LED circuit

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In electronics, the LED or LED circuit is an electrical circuit used to power a light-emitting diode (LED). The circuit must provide enough current to turn on the LED at the required brightness, but must limit the current to prevent LED damage. The voltage drop across the LED is approximately constant over the current wide operating range; Therefore, a small increase in applied voltage greatly increases the current. A very simple circuit is used for low power LED indicators. More complex, current source circuitry is required while driving high-power LEDs for lighting to achieve the correct current regulation.

Video LED circuit

Basic network

The simplest circuit to drive an LED consists of a voltage source and two connected components in series: current limiting resistors, sometimes called balast resistors, and LEDs. Optionally, switches can be introduced to open and close the circuit. Although simple, this circuit is not the most energy efficient circuit to drive the LED, because energy is lost in the resistor. More complicated circuits improve energy efficiency.

An LED has a voltage drop specified on the intended operating current. Ohm's Law and Kirchhoff circuit law are used to calculate the exact resistor value to get the desired current. The value is calculated by reducing the LED voltage drop from the supply voltage and dividing it by the desired operating current. If the supply voltage is equal to the LED voltage drop, no resistor is required.

This basic set is used in a variety of applications, including many consumer appliances such as mobile phone chargers.

Maps LED circuit

Resource considerations

The voltage characteristics versus LED current are similar to any diode. Current is roughly an exponential voltage function in accordance with the Shockley diode equation, and small voltage changes can cause major changes in currents. If the voltage is below or equal to the threshold, no current flows and the result is a non-lit LED. If the voltage is too high, the current exceeds the maximum rating, overheats and potentially damages the LED.

As the LED heats up, the voltage drop decreases (the band band decrease). This can drive the current to increase.

It is therefore important that resources provide suitable flows. The LED should only be connected to a constant current source. Series resistors are a simple way to passively stabilize LED currents. The active constant current regulator is commonly used for high power LEDs, stabilizing light output through various input voltages which can increase battery life. The low drop-out current regulator (LDO) also allows the total LED voltage to be a higher fraction of the power supply voltage. Switch mode power supplies are used in some LED flashlights and household LED lights.

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Series inhibitors

Series resistors are a simple way to stabilize LED currents, but energy is wasted in resistors.

LED Thumbnail Indicator is usually driven from low voltage DC through current limiting resistor. The 2 mA, 10 mA, and 20 mA currents are common. The sub-mA indicator can be made by driving an ultrabright LED at a very low current. Efficiency tends to decrease at low currents, but the indicators are running at 100? A is still practical.

In coin-powered cell type LED lights, the resistance of the cell itself is usually the only current-limiting device. Therefore the cell should not be replaced with a lower type of resistance.

LEDs can be purchased with a built-in series resistor. This can save printed circuit board space and is especially useful when building prototypes or filling PCBs in ways other than those intended by the designer. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting LED intensity.

Series resistor count

Rumus untuk menghitung resistensi yang benar untuk digunakan adalah

${\ displaystyle R = {V_ {s} -V_ {f} \ over I}}  ÂÂ$

where V _s is the power supply voltage, e.g. 9-volt battery; << sub> f is an advanced LED voltage (also referred to as "voltage drop") in LED; and i is the desired LED current. Both V <> f and the recommended maximum value for I are obtained from the LED manufacturer's specifications. The above formula requires a current in the amperage, although this is usually provided by the manufacturer in milliamperes, such as 20 mA. Many circuits operate LEDs less than the recommended maximum current, to save power, to allow the use of standard resistor values, or to reduce brightness.

Typically, the forward voltage of the LED is between 1.8 and 3.3 volts. It varies by color of LED. A red LED usually drops 1.8 volts, but the voltage drop usually rises when the light frequency increases, so the blue LED can drop from 3 to 3.3 volts.

The formula is Ohm's law application in which the supply voltage is offset by a voltage drop across the diode, which varies slightly above the useful current range.

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Array LED

Strings of some LEDs are usually connected in series. In one configuration, the source voltage must be greater than or equal to the number of individual LED voltages; usually the LED voltage adds up to about two thirds of the supply voltage. A single current divider resistor can be used for each string.

Parallel operation is also possible but can be more problematic. The parallel LED must have a suitable forward voltage ( V <> f ) to have the same branch current and, therefore, the same light output. Variations in the manufacturing process can make it difficult to get a satisfactory operation when connecting multiple LED types in parallel.

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LED Display

LEDs are often arranged so that every LED (or any LED string) can be turned on and off individually.

Direct drive is the simplest approach to grasp - it uses many single-LED (or single-string) independent circuits. For example, one can design a digital clock in such a way that when the clock displays "12:34" in the seven-segment display, the clock will activate the corresponding segment directly and let it light up until something else needs to be displayed.

However, multiplex viewing techniques are used more often than direct drivers, because they have a lower net hardware cost. For example, most people designing a digital clock design it so that when the clock displays "12:34" in the seven-segment display, even in an hour alternating hours on the right segment of one of the digits - all of the other digits are dark. The clock scans the numbers fast enough to give the illusion that it's "constantly" showing "12:34" for a full minute. However, each "on" segment is actually being pulsed and dying many times per second. Such multiplex displays have lower hardware costs, but the resulting pulse operation makes the display more dimmed than directly driving the same LED independently.

The extension of this technique is Charlieplexing where the ability of some microcontrollers to do tri-state on their output pins means the larger number of LEDs can be driven, without the use of a hook. For pin N, it is possible to push n ² -n LED

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Polarity

Unlike incandescent light bulbs, which illuminate regardless of electrical polarity, LEDs will only light up with the correct electrical polarity. When the voltage at the p-n junction is in the right direction, a significant current flows and the device is said to be forward biased . If the voltage is a polarity, it is said to be rear biased , very little current flows, and no light is emitted. LEDs can be operated at alternating current voltages, but they will only light up with positive voltage, causing the LED to turn on and off at the AC supply frequency.

Most LEDs have low voltage feedback ratings, so the LEDs will also be damaged by the back voltages applied above this threshold. The cause of the damage is the excessive current resulting from the breakdown of the diode, not the voltage itself. LEDs that are driven directly from the AC supply over the reverse damage voltage can be protected by placing diodes (or other LEDs) in reversed parallel.

Manufacturers will usually suggest how to determine the polarity of the LEDs in the product datasheets.

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Pulsed LED Operation

Many LED systems are pulse on and off, by applying power periodically or intermittently. As long as the flicker level is greater than the threshold of the human flicker photoler, and the LED is relatively still against the eye, the LED will appear to continue to burn. Varying the ratio of on/off pulses is known as pulse width modulation. In some cases PWM-based drivers are more efficient than constant or constant current voltage drivers..

Most LED data sheets determine maximum safe DC current for continuous operation. Often they define some higher maximum pulsed pulses that are safe for short pulses, as long as the LED controller keeps the pulses short enough and then turns power to the LED long enough for the LED to cool down.

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LED as light sensor

In addition to emissions, LEDs can be used as photodiodes in light detection. This capability can be used in a variety of applications including ambient light detection and two-way communication.

As a photodiode, the LED is sensitive to the wavelength equal to or shorter than the primary wavelength it emits. For example, green LEDs are sensitive to blue light and some green lights, but not for yellow or red lights.

This LED implementation can be added to the design with only slight modifications to the circuit. An LED can be multiplexed in such circuits, so it can be used for both light and sensing emissions at different times.

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See also

• Joule thieves - switch mode switch power is minimal
• Switch mode power supply (SMPS) application

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References

Source of the article : Wikipedia