A light-emitting diode, or LED for short, is a semiconductor device that converts electrical energy into light energy. When a certain forward current passes through the tube, the energy can be released in the form of light. The luminous intensity is approximately proportional to the forward current. The luminous color is related to the material of the tube.
First, the main characteristics of LED
(1) The working voltage is low, and some only need 1.5-1.7V to turn on the light; (2) The working current is small, the typical value is about 10mA; (3) It has unidirectional conductive characteristics similar to ordinary diodes, but the dead zone The voltage is slightly higher; (4) It has similar voltage stabilization characteristics as silicon zener diodes; (5) The response time is fast, the time from voltage application to light emission is only 1-10ms, and the response frequency can reach 100Hz; then the service life is long , Generally up to 100,000 hours or more.
At present, the commonly used light-emitting diodes are red and green phosphorescent phosphor (GaP) LEDs, which have a forward voltage drop of VF = 2.3V; red phosphorescent arsenic phosphor (GaASP) LEDs, whose forward voltage drop is VF = 1.5-1.7V; and for yellow and blue LEDs using silicon carbide and sapphire materials, the forward voltage drop VF = 6V.
Due to the steep forward volt-ampere curve of the LED, a current-limiting resistor must be connected in series to avoid burning the tube. In a DC circuit, the current-limiting resistance R can be estimated using the following formula:
R = (E-VF) / IF
In AC circuits, the current-limiting resistance R can be estimated by the following formula: R = (e-VF) / 2IF, where e is the effective value of the AC power supply voltage.
Second, the test of light-emitting diodes
In the case of no special instrument, the LED can also be estimated by a multimeter (here MF30 multimeter is taken as an example). First, set the multimeter to Rx1k or Rx100, and measure the forward and reverse resistance of the LED. If the forward resistance is less than 50kΩ, the reverse resistance is infinite, indicating that the tube is normal. If both the forward and reverse directions are zero or infinite, or the forward and reverse resistance values are close, it means that the tube is defective.
Then, it is necessary to measure the light emission of the LED. Because its forward voltage drop is above 1.5V, it cannot be measured directly with Rx1, Rx1O, Rx1k. Although Rx1Ok uses a 15V battery, the internal resistance is too high, and the tube cannot be turned on to emit light. However, the double meter method can be used for testing. Two multimeters are connected in series and both are placed in Rx1 position. In this way, the total battery voltage is 3V and the total internal resistance is 50Ω. The working current provided to the L-print is greater than 10mA, which is enough to make the tube turn on and emit light. If a tube does not glow during the test, it indicates that the tube is defective.
For VF = 6V LED, you can use another 6V battery and current limiting resistor for testing.
Post time: Mar-19-2020
