LEDs are quite often used in electrical engineering, for example, as indicators. In order for a diode to work and emit light, it has to be plugged into an electrical circuit correctly. To do this, you need to determine the polarity of the LED. Let's look at ways to help you do this.
When buying a large batch of LED devices, it is worth requesting technical documentation from the seller. This will help to know exactly many of the characteristics of the product, not excluding polarity. For a small number of LEDs the passport is usually not given. But by the exact name of the brand of the element, it will not be difficult to find the specifications on the Internet.
In the wiring diagram, LEDs are depicted in two ways.
The triangle indicates the anode, the vertical line - the cathode. The two arrows symbolize the glow.
If technical documentation is not available, it is worth looking closely at the element to begin with. It often helps to understand where the plus side of the LED is. The most common type of LED device - a cylindrical diode of at least 3.5 mm in size - has one contact longer. This design is invented to indicate polarity. The long pin is the positive anode.
You can recognize the plus and minus if you can see what the LED has inside. You can see through the transparent shell that the area of the anode (positive contact) is smaller than that of the cathode (negative contact).
If there is a bevel on the body of the LED, it is a sign of the cathod
The higher the size and power of the LED product, the better the chances of determining the polarity "by eye".
Find the anode and cathode of LED elements with power over 1W.
High-power LEDs are used in electrical engineering. How do you quickly determine their polarity? Quite simple. It is enough to look closely at the diode. In the manufacture, the contacts of elements with a power over 0.5 W are marked. The anode is marked with a "+".
If the LED is made in a SMD package, it is impossible to see what is inside it. As a rule, manufacturers care about electrical engineers and make certain markings. The polarity can be recognized by the cut on the housing, the heat sink or the pictograph. The first two ways are more suitable for larger sizes.
On the body of such diodes, you can find a design slice. This indicates the negative contact (cathode). On the opposite side, respectively, the positive anode will be located.
The heat sink on the reverse side of the case also indicates polarity. It is offset to the anode.
Small SMD diodes (e.g., size 1206) have special pictograms as a clue. They are in the shape of a triangle, the letter P or T. The protrusion indicates the cathode.
The most reliable way to recognize polarity is to use special instruments. With a conventional multimeter you can mark the contacts of diodes with a high degree of accuracy. At the same time you will find out if the element is working and the color of the glow. The tester can be used in 3 ways.
First, check the LED device on the mode "resistance test - 2 kOhm". You should touch the contacts of the LED with the stylus of the multimeter. If the red positive probe of the tester touches the anode of the diode, and the black negative - the cathode, the screen will show a value of 1600-1800 Ohm. In the opposite case the tester will show one. So the probes need to be reversed. If this does not help, then the element is defective. You will not be able to find out the color of the glow with this method.
Second, you can set the multimeter to the "diode test" mode. If the red wire touches the anode and the black wire touches the cathode, the element will glow. The screen will show a number between 500 and 1200 mV.
Third, many testers allow you to measure without styli at all. A multimeter should have a special department for testing PNP and NPN transistors. These have connectors labeled "E" and "C". When testing a cell in the PNP zone, if the cathode is inserted into the "C" socket and the anode into the "E" socket, the LED will start to emit light. Therefore, the polarity is correct. When working in the NPN section the glow will appear with the opposite pin placement: cathode in "E" and anode in "C". This is probably the fastest way to determine the pin assignment. By the way, if the LED in question does not have long pins, you can put needles in the connectors, and the LED element neatly connect to them.
The next obvious method for recognizing cathode and anode would be to connect to the power supply. This method, like the previous one, also lets you know if the LED element is working properly.
Naturally you need a voltage source for the experiment. A power supply with stepless regulation is excellent. Connect the LED and gradually increase the voltage. If it still does not light up at 3-4V, you must have gotten the polarity wrong.
If you do not have such a power supply at hand, you can use a battery or rechargeable battery from a cell phone. Because their voltage can be up to 12 V, the LED should not be connected directly. A resistor should be included in the circuit to prevent breakage. You can choose a suitable resistor in the article "Calculation of the resistor for the LED"
The next obvious method for recognizing cathode and anode would be to connect to the power supply. This method, like the previous one, also lets you know if the LED element is working properly.
Naturally you need a voltage source for the experiment. A power supply with stepless regulation is excellent. Connect the LED and gradually increase the voltage. If it still does not light up at 3-4V, you must have gotten the polarity wrong.
If you do not have such a power supply at hand, you can use a battery or rechargeable battery from a cell phone. Because their voltage can be up to 12 V, the LED should not be connected directly. A resistor should be included in the circuit to prevent breakage.
The resistor should be soldered to one of the pins of the LED element. Touch the pins of the power supply with the resulting construction. If the polarity is correct, the diode will start to emit light. Otherwise, you should swap the pins.
If you have a flat dead watch or motherboard battery (CR2032 type), you can do without the resistor. The voltage of such power supplies does not exceed 6 V, which is safe for the LED. The battery is clamped between the pins of the diode and the polarity can be determined by its glow or lack thereof.
The described methods have their strengths and weaknesses. According to the technical documentation and visually it is impossible to check the performance of the LED. Checking with a voltage requires special care. And a powerful LED can not always be tested with a multimeter. To be successful, it is worthwhile for an electrical technician to master all methods and apply them as needed.
