In this paper, the virtual instrument is applied to the measurement of LED junction temperature and light decay. The LED junction temperature and light decay monitoring system developed by LabVIEW is based on computer, coupled with sensors, data acquisition card and corresponding software. The voltage, operating current, and illuminance changes of the LEDs are monitored to automatically, continuously, and online measure and record 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, thus becoming 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, there have been many reports, such as forward voltage method, pin method, blue and white ratio method, etc. [2]. Currently used is forward voltage method, which is using LED electric transport. The temperature effect determines the junction temperature by measuring the voltage at a constant operating current. The smaller the forward voltage, the higher the junction temperature and the substantially 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, the overall design of the system
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, system signal acquisition schematic
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 program 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.
2.2 data acquisition card
This system uses the USB-6009 multi-function data acquisition card from National Instruments of the United States, with a maximum acquisition rate of 48kS/s, 8 analog input channels, 14-bit resolution, 12 digital I/O lines, 2 Analog output, and 1 counter. In order to obtain more accurate data acquisition and avoid external interference, the system uses differential method for voltage acquisition. Three acquisition channels are formed by six input interfaces, and three voltage signals are simultaneously acquired. The corresponding channel can be configured by software, such as sampling frequency and voltage acquisition range.
2.3 Working current collection
Since the signal input of the acquisition card is a voltage signal, a resistor is connected in series in the LED circuit, and the current is converted by collecting the voltage drop across the resistor. The 0.1Ω small resistor is used here to ignore the impact on the system. The purpose of monitoring the operating current is to directly observe that the LED lamp is operating in a constant current state, and the change in junction voltage depends only on the change in junction temperature.