Detection and testing methods for single-color LEDs

1. Main idea:
Because the light-emitting diode has unidirectional conductivity, we use the R × 10k range to measure its forward and reverse resistance. Generally, the forward resistance should be less than 30k ohms, and the reverse resistance should be greater than 1M ohms. If both the forward and reverse resistances are zero, it means that the internal circuit is short-circuited. If both the forward and reverse resistances are infinite, it proves that the internal circuit is open. Two points need to be explained: First, for the tube core of the same material, the light color is different due to the different doping substances; second, LED is a current-controlled device, VF changes with IF, and the VF value marked is for reference only.

2. Distinguishing electrodes
The positive and negative electrodes of light-emitting diodes can be distinguished based on their appearance. The tubes produced in the early stage had metal tube sockets, which were covered with optical lenses. There was a protrusion on the side of the tube, and the positive electrode was close to the protrusion. The LEDs currently produced are all encapsulated with transparent or translucent epoxy resin, and the epoxy resin is used to form a lens to magnify and focus. The longer lead of this type of tube is the positive electrode.

3. Example test:
The steps to measure the quality of an unknown LED are as follows:

The first step is to determine the positive and negative poles. Using the R × 10k range of the MF30 multimeter, the forward resistance is 26k ohms and the reverse resistance is close to infinity. When measuring the forward resistance, the black test lead is connected to the positive pole.

The second step is to set both MF30 multimeters to R × 1 and use dual meters to measure. The tube under test emits a bright red light. If the polarity of the light-emitting diode is reversed, the tube will not emit light when a reverse voltage is applied.

Then set the two multimeters to R × 10, and the tube glows dimly. This is because the total resistance R0 ′ = 2 × 250 = 500 ohms, which provides a small forward current. At this time, IF ≈ 3 - 1.7/500 = 2.6 mA

IV. Notes:
1. When using the dual meter method, the ohm zero of the two multimeters must be adjusted first.
2. In order not to damage the light-emitting diode to be measured, the IM ′ value should be calculated before measurement. If IM ′ ≥ 50mA, the R × 10 position should be selected.
3. The light-emitting diode itself still has a voltage drop of 1.5 to 2.5V, so the above results all have a certain margin.
4. If the forward voltage of the light-emitting diode to be measured is unknown, the IM ′ value is also unknown. It is recommended to first set both meters to the R × 10 position. If the light is very dim, then change to the R × 1 position.