What is the average resistance in diode

LED - light emitting diodes

Light-emitting diodes convert electrical energy into light. They work like semiconductor diodes that generate light in the forward direction. The abbreviation LED is the abbreviation for "Light Emitting Diode", which means "light emitting diode" in German. They are used as signal and light transmitters in different areas.

Light-emitting diodes are available in different colors, sizes and designs. The most common designs have a 3 mm or 5 mm diameter. Then there are jumbo LEDs and mini LEDs up to SMD size.

A light-emitting diode switches very quickly from the illuminated to the non-illuminated state. The light beam can be clocked up to the MHz range. However, this is only visible to the human eye as luminous pulp. The brightness of the LED is then lower than it should be with the set current.
The lifespan is an incredible 106 Hours. This is very long compared to normal lamps.


Like any other diode, the LED is polarity dependent. One connection side is the anode, the other side the cathode. If you look into the light-emitting diode, the thicker side is the cathode. From the outside, you can recognize the cathode by the shorter connection or on the flattened side of the housing edge on the underside.

Beware of incorrect polarity! Light-emitting diodes can only tolerate a very low reverse voltage. They can be destroyed with low blocking voltages of 5 to 6 V.

Colors and semiconductor material

The classic colors are red, green, yellow and orange. But there are also blue and white. Depending on the color, the semiconductor crystal of a light-emitting diode consists of different materials. The color of the light or the wavelength of the light is determined by the semiconductor crystal and the doping. The crystal consists of an n- and a p-layer. Therefore it hardly differs from a normal semiconductor diode.
LEDs differ not only in their color, but also in their electrical properties. Sometimes you can't swap the colors with each other. The forward voltage is different and strongly dependent on the semiconductor material.
Red light-emitting diodes (λ = 0.66 µm) are particularly efficient. Infrared light-emitting diodes (λ = 0.9 to 0.94 µm) have the highest efficiency.

Depending on the color, the LED is made up of different mixed crystals:

  • Gallium arsenide (GaAs)
  • Gallium Arsenide Phosphide (GaAsP)
  • Gallium phosphide (GaP)
  • Aluminum-indium-gallium-phosphide (AlInGaP) for red, red-orange, amber
  • Indium-Gallium-Nitrogen (InGaN) for green, cyan, blue, white
  • Gallium Nitride (GaN) for blue

White light emitting diodes

There are two different ways to create white light with LEDs.

mix colors

One way is to mix red, green and blue LEDs. This creates a white light with an adjustable color space. In this form, three light-emitting diodes are combined in a common housing. You can recognize these housings by the six connections that are attached to the LED chip in a star shape. The color temperature and even the color can be set very finely.
The generation of white light with light-emitting diodes of different colors is the most expensive method.

Filter colors

The second option is UV light-emitting diodes or blue LEDs with the semiconductor material indium gallium nitride (InGaN), the housing of which is coated with various phosphors. With a blue LED that would be a yellow phosphor layer. This layer acts as a filter for the blue photons, which are emitted as yellow light. The blue and yellow light mix into a white light. These LEDs are called white diodes.
Other high-power LEDs are made from aluminum gallium indium phosphide (AlGaInP). They shine with a wavelength of 610 or 615 nm. Their color is more orange-yellow. It is warmer than that of blue LEDs.
The color temperature is determined during manufacture. Nothing can be changed afterwards. These pseudo-white LEDs are available with warm, neutral and cold white light with color temperatures between 2,500 and 10,000 Kelvin.

How an LED works

A light-emitting diode consists of an n-conducting basic semiconductor. A very thin p-conducting semiconductor layer with a large hole density is applied to it. As with the normal diode, the boundary layer is flooded with free charge carriers. The electrons recombine with the holes. The electrons release their energy in the form of a flash of light. Since the p-layer is very thin, the light can escape. Light emission is perceptible even at low currents. The light intensity increases proportionally with the current intensity.
Since only a small amount of light emanates from the semiconductor crystal, the metal under the crystal is hemispherical. This diffuses the light. The lens-shaped housing bundles the light. Light-emitting diodes can shine very brightly with just a few milliamps of current.

Circuit symbols



Standard LEDs

Standard light-emitting diodes have a diameter of 5 mm. They are the most commonly used light-emitting diodes in electronic circuits. They start to glow at 8 to 12 mA. If you increase the current, they shine brighter. The maximum luminosity is reached at 20 mA. The difference to 15 mA is only minimal. Usually a current of 10 mA is already sufficient to make them glow sufficiently.

Standard LED (IF./ ILED = 10 mA)
coloursemiconductorUF./ ULED

The table provides information about the flow voltage UF. depending on the color. The exact flow voltage UF. and the flow rate IF. can be read from the data sheet of the LED. Caution, there may be differences depending on the manufacturer.

Low current LEDs

Low-current light-emitting diodes have a diameter of 3 or 5 mm. They light up at 2 mA with up to 5 mcd. If you increase the current, they shine brighter. The maximum luminosity is reached at 20 mA.
Low-current LEDs have the property that they still light up at 2 mA, which standard LEDs do not. They stop glowing at 8 to 10 mA (depending on the manufacturer).
Low-current does not mean that an LED lights up as brightly at 2 mA as it does at 20mA, but that this LED can be operated up to 2 mA and at least lights up weakly.

Low-Current-LED (I.F./ ILED = 2 mA)
colourUF./ ULED

The table provides information about the flow voltage UF. depending on the color. The exact flow voltage UF. and the flow rate IF. can be read from the data sheet of the LED. Caution, there may be differences depending on the manufacturer.

Overview: light emitting diodes / LEDs

TypeUF. at IF. = 20 mAI.F max/ mAUR./ VPdead/ mWcolour
CQY 261.7V1003210red
CQY 282.4V603210green
CQY 292.4V603210yellow
CQY 651.7V403100red
CQY 662.5V403100green
CQY 672.5V403100yellow

The light-emitting diode in use

Light-emitting diodes react very sensitively to an excessively high forward current. A light-emitting diode must therefore never be connected directly to a voltage. A light-emitting diode must always be connected to a series resistor or a current-limiting component. Alternatively, if the operating voltage fluctuates, the light-emitting diode can be supplied with constant current via an FET.

The forward current IF.that flows through the light-emitting diode. When determining the resistance, the respective forward voltage UF. must be taken into account.

The formula calculates the series resistance R.V. over the total voltage Utotal minus the forward voltage UF. by the forward current I.F..
A light-emitting diode already burns at a fraction of the maximum forward current. In addition, light emitting diodes do not necessarily have to shine with their full luminosity. Usually just a few mA are enough to generate sufficient brightness.

The exact calculation method with circuit, explanation and other formulas can be read under series resistor for an LED. If you need the result faster, you can use the LED series resistor calculator.

Why is a series resistor required?

LEDs must always be operated with a series resistor. This also applies if an operating voltage is available that corresponds to the LED flow voltage. On the one hand, the series resistor serves to limit the voltage, in which the operating voltage is divided between the series resistor and the LED. A fixed, previously known voltage drop occurs at the LED. That is the forward voltage of the light-emitting diode, which however is not too constant and suffers from specimen variance. The rest of the operating voltage then drops at the series resistor.
But what is even more important, the series resistor limits the current that flows through the LED. The reason why a current limitation is necessary is quickly explained.

A light-emitting diode is not an ohmic consumer whose resistance is always the same. A light-emitting diode is a semiconductor whose resistance drops to zero when a voltage is applied. This means that the current increases theoretically infinitely. This means that the light-emitting diode is a very power-hungry semiconductor. But the light-emitting diode cannot withstand too much electricity. Too much current destroys the light-emitting diode.
The temperature rises before the destruction. The light emitting diode gets warmer. It is well known that warm semiconductors conduct better than cold ones. So there follows another increase in current, which causes the LED to become hot and ultimately destroyed. This effect does not have to occur inevitably or immediately. To a certain extent, it depends on what type of voltage source is used and how long the light-emitting diode is operated on it. So many inexperienced users will never be confronted with this problem. If you only briefly operate a light-emitting diode without a series resistor, you will not destroy it immediately. It can also go well for several hours.

Typically, the effect of temperature and current increases is caused by voltage fluctuations or normal temperature increases during operation. A series resistor must be used to prevent this in order to limit the current.
But there can also be constellations where a series resistor is superfluous. For example, if the voltage source has a high internal resistance, as occurs in batteries and accumulators. Here the internal resistance of the voltage source acts like a series resistor for the LED. In practice it looks like this that LED flashlights are only connected in series from a white LED and several batteries. And that without a series resistor for the LED. Here the internal resistance of the batteries limits the current through the LED. In addition, such a flashlight is not operated for too long.

Regardless of whether certain operating constellations also work without a series resistor, the current is ideally limited with a constant current source. Since an LED is a current-dependent semiconductor, the easiest way to do this is with a series resistor that limits the current to the value specified by the manufacturer (usually 20 mA). As a rule, a smaller current will do the job (e.g. 10 to 15 mA). The greater the current, the brighter the LED lights up.


  • Display of operating states
  • 7-segment display
  • Lamp replacement
  • Chases
  • Laser pointer
  • Light barriers

Overview: semiconductor diodes

Overview: optoelectronic components

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