In electronics, an LED circuit or LED driver is an electrical circuit used to power a light-emitting diode (LED). The circuit must provide sufficient current to light the LED at the required brightness, but must limit the current to prevent damaging the LED. The voltage drop across a lit LED is approximately constant over a wide range of operating current; therefore, a small increase in applied voltage greatly increases the current. Datasheets may specify this drop as a "forward voltage" (
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The simplest circuit to drive an LED is through a series resistor. It is commonly used for indicators and digital displays in many consumer appliances. However, this circuit is not energy-efficient, because energy is dissipated in the resistor as heat.
The LED's
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If the supply voltage is close or equal to the LED's
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The voltage versus current characteristics of an LED is 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 or equal to the threshold no current flows and the result is an unlit LED. If the voltage is too high, the current will exceed the maximum rating, overheating and potentially destroying the LED.
LED drivers are designed to handle fluctuation load, providing enough current to achieve the required brightness while not allowing damaging levels of current to flow. Drivers may be constant current (CC) or constant voltage (CV). In CC drivers, the voltage changes while the current stays the same. CC drivers are used when the electrical load of the LED circuit is either unknown or fluctuates, for example, a lighting circuit where a variable number of LED lamp fixtures may be installed.
As an LED heats up, its voltage drop decreases (band gap decrease[1]). This can encourage the current to increase.
An active constant current source 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. Active constant current is typically regulated using a depletion-mode MOSFET (metal–oxide–semiconductor field-effect transistor), which is the simplest current limiter.[2] 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 (e.g. buck, boost, and buck-boost converters) are used in LED flashlights and household LED lamps. Power MOSFETs are typically used for switching LED drivers, which is an efficient solution to drive high-brightness LEDs. Power integrated circuit (IC) chips are widely used to drive the MOSFETs directly, without the need for additional circuitry.[2]
Series resistors are a simple way to stabilize the LED current, but energy is wasted in the resistor.
Miniature indicator LEDs are normally driven from low voltage DC via a current-limiting resistor. Currents of 2 mA, 10 mA and 20 mA are common. Sub-mA indicators may be made by driving ultrabright LEDs at very low current. Efficiency tends to reduce at low currents, but indicators running on 100 μA are still practical.
In coin cell powered keyring-type LED lights, the resistance of the cell itself is usually the only current limiting device.
LEDs with built-in series resistors are available. These may save printed circuit board space, and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity.
The value for the series resistance may be obtained from Ohm's law, considering that the supply voltage is offset by the diode's
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