How to choose a current limiting resistor for an LED

Modern LED light sources are well suited for long-term operation in difficult conditions. However, for current protection, a limiting electrical resistance is used. Accurate calculation of the resistor for the LED will help to select the functional components of the circuit without errors.

Application of a current limiting resistor for an LED

LEDs are used for decorative decoration, to ensure good visibility in a darkened corridor and to solve other practical problems. They are much more economical than classic incandescent bulbs. High strength prevents contamination of the environment with harmful chemical compounds, which is possible after damage to the bulb of the gas-discharge light source.

Taking into account the one-sided conductivity of the semiconductor junction, the need to connect the LED to a storage battery, another DC power source, is understandable. The voltage of a standard household network is rectified, reduced to a nominal level. The resistor is used to limit the current.

Features of work and calculations

Despite the significant advantages, attentive users recommend paying attention to the significant disadvantages of LED devices:

• semiconductor technologies determine nonlinear current-voltage characteristics (VAC);
• an increase in voltage above a certain threshold is accompanied by degradation of the p-n junction;
• at a certain level (with direct or reverse connection), a sharp increase in current strength damages the product.

Of particular importance is the low intrinsic resistance in the operating mode. A relatively small change in the basic parameters of the power supply can damage the semiconductor junction. For this reason, a current limiting resistor is added to the circuit.

An additional passive element increases energy consumption. For this reason, it is recommended to use such solutions in combination with low power LEDs, or to create devices with small duty cycles.

Mathematical calculation

In the simplest circuit, a current-limiting resistor (R) and an LED are connected in series to a constant current source (I) with a certain voltage (Ui) at the output terminals. You can calculate the electrical resistance using the well-known formula of Ohm's law (I = U / R).

Kirchhoff's second postulate is also useful. In this example, it defines the following equality: Ui = Ur + Ucwhere Ur (Uc) - voltage across the resistor (LED), respectively. By simply transforming these expressions, you can get the basic dependencies:

• Ui = I * R + I * Rc;
• R = (Ui - Uc) / I.

Here Rc denotes the differential resistance of a semiconductor device, which changes non-linearly depending on voltage and current. On the reverse part of the current-voltage characteristic, the blocking area can be distinguished. Significant increase Rc in this area prevents the movement of electrons (Iobr = 0). However, with a subsequent increase in voltage at a certain level (Urev-m), a breakdown of the p-n junction occurs.

Since the driver provides DC power, you need to carefully study the corresponding "direct" connection. Features of the I - V characteristic:

• on the first section to Uн resistance decreases smoothly and current increases accordingly;
• from Uн before Um - working area (radiation in the light range);
• further - a sharp decrease in resistance provokes an exponential increase in current strength with the subsequent failure of the product.

LEDs are calculated based on the operating voltage. Uc... Manufacturers indicate this parameter in the accompanying documentation. To calculate the electrical resistance of a suitable current limiting resistor, use the formula: R = (Ui - Uc) / I.

Graphical calculation

If you take the CVC, you can apply a graphical technique. The original graphic and digital information is taken from the passport, or on the official website of the manufacturer. Algorithm of actions (example):

• according to the initial data, the nominal LED current (In) is 25 mA;
• a dotted line is drawn from the corresponding point (1) on the vertical axis of ordinates to the intersection with the I – V characteristic curve (2);
• note the voltage of the power supply (Ui = 5.5 V) on the abscissa axis (3);
• draw a line through points (2) and (3);
• the intersection with the y-axis will show the maximum allowable current (Im = 60 mA).

Further, according to the classical formula, it is not difficult to calculate which resistor is needed for the LED in this case: R = Ui / Im = 5.5 / 0.06 ≈ 91.7... In the series, you need to select the closest denomination with a small margin - 100 Ohm. This solution will slightly reduce the efficiency. But in a gentle mode, functional components will heat up less. The load on the semiconductor junction will be reduced accordingly. Expect to increase the lifetime of the light source.

To select the correct resistor, you need to know the power (P). Standard values ​​(W): 0.125; 0.25; 0.5; one; 2; 5. Calculations can be done for any known parameters using the formulas: P = Im2 * R = Ur2 / R... If we take the initial data of the example under consideration: P = 0.06 * 0.06 * 100 = 0.36 W. Taking into account the typical model range, it is necessary to choose a 100 Ohm resistor with a dissipation power of 0.5 W.

The tolerances for the accuracy of the electrical resistance of the resistors are from 0.001 to 30% of the nominal value. In the marking according to international standards, the corresponding classes are designated in Latin letters (D - 0.5%; G - 2%; J - 5%).

Connecting an LED through a resistor

Taking into account the presented data, several important intermediate conclusions can be drawn:

• resistive protective circuits are used at low power;
• they do not perform stabilization functions;
• the passive element is not capable of damping impulse voltage surges.

Acceptable performance indicators can be obtained by creating:

• sensors;
• indicators;
• signaling devices.

For a small local aquarium lighting, this solution is suitable. However, prolonged consumption of large amounts of energy is unlikely to be acceptable. The lack of stabilization is manifested by a noticeable change in brightness with increasing / decreasing voltage.

Experts recommend using power supplies with reliable current stabilization with a total consumption of more than 1.5-2 W. These devices (dimmers) are used to connect groups of lighting devices and high power semiconductor devices.

Resistor calculation for LED

You can make the necessary calculations online using a specialized calculator. Full use of such programs is offered free of charge.

However, Internet access is not always available.After studying a fairly simple technique, anyone can quickly select a resistor for an LED without looking for the appropriate software.

For a visual demonstration of the algorithm, you need to consider connecting a protective resistor to the power supply circuit (5 V) of a specific LED (Epistar 1W HP).

Technical specifications:

• dissipation power, W - 1;
• current, mA - 350;
• forward voltage (typical / max.), V - 2.35 / 2.6.

To limit the LED current, taking into account the manufacturer's recommendations, a resistor with an electrical resistance R = (5-2.35) / 0.35 = 7.57 Ohm is suitable. According to the E24 standard, the closest values ​​are 7.5 and 8.2 ohms. If you use the standard rules, you will have to choose a larger value, which differs from the calculated one by almost 8.5%. An additional margin of error will be created by a 5% tolerance for mass-produced inexpensive products. With such a deviation, it is difficult to obtain acceptable circuit characteristics in terms of protective functions and power consumption.

The first way to solve the problem is to select several lower resistors. Next, a series, parallel or combined connection option is used to obtain the required equivalent resistance of the circuit section. The second method is to add a trimmer.

Power dissipation calculation

In any of the options, when choosing the electrical resistance of the circuit, a slightly lower current should be set in order to extend the life of the LED. To prevent heat damage, the product is used within the recommended temperature range. For Epistar 1W HP - from -40 ° C to + 80 ° C. If necessary, use the installation on a specialized "star" radiator. This addition increases the effective heat dissipation area.

For accurate selection, the power dissipation of the resistor is estimated: P = I2 * R = (0.35) 2 * 7.57 = 0.1225 * 7.57 ≈0.93 W. The reserve for this parameter is made at least 20-25%. The 1 W rating is not enough, so the next rating in the standard row is chosen - 2W.

The efficiency of the assembled circuit is checked by the ratio Uc / Ui = 2.35 / 5 = 0.47 (47%). The final result shows that more than half of the electricity in this case is wasted. In fact, the indicator is even worse, since not all consumption power is consumed by the LED for radiation in the visible part of the spectrum. A significant part is electromagnetic waves in the IR range.

Parallel connection

At any point in the series circuit, the current is the same. This simplifies the calculation, prevents emergencies. When one element fails, all LEDs are turned off. Therefore damage due to voltage increase is excluded. The noted reasons explain the popularity of this method in the creation of strip lamps and other designs.

The use of a parallel connection provides certain advantages. In this version, the product retains partial functionality if one circuit is damaged. This solution ensures the same voltage at the points of connection to the power source of each branch.

Parallel connection is suitable for organizing independent control schemes. The principles of the New Year's garlands are based on this technology. Separate branches are connected to the power source according to the algorithm specified by the program.

You cannot use one resistor for several parallel diodes. Careful selection of the resistance is due to the need for precise current regulation. In some situations, errors of 0.1-0.5 A cause breakdowns, a radical reduction in the resource.

The actual technical characteristics of LEDs differ significantly even in the same consignment. For this reason, each circuit is protected with a separate resistor.

Features of cheap ICE

Low cost alone is not proof of poor quality. Scaling up production and improving technological processes reduces costs.However, in the corresponding market segment, there are products from manufacturers that do not actually correspond to the declared characteristics.

To determine possible problems, pay attention to the following parameters:

• in cheap models, the main parts of the structure are made of aluminum;
• copper analogs are heavier, remove heat more efficiently, and are resistant to mechanical stress;
• in a quality product, the crystal size corresponds to the standard (0.762 x 0.762 mm or other);
• the disadvantages are indirectly evidenced by the distortion of the proportions of the working area (rectangle instead of square);
• to increase reliability, responsible manufacturers are increasing the number of conductors, using threads made of noble metals.

High-quality LEDs create a luminous flux of 150-220 lumens per 1 W of consumption. Counterfeits - no more than 50-70 lumens. When in doubt, the protection components should be selected with particular care.