LED circuit

Electrical circuit used to power a LED

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 an LED is approximately constant over a wide range of operating current; therefore, a small increase in applied voltage greatly increases the current. Very simple circuits are used for low power indicator LEDs. More complex, current source circuits are required when driving high power LEDs for illumination to achieve correct current regulation.

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. An LED has a voltage drop specified at the intended operating current. Ohm's law and Kirchhoff's circuit laws are used to calculate the appropriate resistor value, by subtracting the LED voltage drop from the supply voltage and dividing by the desired operating current. With a sufficiently high supply voltage, multiple LEDs in series can be powered with one resistor. If the supply voltage is close or equal to the LED forward voltage, then no reasonable value for the resistor can be calculated, so some other method of current limiting is used.

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). 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. 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.

Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs will only light with the correct electrical polarity. When the voltage across the p n junction is in the correct direction, a significant current flows and the device is said to be forward biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated with alternating current, but they will only light on the half of the AC cycle where the LED is forward biased. This causes the LED to turn on and off at the frequency of the AC supply. Most LEDs have relatively low reverse breakdown voltage ratings compared to standard diodes, so it may be easier than expected to enter this mode and cause damage to the LED due to overcurrent. However, the cut in voltage is always less than the breakdown voltage, so no special reverse protections are necessary when driving an LED directly from an AC supply when properly current limited for forward biased operation. The manufacturer will normally advise how to determine the polarity of the LED in the product datasheet. However, there is no standardization of polarity markings for surface mount devices.

Many systems pulse LEDs on and off, by applying power periodically or intermittently. So long as the flicker rate is greater than the human flicker fusion threshold, and the LED is stationary relative to the eye, the LED will appear to be continuously lit. Varying the on/off ratio of the pulses is known as pulse width modulation (PWM). In some cases, PWM based drivers are more efficient than constant current or constant voltage drivers. Most LED data sheets specify a maximum DC current that is safe for continuous operation. Often they specify some higher maximum pulsed current that is safe for brief pulses, as long as the LED controller keeps the pulse short enough and then turns off the power to the LED long enough for the LED to cool off.

In addition to emission, an LED can be used as a photodiode in light detection. This capability may be used in a variety of applications including ambient light detection and bidirectional communications. As a photodiode, an LED is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. For example, a green LED is sensitive to blue light and some green light, but not to yellow or red light. This implementation of LEDs may be added to designs with only minor modifications in circuitry. An LED can be multiplexed in such a circuit, such that it can be used for both light emission and sensing at different times.