Understanding LCD Electronics
An LCD electronics overview for LCD technology
Below is an introductory guide to understanding LCD electronics and how Liquid Crystal Displays operate.
Alternating Current and Direct Current
In short DC stands for Direct Current and AC stands for Alternating Current.
Alternating Current (AC) is not polarized. This means that AC does not have a positive and negative side like a battery. AC voltage is measured in Cycles per second or Hertz (HZ). For example, 60 cycle or 60 hz. AC power alternates (changes) from a positive voltage to a negative voltage 60 times a second ( see Figure 1). Most of the outlets in your house are rated at 120VAC.
Every time AC voltage switches from positive to negative an electrical field is generated in the atmosphere around the wire or component. When the electrical field comes into contact with metal, or wire, a current is generated in the metal or wire. If this effect is not desired the result is interference (called EMI or noise), the same effect is used to couple signals, drive electrical motors and transmit radio signals. This electrical field sustains the voltage over long distances. That is why power lines carry AC.
EL backlights use Alternating Current. The advantage to this is that they cannot be connected wrong. There are no black or red wires. It does not matter which wire is connected it will still work.
Direct Current flows from Positive to Negative in only one direction. When the switch is open (aka open circuit), current cannot flow to the negative end of the battery, and the light is off. When the switch is closed (called a closed circuit), current flows through the filament to the negative end of the battery, thus causing the filament to heat up and emit light ( Figure 2).
Direct Current is always measured with respect to ground. (Ground is zero volts, so voltage is measured by starting at zero) DC has a distinct polarity, either Positive or Negative. A positive voltage is considered to be more positive than ground. A negative voltage is considered to be more negative than ground. While current always flows from positive to negative, we characterize the flow from the source and always assume that current flows towards ground. A negative voltage is considered to flow from the source towards ground ( Figure 3). DC voltage is used in applications where stability in the supply voltage is needed or where the supply is from a battery. DC voltage has to travel from the source to the termination. The ground is considered to be the termination or ending point. All current flows towards the ground plane, the ground plane is what is used to complete the circuit.
Over long distances Direct Current is absorbed by the transmitting medium, and as a result less current finds it way to the termination resulting in a reduction of power. DC voltage does not create electrical interference. The farther DC travels the less electricity arrives on the other end. Think of this as a sprinkler hose with hundreds of little holes in it that allow water to spray out. One end of the hose is connected to the water source, and on the other end of the hose very little water spills out. This is how it is with Direct Current that is carried a long distance. When the electricity reaches the house, it is converted from Alternating Current to Direct Current.
A flashlight has two wires. The wires are connected to a DC battery on one end and a light bulb on the other end. As in Figure 4, one wire is always positive and one is always negative. This is the type of current that is used by LED backlights, which we will discuss in the LCD Manual.
Static LCD displays are AC driven. In fact, the less DC voltage (or DC component) that exist the better. DC voltage will destroy a segmented display.
Reference Coordinates for a LCD display
Engineers will design their product around the dimensions of the LCD display. For this reason, it is critical to make sure there is a reference system in place. In the construction industry, dimensions are referenced as: length, width and height.
In the LCD industry, coordinates are referred to as X, Y and Z.
All physical matter can be measured in three dimensions. The most common way to describe this is to measure the width, length and height of the object.
In electronics people tend to use the letters X (for length),Y (for width), and Z (for height).
X is the horizontal or the length. Y is the depth or width. Z is the vertical or height.
Let’s look at the example of the truck (left). The Z distance is from the front of the truck (the headlight) to the back of the truck (tail light), and the other distances are listed.
Common Coordinates of an LCD
The dimensions of a character COB (Chip on Board) LCD can be seen below.
One critical component is the dimensions between the mounting holes of the display. This is the first item we look at when helping a customer locate a replacement LCD.
LCD Electronics: Frequency
The number of times per second a signal, or voltage, completes a full cycle from zero volts to maximum positive voltage through zero to maximum negative voltage and back to zero again. Frequency is measured in Hertz and/or cycles per second. ( Figure 5)
The frequency in your house is 60Hz. This means the voltages switches between positive voltage and negative voltage 60 times a second. So the voltage is +120V then –120V then back to +120V. This happens 60 times a second.
EL and CCFL backlights are just like fluorescent lights. They use AC current and operate at a high frequency (Hertz). This means the current switches back and forth between positive and negative many times a second.
In the data sheet below ( Figure 6) the LED backlight voltage (V) and current (I) is in DC. The EL backlight power is 110V ACand it shows the frequency to be 400Hz. That means that the voltage on this EL backlight switches between positive and negative 400 times a second.
Segment displays must operate at a specific frequency to provide the sharpest contrast and to avoid any ghosting.
EL backlights also require a specific frequency to provide an even light flow.
The disadvantage to AC current is that it generates what is known as ‘noise’. Noise is radiated out from the light and can interfere with the LCD electronics that are nearby. This noise is also called EMIbut most people just call it noise.
Many customers must use a LED backlight because it does not create any noise. They would like to use the EL but it will cause their circuit not to work correctly.
LCD Electronics: Metrics
In the world of LCD displays, there are two standards: the US and the rest of the world. The US still prefers inches, feet etc. over meters. Although the majority of engineers in the US prefer metric over the US’s SAE system.
No matter which system you use, Focus Displays will work with you to supply you a standard or custom LCD display.
In metrics there are no multiple fractions as everything is divisible in units of 10. The basic metric unit is the meter, which is slightly larger than three feet (1 yard).
Metrics is one word that most American’s do not understand or use in daily life. But in order to work with LCD’s, or any weights and measures used outside of the United States (such as tools, cars, money, travel, water, etc.) one must have an understanding of metrics. The United States and parts of Canada are the only places in the world not to use metrics.
In America a typical linear measure is in inches or feet. There are 12 inches in each foot, and 3 feet in each yard. Then one mile is 1760 yards, or 5,280 feet, or 63,360 inches.
Problems arise when you want to add or subtract numbers. Let’s say you have 7/32 of an inch and you want to add it to 3/16 of an inch. You must now mathematically convert the fractions into common denominators and then you can add them. Or, maybe you want to see how many yards a perimeter measures. When the sides are 10 feet 5 1/4 inches by 11 feet 3 1/8 inches, the math becomes cumbersome.
In metrics all the measurements are divisible by units of 10. The one basic linear unit is the meter. No inches, feet, yards, miles, etc.
Meters come in different sizes. Common examples are: centimeter (c), a millimeter (m), even a picometer (p). The only one we normally use in working with LCD’s is the millimeter.
When a meter is broken down into 1,000 little sections of equal length, each one of these sections is called a millimeter (mm).
With regard to LCD’s, a multiplier is a letter that goes in front of another word, or follows a number, to express its increased or diminished size. There are three main letters we will use.
The capital letter K (Kilo, pronounced kee low) is taking something and multiplying by one thousand to make it much larger. While the small letter m (milli, pronounced mill ee) is taking something and multiplying it by one-one thousandth in order to cut it into many smaller pieces. A third letter used is the Greek m (pronounced meuw), this signifies taking something and multiplying it by one-one millionth in order to cut it into many smaller pieces.
In the following examples of this concept, we will think in terms of dollars instead of meters.
If the cost of a car is $12,000, you could say that it cost $12K. Twelve times 1,000 is equal to 12,000.
If you look at your checking account, and find that you have a $500 balance, you have $1/2 K. One half of a K, is one half of 1,000, which is 500.
Given that metrics are based upon decimals, and not fractions, it is a simple process to signify partial amounts using multipliers. As in the above example, 500 equals 1/2K, which in turn is the same as .5K. Notice the following examples of this principle.
4,500 is the same as 4.5K or 2,700 is the same as 2.7K or 800 is the same as .8K.
Now we will look at the letters that cause our amounts to go decrease. Instead of having large letters for larger numbers, we use small letters to show smaller numbers. This is where you take something and break it up into 1,000 little sections that are all the same size.
Suppose you take an apple and cut it up into 1000 small pieces, with each piece being the same size. If you picked up one piece of the apple you would now be holding a milli-apple or mApple.
If you had 10 pieces of that apple, you would have 10 milli-apples or 10mApples.
LCD’s use very small amounts of current, and are not referred to whole amps of current, but rather milliamps, or mAmps. If an amp is cut into one thousand equal pieces, a mAmp would be only one of those pieces. And 20 mAmps would be 20 of those pieces, or 20/1,000 of an amp.
LCD Electronics: Voltages
As stated above, volts come as either positive or negative. Sometimes a LCD will need both a positive and a negative voltage to work. If a data or spec sheet calls out a voltage of 5V, it is assumed to be a plus 5 volts (+5 volts). The data sheet will put a negative (–) sign in front of the voltage amount if it is negative.
Zero (0) volts are just like the name, and have no voltage. Zero volts is called ground.
One way to think of this is like a checking account. A checking account can be in one of three conditions. Positive (you have money), Negative (you are overdrawn) and zero or ground (there is no money.)
The difference between +5V (plus 5 volts) and –5V (negative 5 volts) is 10V.
For example, say you have 5 dollars (+ 5 V) in your checking account and you know there is a check coming for deposit. So you write a check to someone for 10 dollars (-10 V). When the check you are expecting is delayed, your checking balance is now –5 dollars. So the difference between 5 dollars and –5 dollars is 10 dollars.
LCD Electronics: Temperature
Add room temperature is 25°C or 77°F
There are two main temperature scales that are used in the world. Fahrenheit (F) is the standard used in the United States. Celsius (C) is the standard used by the rest of the world. For the purposes of our business we will us the Celsius scale.
When a particular temperature is required, there is usually a C or an F on the end so that you will know which scale to use. If there is no specific designation as to which temperature scale to use then you need to clarify this with the customer. Temperatures ranges that come from the factory are assumed to be Celcius (C).
On our specs if there is no C or F after the temperature, always assume C.
LCD Electronics: Voltage and Current
In this section we will introduce the electrical concepts of voltage and current. These are two basic components that are related to our product.
To help us gain an understanding of voltage and current, we will introduce the concepts with an illustration using water as the example. For the basic concept, think of voltage as how much water volume we have in gallons, and current as how fast the water moves in gallons per minute.
The symbol for voltage is V, and is measured in units called volts.
Think of a single volt as a gallon of water. When a landscaping job is developed, the various plants will require differing amounts of water. A cactus may need only one gallon of water every 7 days, while a patch of grass may require a gallon of water every other day.
The symbol for current is I, and is measured in units called amps.
Current is how the flow of electricity is measured. A high current is more electricity, and a low current is less electricity. Your electric bill is based on how much current is sent to your home in a given period of time.
LCD Electronics: AMP
Current, then, is how many ‘volts’ are moving along a wire or battery.
Using our water analogy again, we can say that voltage (volts) is how much water is needed to make a plant grow, and current (amps) is how fast the water is moving along the irrigation line.
Our cactus needs a gallon a week to grow, but it must be spread out over the whole 7 days. This is done by regulating the flow with a drip system. The flow (current) is so slow that it takes a week for the whole gallon of water to be dispensed. This slow distribution of water is a low current or flow.
The patch of grass needs a gallon of water every two days. In Arizona it is a waste of water to run your sprinklers during the day, so you turn them on for a few minutes at night. Therefore the flow of water (current) is much higher, because you move the one-gallon of water through the sprinkler in a few minutes, instead of over a week. This is a high current or flow of electricity.
As a general rule, voltage is not the driving force in electronics. It is the current. 120V(volts) at 1mA (read 1 milliamp or 1/1000 of a amp) is barely enough to turn on a small light bulb. On the other hand 120V(volts) at 13 Amps is enough to run a TV or microwave.
Delivering 120V at 1mA is so low a current that if you touched the wires you would not feel any electrical charge. On the other hand if you were to touch the wires of 120V at 13 amps, you probably would not remember anything.
LCD’s use small amounts of volts and amps. Think of a LCD as the cactus from our illustration, the LCD’s usually run on 5 volts or less, and the current is around 50mA at the most.
FocusLCDs Design Solutions
Focus Displays is able to supply you both standard, in stock, LCD displays such as TFTs, OLED, characters, segments, and graphics. We also offer an in-house, US-based, LCD engineering department to help you with your LCD design.
If you find that one of our standard LCDs will not work for your design or you need a replacement LCD, then we are able to design and supply a custom LCD to fit your design requirements.
Call a human today at FocusLCDs.com at 480-503-4295 or use our online quick LCD elector tool to choose a LCD that is in stock and can ship the same day.