LCD modules containing a LED backlight. LEDs operate on DC (Direct Current) and must be connected with the correct polarity.

The positive side (called the anode) of the LED needs to be connected to the positive side of the power supply. You also need to connect the ground side (called the cathode) to the ground of the power supply. If you reverse the polarity, the backlight will not turn on and in some cases you can damage the backlight.

The backlight does not work on my LCD Display:

When we are contacted by a customer looking for a replacement LCD that has been discontinued by another LCD manufacture, we are able to build a sample to match their current display, but sometimes after the customer receives the Focus LCDs sample, they tell us that the display works, but the backlight is not working. The majority of the time this is due to the polarity of the LED being reversed. Correcting the polarity is not a major problem to fix and many times can be completed by the customer.

Is the LED backlight damaged if the polarity is reversed?

Will the LED backlight be burned out or destroyed if connected in the reverse order? No, the majority of LCD displays contain a protection circuit to shield the LED from incorrect polarity.

It is possible to damage the LED backlight if too much power is supplied, also overpowering the backlight can greatly shorten its half-life.

How to change the polarity of the LED backlight?

If you find that your replacement LCD with LED backlight operates correctly, but the backlight does not turn on, this is a good indication that the LCD’s LED backlight polarity is reversed.

The most common solution is to reverse or remove jumper settings that are on the back of the PCB. Character and most monochrome graphic LCDs have jumper settings on the back of the PCB that can be unsoldered to break the circuit. If you are not sure which jumper settings control the backlight or if you don’t know what a jumper is, then contact our US-based technical support and we can help.

Will I have to change the jumpers on all my LCDs?

No, once you tell us which jumper setting is the correct one, we will build your production units to match that configuration.

Have questions on your LCD design? Contact us today for solutions at 480-503-4295.