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LED circuit-Power source considerations-circuit board pcb

LED circuit-Power source considerations-

circuit board pcb

Power source considerations

The voltage versus current characteristics of an LED are similar to any diode. Current is approximately an exponential function of voltage according to the Shockley diode equation, and a small voltage change may result in a large change in current. If the voltage is below the threshold or on-voltage no current flows and the result is an unlit LED. If the voltage is too high the current exceeds the maximum rating, overheating and potentially destroying the LED.

It is therefore important that the power source provides an appropriate current. LEDs should only be connected to constant-current sources. Series resistors are a simple way to passively stabilize the LED current. An active constant current regulator is commonly used for high power LEDs, stabilizing light output over a wide range of input voltages which might increase the useful life of batteries. Low drop-out (LDO) constant current regulators also allow the total LED voltage to be a higher fraction of the power supply voltage. Switched-mode power supplies are used in some LED flashlights and household LED lamps.

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


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