# Advice 1: How to choose a resistance for the LEDs

To connect the led directly without a resistor, it is possible only to a power source having a high internal resistance. In all other cases current limiting is needed to prevent diode failure. The definition of the nominal current-limiting resistor is calculated.
Instruction
1
To determine the rated current of the led is empirically impossible. This device parameter, you must ask the seller when buying it. If you know the diode type, type it in any search engine - it is likely that it will be located reference data, including rated current.
2
In case no data on the led there, we can assume that the device in the package type SMD rated current equal to 3 mA, round with a diameter of 3 mm to 5 mA, the rectangular cross-section 3 to 5 mm to 10 mA, in a round with diameter 5 or 10 mm to 20 mA.
3
The voltage drop across the led depends on its color. In the infrared it is 1.2 V, red for 1.8, green for 2.2, blue, white and purple - from 3 to 4.
4
The voltage drop across current-limiting resistor is define by the following formula: Ur=Up-Ud, where:
Ur - the voltage drop across the resistor;
Up - voltage power source;
Ud - the voltage drop across the led, V.
5
To calculate the resistance of the resistor, use Ohm's law. Before calculating put of the nominal led current in amperes, what is its value expressed in milliamps, divide by 1000. For example, 20 mA=0,02 A. Then, determine the value of the resistor according to the following formula: R=Ur/IR, where:
R is the desired resistance of the resistor in Ohms,
Ur - the voltage drop across the resistor, calculated by the previous formula;
Inom - nominal current of the led A.
6
The last step is the calculation of power resistor. To do this, multiply the voltage drop across the resistor to current flowing through it (also pre-translated in amps): P=Ur*Inom where:
Ur - the voltage drop across the resistor;
Inom - nominal current of the led A. to Use a resistor of higher power is possible, less not.
7
To improve the overall efficiency of the system if too large a voltage LEDs with the same current rating can be connected in a sequential group so that this chain had about 2/3 of the supply voltage, and the resistor is of the order of 1/3. In this case, before performing the calculation of the nominal voltage of the diodes need to be added.

# Advice 2: How to choose a resistor for the led

Led is a semiconductor device that has firmly entered into our life and slowly began to replace the traditional light bulbs. He has low power consumption and small size, which positively affects the fields of its application.
Instruction
1
Remember that any led that is included in the network must be connected in series resistor required to limit current flowing through the semiconductor device. Otherwise, the probability is that the LEDs may quickly fail.
2
Therefore, before assembling the circuit containing the LEDs, carefully calculate the resistance value, which is determined as the difference between the supply voltage and direct voltage, which is calculated for a particular type of diode. It ranges from 2 to 4 Volts. The difference divide by the current of the device and eventually get the desired amount.
3
Remember, if the resistance value of the resistor accurately select fails, you better take a resistor with a slightly higher value than the desired value. The difference you'll hardly notice, because the brightness of the emitted light will decrease by a small part. Also the amount of resistance can be calculated using Ohm's law, in which the voltage flowing through the diode, is divided by the current.
4
When connecting several LEDs in series, it is also necessary to set the resistance that is calculated in the same way. Remember that there is the total voltage of all the diodes, which is taken into account in the formula for determining the parameters of the resistor.
5
Also, do not forget to connect the LEDs in parallel through a single resistor is prohibited. This is due to the fact that all devices have different dispersion parameters and some of the diodes will glow brighter, therefore it will carry a large amount of current. In the end it will lead to the fact that it will fail. Therefore, when parallel connection of the resistance set for each led separately.
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