The voltage drop of a light emitting diode is relatively constant, usually about 1.6V for red, 2V and 3V for green, about 2.2V for yellow and orange, and about 3.2V for blue. For the commonly used diode size of a few millimeters, its operating current is generally between 2 mA and 20 mA, the higher the current, the higher the brightness, the power supply voltage minus the diode's voltage drop, and then divided by the set operating current, which results in the current limiting resistor resistance value.
1、Design principles of light emitting diode circuits:
The reverse breakdown voltage of a light emitting diode is about 5 volts. Its forward volt-ampere characteristic curve is very steep, the use of current-limiting resistors must be connected in series to control the current through the tube. Current limiting resistor R can be calculated using the following formula: R = (E - UF) / IF where E is the supply voltage, UF for the LED forward voltage drop, IF for the LED's general operating current.
Note: In the actual design, often improve the current-limiting resistor value to reduce the operating current method to extend the service life, the empirical value of 3-8MA.
2、Characterization:
（1）Brightness is not linearly related to current, it doesn't change much when the current is high up to a certain value. As long as the current exceeds the maximum forward current will burn. Special mainly depends on the information, the general current selected in the 3-20mA.
（2）Light-emitting diodes are current-type devices, usually static display with 10 mA has enough brightness, the limit is below 50 mA. 36V string a 2K or so, 48V string a 3K or so.
（3）The operating voltage of LEDs (Light Emitting Diodes) varies with the material used to manufacture them.
3、Positive and negative judgment of light emitting diodes:
（1）Direct insertion of light-emitting diodes: the longer of the two leads for the positive pole, should be according to the power supply positive. Some light-emitting diodes have two leads of the same length, but there is a small raised tongue on the shell, near the small tongue of the lead is positive.
（2）SMD light-emitting diodes: the one with a bar is the negative pole
4、Work parameters:
（1）Forward working current If: it refers to the value of forward current when the light-emitting diode emits light normally. General LED light-emitting diode working current in a dozen mA to dozens of mA, and low-current LED working current in the following 2mA (brightness and ordinary light-emitting diode the same).
（2）Forward operating voltage VF: The operating voltage given in the general light-emitting diode parameter table is obtained at a given forward current. Generally measured at IF = 20mA. Light emitting diode forward operating voltage VF in 1.4 ~ 3 V. In the external temperature increases, VF will drop.
R≈V/I，For general application take I = 3 to 5 mA, then R = ? ---Empirical value
（3）Commonly used parameters:
Ordinary light-emitting diode forward saturation voltage drop of 1.6V ~ 2.1V, forward operating current of 5 ~ 20mA
Super bright light emitting diodes come in three colors, however all three have different voltage drops. The voltage drop of the red color is 2.0 - 2.2 V, the voltage drop of the yellow color is 1.8 - 2.0 V, and the voltage drop of the green color is 3.0 - 3.2 V. The rated current for normal light emission is 20 mA.
5、Comparison with other luminous lamps:
Compared with small incandescent bulbs and neon lamps, light-emitting diodes are characterized by: low operating voltage (some only a few volts); operating current is very small (some only a few milliamps can be light-emitting); good shock and vibration resistance, high reliability, long life; through the modulation of the strength of the current through the modulation of the light-emitting intensity of light can be easily modulated.