Segment LCD Displays: Avoiding Excessive DC Component

This article, a continuation in Focus Display Solutions LCD Design Guide series, covers minimal recommended DC component values for a (static) segment LCD Displays module, and is valid for all types of standard and custom segment LCD Displays including 4:1 mux, 3:1 mux and direct drive interfaces.

Segment LCDs: DC component and dissimilar metals

I worked at a hardware store my senior year in High School and was assigned to the plumbing department. No training was provided, but how hard could plumbing be? Well . . . at least I could smile and act like I understood.

One day a customer asked me for an elbow to mate to the copper pipe in his house. This was easy and I went in search of a ¾ inch elbow. I returned with two different elbows: galvanized and black pipe and then asked which one he preferred.

He looked at me, shook his head before walking off and said “I want to repair, not destroy the pipes in my house.” I had no idea what he was talking about until someone explained the dangers of accelerated corrosion caused by mixing dissimilar metals.

Segment LCD displays can be damaged and their life greatly shortened by allowing too high of a DC (Direct Current) component to be applied for an extended period of time.

What is a DC Component in a segmented LCD?

A DC component, DC offset, DC coefficient or DC bias is the difference between the amplitude of an AC signal. The greater the difference, the larger the DC component. For the majority of electronics products, DC component is undesirable.

Below is a graph showing DC offset next to a sine wave.Additional information can be found at EHow.

Segment LCD Displays are AC driven

Segment LCD Displays are controlled by the level of the AC (Alternating Current) voltage across each segment; an excessive DC component will damage the display and should not exceed 50mV. (Think golf, the lower the number the better).

The Wikihow website offers a good article on how you can measure the DC offset of your circuit: http://www.wikihow.com/Measure-DC-Offset

Results of excessive DC component on a segmented LCD

Extended operation of an excessive DC component on a static display, or segment LCD, can damage the display by electro-plating the electrode contact. Noticeable results of a damaged display can include a blurry segment (decrease in contrast) and the segments can become burned in.

AC voltage must be applied across all segments

To maximize the life of your segment LCD Display, the AC voltage must be applied across all the segments of your display. Avoid any ‘floating’, disconnected segments.

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Focus Display Solutions offers US based Engineer design and support for your segment LCD display. Focus Display Solutions is a designer and supplier of TFT’s, OLED’s, Character, Dot-matrix and segment LCDs