LED Arrays-display-led Printed Circuit Board
LED arrays-display-led printed circuit board
Strings of multiple LEDs are normally connected in series. In one configuration, the source voltage may be greater than or equal to the sum of the individual LED voltages; typically the LED voltages add up to around two-thirds of the supply voltage. A single current-limiting resistor may be used for each string.
The other configuration is to run the sum of the supply voltage at approximately 75 – 85% of the combined LED voltages. This uses the LEDs' combined inherent resistance as a serial resistor. While small voltage drops to each LED generally make no discernible loss of intensity or brightness, with sufficient LEDs in series a noticeable drop in brightness begins to show. In assuming that the supply voltage is 12 V, and each LED is 3 V, by using a string of either five or six LEDs whose combined voltage is 15 V or 18 V, they effectively may be under-driven, in favour of absence of power losses in terms of waste heat from resistors as well as simple circuitry.
Parallel operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have similar branch currents and, therefore, similar light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.
Main article: LED display
LEDs are often arranged in ways such that each LED (or each string of LEDs) can be individually turned on and off.
Direct drive is the simplest-to-understand approach -- it uses many independent single-LED (or single-string) circuits. For example, a person could design a digital clock such that when the clock displays "12:34" on a seven-segment display, the clock would turn on the appropriate segments directly and leave them on until something else needs to be displayed.
However, multiplexed display techniques are more often used than direct drive, because they have lower net hardware costs. For example, most people who design digital clocks design them such that when the clock displays "12:34" on a seven-segment display, at any one instant the clock turns on the appropriate segments of one of the digits -- all the other digits are dark. The clock scans through the digits rapidly enough that it gives the illusion that it is "constantly" displaying "12:34" for an entire minute. However, each "on" segment is actually being rapidly pulsed on and off many times a second. Such multiplexed displays have net lower hardware costs, but the resulting pulsed operation makes the display inevitably dimmer than directly driving the same LEDs independently.
An extension of this technique is Charlieplexing where the ability of some microcontrollers to tri-state their output pins means larger numbers of LEDs can be driven, without using latches. For N pins, it is possible to drive n2-n LEDs