LED circuit

In electronics, the basic LED circuit is an electric power circuit used to power a light-emitting diode or LED. The simplest such circuit consists of a voltage source and two components connected in series: a current-limiting resistor (sometimes called the ballast resistor), and an LED. Optionally, a switch may be introduced to open and close the circuit.[1] The switch may be replaced with another component or circuit to form a continuity tester.[2]

(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 and Kirchhoff's circuit laws are used to calculate the resistor that is used to attain the correct current.[3][4] 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.[5]

This basic circuit is used in a wide range of applications, including many consumer appliances.[6]

Contents

Power source considerations

The voltage versus current characteristics of an LED are much like any diode. 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 decrease[7]). 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 are used in some LED flashlights.

Series resistor

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. 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 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. 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

\mbox{resistance} (R) = \frac {\mbox{power supply voltage} (V_s) - \mbox{LED voltage drop}(V_f) } {\mbox{LED current}(I)},

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 {V_f \over I} \;\Omega resistance, and applying Kirchhoff's voltage law (KVL) (R is the unknown quantity):

V_s=V_r%2BV_f=R I%2B{V_f \over I} I

R I=V_s-V_f \;

R={V_s-V_f \over I}

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 limiting 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.[8]

Polarity

Unlike incandescent light bulbs, which illuminate regardless of the electrical polarity, LEDs 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 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 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.

The manufacturer will normally advise how to determine the polarity of the LED in the product datasheet. However, these methods may also be used:[9]

sign: + –
terminal: anode (A) cathode (K)
leads: long short
exterior: round flat
wiring: red black
marking:* none stripe
pin:* 1 2
PCB:* round square

(*)Less reliable methods of determining polarity

Mains supply

LEDs, by nature, require direct current (DC) with low voltage, as opposed to the mains electricity from the electrical grid which supplies a high voltage with an alternating current (AC).

A CR dropper (resistor-capacitor circuit) followed by a full-wave rectifier is the usual electrical ballast 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 and modified sine wave (MSW) sources, such as many inverters, causes heavily increased resistor dissipation 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 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 converter), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.

LED as light sensor

An LED can be used as a photodiode used for light detection as well as emission. This capability has been demonstrated and used in a variety of applications including ambient light detection and bidirectional communications.[10][11] This implementation of LEDs is important because functionality can be added to designs with only minor modifications, usually at little or no cost.[10]

An LED is simply a diode that has been doped 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, 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.[10]

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 control keypad backlight 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.[10]

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.[10]

One application for bidirectional communication with a single LED is fiber optic communications. In typical plastic optical fiber communications, a single optical fiber is used only for communication in one direction. This is because a single LED transmitter is placed at one end of the fiber, and a photodiode receiver 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.[11]

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, 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 because the user must press a button to reveal personal information, and is similar in cost.[10]

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, in contrast to a telephone. This means that it takes a considerable time for two devices to "talk" to each other.[11]

See also

References

  1. ^ Singmin, Andrew (1997). "3. Building a Project Using a Basic LED Circuit". Beginning electronics through projects. Oxford [England]: Newnes. p. 29. ISBN 0-7506-9898-5. "As you can see in Figure 3-1, there are just four components in an LED circuit. They are a battery, a switch, an LED, and a resistor" 
  2. ^ Cave, John; Caborn, Colin; Mould, Ian (2000). Design and Technology. Cheltenham: Nelson Thornes Ltd. p. 116. ISBN 0-17-448277-9. "A fuse or filament bulb placed to complete the circuit will show whether the bulb or fuse is good." 
  3. ^ Meade, Russell L. (2004). Foundations of Electronics: Circuits & Devices Conventional Flow. Clifton Park, NY: Thomson Delmar Learning. p. 1051. ISBN 1-4018-5976-3. "The value of the current limiting resistor connected in series with the LED depends on the amount of supply voltage." 
  4. ^ Applied electronics. p. 270. http://books.google.com/books?id=v9dSggu4hB8C&pg=PA270. 
  5. ^ Walsh, Ronald A. (2000). Electromechanical design handbook. New York: McGraw-Hill. pp. 6–242. ISBN 0-07-134812-3. "The light-emitting diode is normally fed from a supply voltage source that is higher than the LED can sustain without burnout." 
  6. ^ Catsoulis, John (2003). Designing embedded hardware. Sebastopol, CA: O'Reilly. ISBN 0-596-00362-5. "This simple LED circuit (or variations of it) drives the LEDs on your PC's front panel, your VCR and DVD player, your cell phone, and a host of other appliances." 
  7. ^ Van Zeghbroeck, Bart J. (1997). "2.2.5". 2.2.5 Temperature dependence of the energy bandgap. Ece-www.colorado.edu. http://ece-www.colorado.edu/~bart/book/eband5.htm. Retrieved 2009-02-15. 
  8. ^ "Electrical properties of GaN LEDs & Parallel connections" (PDF). Application Note. Nichia. Archived from the original on 2007-08-09. http://web.archive.org/web/20070809062214/http://www.nichia.co.jp/specification/appli/electrical.pdf. Retrieved 2007-08-13. 
  9. ^ a b "Plastic infrared light emitting diode". Fairchild Semiconductor. 2001-10-31. http://www.fairchildsemi.com/ds/QE%2FQED233.pdf. Retrieved 2009-05-15. 
  10. ^ a b c d e f Dietz, Paul, William Yerazunis, Darren Leigh (2003). "Very Low-Cost Sensing and Communication Using Bidirectional LEDs". Mitsubishi electric research laboratories. http://www.merl.com/papers/docs/TR2003-35.pdf. 
  11. ^ a b c Bent, Sarah, Aoife Moloney and Gerald Farrell (2006). "LEDs as both Optical Sources and Detectors in Bi-directional Plastic Optical Fibre Links". Irish Signals and Systems Conference, 2006. IET: 345. http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4123923&isnumber=4123844.