LED supply voltage. How to know the voltage

Calculation of the LED supply voltage is a necessary step for any electric lighting project, and, fortunately, this is easy to do. Such measurements are necessary to calculate the power of the LEDs, because you need to know its current and voltage. LED power is calculated by multiplying current by voltage. In this case, you must be extremely careful when working with electrical circuits, even when measuring small quantities. In the article, we will consider in detail the question of how to find out the voltage in order to ensure the correct operation of LED elements.

LED operation

LEDs exist in different colors, they are two and three-color, blinking and changing color. So that the user can program the sequence of the lamp, various solutions are used that directly depend on the supply voltage of the LED. To illuminate the LED, a minimum voltage (threshold) is required, while the brightness will be proportional to the current. The voltage on the LED increases slightly with current, because there is internal resistance. When the current is too high, the diode heats up and burns out. Therefore, the current is limited to a safe value.

The resistor is placed in series, since a much higher voltage is required for the diode array. If U is the opposite, the current does not flow, but for high U (for example, 20 V) an internal spark (breakdown) occurs, which destroys the diode.

As with all diodes, current flows through the anode and exits through the cathode. On round diodes, the cathode has a shorter wire, and the casing has a cathode side plate.

The dependence of voltage on the type of lamp

With the increase in the number of high-brightness LEDs designed to provide replacement lamps for commercial and indoor lighting, there is an equal, if not more, distribution of power solutions. With hundreds of models from dozens of manufacturers, it becomes difficult to understand all the permutations of the input / output voltage of the LED and the output current / power, not to mention the mechanical dimensions and many other functions for dimming, remote control and circuit protection.

There are a large number of different LEDs on the market. Their difference is determined by many factors in the production of LEDs. Semiconductor makeup is a factor, but manufacturing technology and encapsulation also play a major role in characterizing LEDs. The first LEDs were round, in the form of models C (diameter 5 mm) and F (diameter 3 mm). Then, rectangular diodes and blocks, combining several LEDs (networks), were received.

The hemispherical shape is a bit like a magnifying glass that defines the shape of a light beam. The color of the radiating element improves diffusion and contrast. The most common designations and LED forms:

• A: red diameter 3 mm in the holder for CI.
• B: 5mm red diameter used in the front panel.
• C: purple 5 mm.
• D: two-tone yellow and green.
• E: rectangular.
• F: yellow 3 mm.
• G: white high brightness 5mm.
• H: red 3 mm.
• K- anode: a cathode indicated by a flat surface in the flange.
• F: 4/100 mm anode connecting wire.
• C: reflective cup.
• L: curved shape acting like a magnifying glass.

Device specification

A set of various LED parameters and supply voltages is in the seller’s specifications. When choosing LEDs for specific applications, it is necessary to understand their difference. There are many different specifications of LEDs, each of which will affect the choice of a particular type. LED specifications are based on color, U, and current. LEDS tend to provide one color.

The color emitted by the LED is determined in terms of its maximum wavelength (lpk), that is, the wavelength that has the maximum light output. Typically, process variations produce peak wavelength changes of up to ± 10 nm. When choosing colors in the LED specification, it is worth remembering that the human eye is most sensitive to shades or color variations around the yellow / orange region of the spectrum - from 560 to 600 nm. This may affect the choice of color or position of the LEDs, which is directly related to the electrical parameters.

LED current and voltage

During operation, LEDs have a predetermined drop U, which depends on the material used. The supply voltage of the LEDs in the lamp also depends on the current level. LEDs are devices controlled by current, and the light level is a function of current, its growth increases the light output. It is necessary to ensure that the device operates in such a way that the maximum current does not exceed the permissible limit, which can lead to excessive heat dissipation inside the chip itself, a decrease in the light flux, and a shortened service life. Most LEDs require an external current limiting resistor.

Some LEDs may include a series resistor, so it indicates what voltage supply the LEDs need. LEDs do not allow a large reverse U. It should never exceed its declared maximum value, which is usually quite small. If there is a chance of a reverse U appearing on the LED, it is better to integrate the protection into the circuit to prevent breakage. Usually it can be simple diode circuits that provide adequate protection for any LED. You do not need to be a professional to learn it.

LED power supply

Lighting LEDs have current power, and their luminous flux is proportional to the current flowing through them. The current is related to the supply voltage of the LEDs in the lamp. Several diodes connected in series have an equal current flowing through them. If they are connected in parallel, each LED receives the same U, but different current flows through them due to the dispersion of the effect on the current-voltage characteristics. As a result, each diode emits a different luminous flux.

Therefore, when selecting the elements, you need to know what supply voltage the LEDs have. For each to work on its terminals, approximately 3 volts are required. For example, a 5-diode series requires approximately 15 volts at the terminals. To supply regulated current with sufficient U, LEC uses an electronic module called a driver.

There are two solutions:

1. An external driver is installed outside the luminaire, with a safe ultra-low voltage power supply.
2. Internal, built into the lamp, i.e. the subunit with the electronic module that regulates the current.

This driver can be powered by a 230 V mains supply (class I or class II) or with a safety ultra-low U (class III), for example, at 24 V. LEC recommends a second solution for power supply, as it provides 5 main advantages.

The advantages of LED voltage selection

The correct calculation of the supply voltage of the LEDs in the lamp has 5 key advantages:

1. Safe ultra-low U, possibly regardless of the number of LEDs. LEDs must be installed in series to ensure the same level of current in each of them from one source. As a result, the more LEDs, the higher the voltage at the terminals of the LEDs. If this device is with an external driver, then the ultra-sensitive safety voltage should be much higher.
2. Integration of the driver inside the lights allows for a complete installation of the system with safe extra low voltage (SELV), regardless of the number of light sources.
3. More reliable installation in the wiring standard for LED lamps connected in parallel. The drivers provide additional protection, especially against temperature increase, which guarantees a longer service life while observing the LED supply voltage for different types and current. More secure commissioning.
4. The integration of LED power into the driver avoids improper handling in the field and improves their ability to withstand hot plugging. If the user connects the luminaire with LEDs only to an external driver that is already on, this can cause overvoltage of the LEDs when they are connected and, therefore, their destruction.
5. Simple service. Any technical problems are more easily seen in LED lamps with a voltage source.

Power and heat dissipation

When the drop in U on the resistance is important, you need to choose the right resistor that can dissipate the required power. A current consumption of 20 mA may seem low, but the calculated power suggests otherwise. So, for example, for a voltage drop of 30 V, the resistor must dissipate 1400 Ohms. Calculation of power dissipation P = (Ures x Ures) / R,

Where:

• P is the value of the power dissipated by the resistor, which limits the current in the LED, W;
• U is the voltage across the resistor (in volts);
• R is the value of the resistor, Ohm.

P = (28 x 28) / 1400 = 0.56 W.

The supply voltage of the 1 W LED would not withstand overheating for a long time, and 2 W would also fail too quickly. For this case, it is necessary to connect in parallel two 2700 Ohm / 0.5 W resistors (or two 690 Ohm / 0.5 W resistors in a row) to evenly distribute heat dissipation.

Thermal control

Finding the optimum power for the system will help you learn more about the heat control that will be needed for the reliable operation of LEDs, since the LEDs generate heat, which can be very dangerous for the device. Too much heat will cause the LEDs to produce less light, and also reduce operating time. For a LED with a supply voltage of 1 W of power, it is recommended to look for a radiator with parameters of 3 square inches for each watt of LEDs.

Currently, the LED industry is growing quite rapidly and it is important to know the difference in LEDs. This is a common question, as products can range from very cheap to expensive. You need to be careful in buying cheap LEDs, as they can work perfectly, but, as a rule, they do not work for a long time and burn quickly due to poor parameters. In the manufacture of LEDs, the manufacturer indicates characteristics with average values ​​in the passports. For this reason, customers do not always know the exact characteristics of LEDs in terms of luminous flux, color, and forward voltage.

Forward Voltage Detection

Before you find out the voltage of the LED, set the appropriate multimeter settings: current and U. Before testing, set the resistance to the highest value in order to avoid burnout of the LED. This can be done simply: clamp the terminals of the multimeter, adjust the resistance until the current reaches 20 mA and record the voltage and current. In order to measure the forward voltage of the LEDs you will need:

1. LEDs to check.
2. Source U of an LED with parameters higher than the constant voltage LED indicator.
3. Multimeter.
4. Alligator clamps to hold the LED on the test wires to determine the supply voltage of the LEDs in the fixtures.
5. Wires.
6. Variable resistor 500 or 1000 ohms.

The primary current of the blue LED was 3.356 V at 19.5 mA. If a voltage of 3.6 V is used, the value of the resistor to use is calculated by the formula R = (3.6 V-3.356 V) / 0.0195 A) = 12.5 Ohms. To measure high power LEDs, the same procedure is performed and the current is set, quickly holding the value on the multimeter.

Measuring the supply voltage of high power smd LEDs with a direct current> 350 mA can be a bit complicated, because when they heat up quickly, U drops sharply. This means that the current will be higher at a given U. If the user does not have time, he will have to cool the LED to room temperature before measuring again. You can use 500 ohms or 1 kOhm. To provide coarse and fine tuning or to connect a variable resistor of higher and lower ranges in series.

Alternative definition of voltage

The first step in calculating the energy consumption of LEDs is to determine the voltage of the LED. If there is no multimeter at hand, you can study the manufacturer's data and find the passport U of the LED block. Alternatively, you can evaluate U based on the color of the LEDs, for example, the white voltage of the white LED is 3.5 V.

After the voltage of the LED is measured, the current is determined. It can be measured directly with a multimeter. Factory data give an approximate estimate of current. After that, you can very quickly and easily calculate the power consumption of LEDs. To calculate the energy consumption of the LED, simply multiply the U of the LED (in volts) by the current of the LED (in amperes).

The result, measured in watts, is the power that LEDs use. For example, if an LED has U 3.6 and a current of 20 milliamps, it will use 72 milliwatts of energy. Depending on the size and scale of the project, voltage and current readings can be measured in smaller or larger units than the base current or watts. Unit conversions may be required. When performing these calculations, remember that 1000 milliwatts is equal to one watt, and 1000 milliamps is equal to one ampere.

LED test with a multimeter

To test the LED and find out if it works and what color to choose, a multimeter is used. It must have a diode test function, which is indicated by the diode symbol. Then, for testing, fix the measuring cords of the multimeter on the legs of the LED:

1. Connect the black cord on the cathode (-) and the red cord on the anode (+), if the user makes a mistake, the LED does not light.
2. They give a small current to the sensors and if it is visible that the LED is slightly lit, then it is working.
3. When checking the multimeter, the color of the LED must be taken into account. For example, the yellow (amber) LED test - the threshold voltage of the LED is 1636 mV or 1.636 V. If a white LED or blue LED is tested, the threshold voltage is higher than 2.5 V or 3 V.

To test the diode, the indicator on the display should be in the range from 400 to 800 mV in one direction and not show in the opposite direction. Normal LEDs have threshold U, described in the table below, but for the same color can have significant differences. The maximum current is 50 mA, but it is recommended that it does not exceed 20 mA. At 1-2 mA, the diodes already glow well. Threshold U LED

If the battery is fully charged, then at 3.8 V the current is only 0.7 mA. In recent years, LEDs have made significant progress. There are hundreds of models with a diameter of 3 mm and 5 mm. There are more powerful diodes with a diameter of 10 mm or in special cases, as well as diodes for mounting on a printed circuit board up to 1 mm long.

Running LEDs from AC Power

LEDs are usually considered direct current devices operating on several volts of direct current. In low-power applications with a small number of LEDs, this is a perfectly acceptable approach, for example, in mobile phones where power is supplied by a DC battery, but other applications, such as a linear strip lighting system, extending 100 m around the building, cannot operate on such a scheme.

A DC drive suffers from distance loss, which requires the use of a higher U drive from the very beginning, as well as additional controllers that lose power. Alternating current makes it easy to use transformers to lower U to 240 V or 120 V AC from kilovolts used in power lines, which is much more problematic for direct current. To start any type of LED with a supply voltage (for example, 120 V AC), electronics are required between the power source and the devices themselves to provide constant U (for example, 12 V DC). The ability to control multiple LEDs is important.

Lynk Labs has developed a technology that allows you to power the LED from AC voltage. A new approach is to develop AC-LEDs that can work directly from an AC power source. Many self-contained LED downlights simply have a transformer between the wall outlet and the fixture to provide the required constant U.

A number of companies have developed LED bulbs that screw directly into standard connectors, but they invariably also contain miniature circuits that convert alternating current into direct current before entering the LEDs.

The standard red or orange LED has a threshold U from 1.6 to 2.1 V, for yellow or green LEDs the voltage is from 2.0 to 2.4 V, and for blue, pink or white it is a voltage from about 3.0 to 3.6 V. The table below shows some typical voltage values. The values ​​in parentheses correspond to the closest normalized values ​​in the E24 series.

The power supply characteristics for the LEDs are shown in the table below.

Designations:

• STD - standard LED;
• HL - LED indicator of high brightness;
• FC - low consumption.

This data is enough for the user to independently determine the necessary device parameters for the lighting project.