Despite the fact that the electrical parameter #1 for an LED is the rated current, it is often necessary to know the voltage at its terminals for calculations. The term "LED voltage" means the potential difference at the p-n junction in the open state. It is a reference parameter and, together with other characteristics, is indicated in the passport for a semiconductor device. But sometimes copies of which nothing is known fall into the hands. How to find the voltage drop across an LED? This will be discussed.
Theoretical Method
An excellent clue in this case is the color of the glow, the external shape and dimensions of the semiconductor device. If the LED body is made of a transparent compound, then its color remains a mystery, which a multimeter will help to solve. To do this, the switch of the digital tester is switched to the “open circuit test” position and the probes touch the LED leads in turn. A healthy element in forward bias will have a slight glow of the crystal. Thus, it is possible to draw a conclusion not only about the color of the glow, but also about the performance of the semiconductor device. There are other ways to test emitting diodes, which are described in detail in.
Light emitting diodes of different colors are made from various semiconductor materials. It is the chemical composition of the semiconductor that largely determines the supply voltage of the LEDs, more precisely, the voltage drop across the p-n junction. Due to the fact that dozens of chemical compounds are used in the production of crystals, there is no exact voltage for all LEDs of the same color. However, there is a certain range of values, which are often sufficient for preliminary calculations of the elements of an electronic circuit. On the one hand, the size and appearance of the package does not affect the forward voltage of the LED. But in other way. through the lens one can see the number of emitting crystals that can be connected in series. The phosphor layer in SMD LEDs can hide a whole chain of crystals. A prime example is the company's miniature multi-chip LEDs, which often have voltage drops well in excess of 3 volts.
In recent years, white SMD LEDs have appeared, in the case of which 3 crystals are connected in series. They can often be found in Chinese 220 volt LED lamps. Naturally, it will not be possible to verify the health of the led crystals in such a lamp using a multimeter. The tester's standard battery delivers 9V, and the minimum trigger voltage of the three-chip white light-emitting diode is 9.6V. There is also a two-crystal modification with a response threshold of 6 volts.
You can find out all the technical characteristics of the LED from the Internet. To do this, you need to download a datasheet for a model that is similar in appearance, be sure to have the same glow color, compare the passport dimensions with the actual ones and write out the nominal values of current and voltage drop. It should be borne in mind that this technique is very approximate, since 20 mA and 150 mA LEDs with a voltage spread of up to 0.5 volts can be made in the same package.
practical method
The most accurate data on the forward voltage drop across an LED can be obtained by making practical measurements. To do this, you will need an adjustable DC power supply (PSU) with a voltage of 0 to 12 volts, a voltmeter or multimeter and a 510 ohm resistor (or more). The laboratory circuit for testing is shown in the figure. 
Everything is simple here: the resistor limits the current, and the voltmeter monitors the forward voltage of the LED. Gradually increasing the voltage from the power source, observe the increase in readings on the voltmeter. When the threshold is reached, the LED will start emitting light. At some point, the brightness will reach the nominal value, and the voltmeter readings will stop increasing sharply. This means that the p-n junction is open, and a further increase in voltage from the PSU output will be applied only to the resistor.
The current reading on the screen will be the nominal forward voltage of the LED. If we continue to increase the power supply of the circuit, then only the current through the semiconductor will increase, and the potential difference across it will change by no more than 0.1-0.2 volts. Excessive excess current will lead to overheating of the crystal and electrical breakdown of the p-n junction.
If the operating voltage on the LED is about 1.9 volts, but there is no glow, then the infrared diode is probably being tested. To verify this, you need to direct the radiation flux to the turned on camera of the phone. A white spot should appear on the screen.
In the absence of an adjustable power supply, you can use the "crown" at 9V. You can also use the network adapter in the measurements, which produces a rectified stabilized voltage, and recalculate the resistance value of the resistor.
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