Testing an LED with a multimeter is the easiest and most correct way to determine if it is working. The digital multimeter (tester) is a multifunctional measuring instrument, the capabilities of which are reflected in the switch positions on the front panel. LEDs are checked for performance using the functions present in any tester. Let's consider the verification methods using the DT9208A digital multimeter as an example. But first, let's touch on the topic of the causes of failure of new and failure of old light-emitting diodes.
The main causes of malfunction and failure of LEDs
A feature of any emitting diode is a low reverse voltage limit, which is only a few volts higher than the drop across it in the open state. Any electrostatic discharge or incorrect connection during circuit setup can cause the LED (abbreviation for English Light-emitting diode) to fail. Super-bright low-current LEDs, used as power indicators for various devices, often burn out as a result of power surges. Their planar counterparts (SMD LED) are widely used in 12V and 220V lamps, ribbons and flashlights. You can also verify their serviceability with the help of a tester.
It should be noted that a small proportion of defective (about 2%) LEDs are supplied from the manufacturer. Therefore, an additional test of the LED with a tester before mounting it on a printed circuit board will not hurt.
Diagnostic methods
The simplest way, which is most often used by radio amateurs, is to check light-emitting diodes with a multimeter for performance using probes. The method is convenient for all types of light emitting diodes, regardless of their design and number of pins. Having set the switch to the “continuity, check for an open” position, touch the leads with the probes and observe the readings. Closing the red probe to the anode, and the black one to the cathode, the serviceable LED should light up. When changing the polarity of the probes, the number 1 should remain on the tester screen.
The glow of the emitting diode during the test will be small and on some LEDs in bright light it may not be noticeable.
To accurately test multi-color LEDs with multiple pins, you need to know their pinout. Otherwise, you will have to randomly sort through the conclusions in search of a common anode or cathode. Do not be afraid to test high-power LEDs with a metal substrate. The multimeter is not able to disable them by measuring in the dialing mode.
Testing the LED with a multimeter can be done without probes using the transistor test sockets. As a rule, these are eight holes located at the bottom of the device: four on the left for PNP transistors and four on the right for NPN transistors. The PNP transistor is turned on by applying a positive potential to the emitter "E". Therefore, the anode must be inserted into the socket marked "E", and the cathode - into the socket marked "C". The correct LED should light up. To test in the holes for NPN transistors, you need to change the polarity: the anode is “C”, the cathode is “E”. This method is convenient to check LEDs with long and solder-free contacts. It does not matter what position the tester switch is in. 
Checking the infrared LED is the same, but has its own nuances due to invisible radiation. At the moment the probes touch the terminals of the working IR LED (anode - plus, cathode - minus), a number of about 1000 units should be displayed on the device screen. When changing the polarity, the screen should be one.
To check the IR diode in the transistor test sockets, you will additionally have to use a digital camera (smartphone, telephone, etc.). The infrared diode is inserted into the corresponding holes of the multimeter and the camera is directed at it from above. If it is in good condition, then IR radiation will be displayed on the gadget screen as a glowing blurry spot.
Checking powerful SMD LEDs and LED arrays for performance, in addition to a multimeter, requires a current driver. The multimeter is connected in series in an electrical circuit for several minutes and the change in current in the load is monitored. If the LED is of poor quality (or partially defective), then the current will gradually increase, increasing the temperature of the crystal. Then the tester is connected in parallel with the load and the forward voltage drop is measured. By comparing the measured and passport data from the current-voltage characteristic, we can conclude that the LED is suitable for use.
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