LED circuit
Encyclopedia
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
, the basic LED circuit is an electric power circuit used to power a light-emitting diode
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or LED. The simplest such circuit consists of a voltage source and two components connected in series
Series and parallel circuits
Components of an electrical circuit or electronic circuit can be connected in many different ways. The two simplest of these are called series and parallel and occur very frequently. Components connected in series are connected along a single path, so the same current flows through all of the...
: a current-limiting resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
(sometimes called the ballast resistor), and an LED. Optionally, a switch may be introduced to open and close the circuit.
The switch may be replaced with another component or circuit to form a continuity tester
Continuity tester
A continuity tester is an item of electrical test equipment used to determine if an electrical path can be established between two points; that is if an electrical circuit can be made...
.
(Although simple, this circuit is not necessarily the most energy efficient circuit to drive an LED, since energy is lost in the resistor. More complicated circuits may be used to improve energy efficiency).
The LED used will have a voltage drop, specified at the intended operating current. Ohm's law
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
and Kirchhoff's circuit laws
Kirchhoff's circuit laws
Kirchhoff's circuit laws are two equalities that deal with the conservation of charge and energy in electrical circuits, and were first described in 1845 by Gustav Kirchhoff...
are used to calculate the resistor that is used to attain the correct current. The resistor value is computed by subtracting the LED voltage drop from the supply voltage, and then dividing by the desired LED operating current. If the supply voltage is equal to the LED's voltage drop, no resistor is needed.
This basic circuit is used in a wide range of applications, including many consumer appliances.
Power source considerations
The voltage versus current characteristics of an LED are much like any diodeDiode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
. Current is approximately an exponential function of voltage (see Shockley diode equation), so a small voltage change results in a large change in current. If the voltage is below the threshold or on-voltage no current will flow and the result is an unlit LED. If the voltage is too high the current will go above the maximum rating, heating and potentially destroying the LED.
As an LED heats up, its voltage drop decreases (band gap
Band gap
In solid state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference between the top of the valence band and the...
decrease). This can encourage the current to increase.
It is therefore important that the power source gives the right voltage. LEDs should only be connected to constant-voltage sources if special care is taken. 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 could 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
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
are used in some LED flashlights.
Series resistor
Series resistorResistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
s 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. The cost of ultrabright LEDs is higher than that of 2 mA indicator LEDs.
In coin cell powered keyring type LED lights, the resistance of the cell itself is usually the only current limiting device. The cell should not therefore be replaced with a lower resistance type.
LEDs can be purchased with built in series resistors. These can save printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
space and are especially useful when building prototype
Prototype
A prototype is an early sample or model built to test a concept or process or to act as a thing to be replicated or learned from.The word prototype derives from the Greek πρωτότυπον , "primitive form", neutral of πρωτότυπος , "original, primitive", from πρῶτος , "first" and τύπος ,...
s 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. Alphanumeric LEDs use the same drive strategy as indicator LEDs, the only difference being the larger number of channels, each with its own resistor. Seven-segment and starburst LED arrays are available in both common-anode and common-cathode form.
Series resistor calculation
The formula to calculate the correct resistance to use is
where power supply voltage (Vs) is the voltage of the power supply, e.g. a 9 volt battery, LED voltage drop (Vf) is the forward voltage drop across the LED, and LED current (I) is the desired current of the LED. The above formula requires the current in amperes, although this value is usually given by the manufacturer in milliamperes, such as 20 mA.
Typically, a LED forward voltage is about 1.8–3.3 volts; it varies by the color of the LED. A red LED typically drops 1.8 volts, but voltage drop normally rises as the light frequency increases, so a blue LED may drop around 3.3 volts.
The formula can be explained considering the LED as a
resistance, and applying Kirchhoff's voltage law (KVL) (R is the unknown quantity):
Multiple LEDs
Strings of multiple of LEDs are normally connected in series. The source voltage must be greater than the sum of the individual LED voltages; typically the LED voltages add up to around two-thirds of the supply voltage. A single current limitingCurrent limiting
Current limiting is the practice in electrical or electronic circuits of imposing an upper limit on the current that may be delivered to a load with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load...
resistor may be used for each string.
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.
Polarity
Unlike incandescent light bulbIncandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe makes light by heating a metal filament wire to a high temperature until it glows. The hot filament is protected from air by a glass bulb that is filled with inert gas or evacuated. In a halogen lamp, a chemical process...
s, which illuminate regardless of the electrical polarity, LED
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s will only light with 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 on an alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
Most LEDs have low reverse breakdown voltage
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
ratings, so they will also be damaged by an applied reverse voltage above this threshold. The cause of damage is overcurrent resulting from the diode breakdown, not the voltage itself. LEDs driven directly from an AC supply of more than the reverse breakdown voltage may be protected by placing a diode (or another LED) in inverse parallel
Antiparallel (electronics)
In electronics, two anti-parallel or inverse-parallel devices are connected in parallel but with their polarities reversed.One example is the TRIAC, which is comparable to two thyristors connected back-to-back , but on a single piece of silicon.Two LEDs can be paired this way, so that each protects...
.
The manufacturer will normally advise how to determine the polarity of the LED in the product datasheet. However, these methods may also be used:
| sign: | + | – |
|---|---|---|
| terminal: | anode (A) | cathode (K) |
| leads: | long | short |
| exterior: | round | flat |
| wiring: | red | black |
| marking:* | none | stripe |
| pin:* | 1 | 2 |
| PCB Printed circuit board A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring... :* |
round | square |
(*)Less reliable methods of determining polarity
Mains supply
LEDs, by nature, require direct currentDirect current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC) with low voltage, as opposed to the mains electricity
Mains electricity
Mains is the general-purpose alternating current electric power supply. In the US, electric power is referred to by several names including household power, household electricity, powerline, domestic power, wall power, line power, AC power, city power, street power, and grid power...
from the electrical grid which supplies a high voltage with an alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC).
A CR dropper (resistor-capacitor circuit) followed by a full-wave rectifier is the usual electrical ballast
Electrical ballast
An electrical ballast is a device intended to limit the amount of current in an electric circuit. A familiar and widely used example is the inductive ballast used in fluorescent lamps, to limit the current through the tube, which would otherwise rise to destructive levels due to the tube's...
with series‒parallel LED clusters. A single series string minimizes dropper losses, while paralleled strings increase reliability. In practice usually three strings or more are used.
An advantage of a capacitor is that it can reduce the high line voltage to an appropriate low voltage, without wasting power, with a very simple circuit; a disadvantage is that there may be a high surge of current for a short time when it is first turned on.
Operation on square wave
Square wave
A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels...
and modified sine wave (MSW) sources, such as many inverters
Inverter (electrical)
An inverter is an electrical device that converts direct current to alternating current ; the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits....
, causes heavily-increased resistor dissipation
Dissipation
In physics, dissipation embodies the concept of a dynamical system where important mechanical models, such as waves or oscillations, lose energy over time, typically from friction or turbulence. The lost energy converts into heat, which raises the temperature of the system. Such systems are called...
in CR droppers, and LED ballasts designed for sine wave use tend to burn on non-sine waveforms. The non-sine waveform also causes high peak LED currents, heavily shortening LED life. An inductor and rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
make a more suitable ballast for such use, and other options are also possible. Dedicated integrated circuits are available that provide optimal drive for LEDs and maximum overall efficiency.
Intensity control
To increase efficiency (or to allow digital intensity control without a more complex digital-to-analog converterDigital-to-analog converter
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold
Flicker fusion threshold
The flicker fusion threshold is a concept in the psychophysics of vision. It is defined as the frequency at which an intermittent light stimulus appears to be completely steady to the observer...
, the LED will appear to be continuously lit.
LED as light sensor
An LED can be used as a photodiodePhotodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
used for light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
detection as well as emission. This capability has been demonstrated and used in a variety of applications including ambient light detection and bidirectional communications. This implementation of LEDs is important because functionality can be added to designs with only minor modifications, usually at little or no cost.
An LED is simply a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
that has been doped
Doping (semiconductor)
In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...
specifically for efficient light emission and has been packaged in a transparent case. Therefore, if inserted into a circuit in the same way as a photodiode
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
, which is essentially the same thing, the LED will perform the same function. As a photodiode, it is sensitive to wavelengths equal to or shorter than the predominant wavelength it emits. For example, a green LED will be sensitive to blue light and to some green light, but not to yellow or red light. Additionally, the LED can be multiplexed in such a circuit, such that it can be used for both light emission and sensing at different times.
Several applications for this technology have been suggested and/or implemented, ranging from use as simple ambient light sensors to full bidirectional communications using a single LED. Most of these applications benefit from this technology because of the cost reduction of using the same component for multiple functions.
Ambient light sensors
LEDs have been used as ambient light sensors. For example, a remote controlRemote control
A remote control is a component of an electronics device, most commonly a television set, used for operating the television device wirelessly from a short line-of-sight distance.The remote control is usually contracted to remote...
keypad
Keypad
A keypad is a set of buttons arranged in a block or "pad" which usually bear digits, symbols and usually a complete set of alphabetical letters. If it mostly contains numbers then it can also be called a numeric keypad...
backlight
Backlight
A backlight is a form of illumination used in liquid crystal displays . As LCDs do not produce light themselves , they need illumination to produce a visible image...
would be turned on by capacitive proximity sensors only in the absence of ambient light. The LED used for the backlight was also used as the ambient light sensor. This resulted in increased functionality for no increase in manufacturing costs.
Bidirectional communications
LEDs can be used as both emitters and detectors of light, which means that a device having only a single LED can be used to achieve bidirectional communications with another device meeting these requirements. Using this technology, any of the ubiquitous LEDs connected to household appliances, computers and other electronic devices can be used as a bidirectional communications port.One application for bidirectional communication with a single LED is fiber optic communications. In typical plastic optical fiber
Plastic optical fiber
Plastic optical fiber is an optical fiber which is made out of plastic. Traditionally PMMA is the core material, and fluorinated polymers are the cladding material...
communications, a single optical fiber is used only for communication in one direction
Simplex
In geometry, a simplex is a generalization of the notion of a triangle or tetrahedron to arbitrary dimension. Specifically, an n-simplex is an n-dimensional polytope which is the convex hull of its n + 1 vertices. For example, a 2-simplex is a triangle, a 3-simplex is a tetrahedron,...
. This is because a single LED transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
is placed at one end of the fiber, and a photodiode receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
is placed at the other end. Thus, two fibers are needed for bidirectional communication. However, if a single LED is placed at each end of a fiber, then the optical fiber can carry information in both directions using half the number of components as a typical system. This reduces system weight, cost and complexity.
Another application of this use of LEDs is a proposed alternative to RFID tags called the iDropper, developed by Mitsubishi Electric Research Laboratories in 2003. The iDropper is a small device that consists of a microcontroller, a battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
, an LED, and a single push-button. The device records or transmits a small amount of data upon command from the user. Compared to RFID tags, the iDropper is more secure
Security
Security is the degree of protection against danger, damage, loss, and crime. Security as a form of protection are structures and processes that provide or improve security as a condition. The Institute for Security and Open Methodologies in the OSSTMM 3 defines security as "a form of protection...
because the user must press a button to reveal personal information, and is similar in cost.
One major limitation of this scheme is the fact that a single LED can only operate as a half-duplex transceiver. A single LED can either transmit or receive information at one time, not both simultaneously. A simple way to put this is that an LED transceiver behaves like a walkie-talkie
Walkie-talkie
A walkie-talkie is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola...
, in contrast to a telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
. This means that it takes a considerable time for two devices to "talk" to each other.