The virtual instrument described herein is applied to measuring LED junction failure in moderate light, LED junction temperature and the light failure monitoring system developed in LabVIEW platform, computer as the core, coupled with the sensor, data acquisition card and corresponding software, the work of the LED The voltage, operating current, and illuminance changes of the LEDs are monitored, enabling automatic, continuous, online measurement and recording of data, and the results are displayed graphically and numerically. The system is simple, practical, high-precision, high-sensitivity, safe and reliable, easy to maintain, has good scalability and application prospects, and can be applied to many environments in the laboratory and outdoors.
1. introduction
White light LED solid light source has the incomparable advantages of traditional light source: energy saving, environmental protection, long life, safety and reliability, etc., becoming the trend of future lighting. However, the PN junction temperature of the LED has an important influence on the performance of the LED, which causes color temperature change, wavelength red shift, forward voltage drop, etc., and affects the non-radiative recombination of electrons and holes, resulting in a decrease in optical radiation power. The key to affecting the entry of LEDs into the general lighting field is an urgent problem to be solved. How to measure the junction temperature of LEDs has been reported, such as forward voltage method, pin method, blue and white ratio method, etc. [2]. Currently used is the forward voltage method, which uses the temperature effect of LED electrical transport to determine the junction temperature by measuring the voltage under a constant operating current. The smaller the forward voltage, the higher the junction temperature and the more linear relationship. The system is based on the LabVIEW graphical software platform and is developed as a virtual instrument. The purpose is to realize the indirect measurement of the junction temperature by monitoring the forward working voltage in real time during the constant current operation of the LED, and to monitor the change of the LED luminous intensity. The light decay of the LED illuminator is monitored by changes in illuminance.
2. Overall system design
2.1 Hardware Design
The LED junction temperature and light decay monitoring system converts the non-voltage signal into a voltage signal, which is read, processed, displayed, and stored by the computer after being collected by the acquisition card. The hardware design part consists of a drive circuit, an illuminometer probe, a data acquisition card, and a computer. The drive circuit is not within the system design range and is the object to be measured. The schematic diagram of system signal acquisition is shown in Figure 1.
Figure 1 Schematic diagram of system signal acquisition
The circuit in this schematic diagram uses a capacitor buck LED driver circuit with good constant current. The load is the same batch of 18 low-power LEDs with the same physical parameters. All signals are collected by the acquisition card, connected to the computer through the USB interface, and then the application completes the control and data recording and analysis of the acquisition card. By using the computer to combine the system software and the acquisition hardware, real-time data acquisition, recording, waveform display, simultaneous acquisition of multiple signals and generation of corresponding data tables can be realized.
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