## 2 Investigation of Temperature Dependence of Current and Luminous Flux  
Investigation of Temperature Dependence of Current and Luminous Flux of LED Lumps Based on Linear Drivers  
It is well known that with the increase in the environment temperature \(T_{c}\) at constant value of LED current, the luminous flux of the LED decreases significantly [12; 13]. Therefore, in LED devices, the effect of temperature on their light parameters is significant. The temperature changes in electrical and lighting parameters are especially noticeable while using LEDs outdoors. Especially in summer, when high temperature conditions can cause noticeable reduction in the luminous flux of LEDs. Most circuits of linear and current stabilization drivers in LED lighting devices have the capability to stabilize the current in both cases when the supply voltage and when the environment temperature is changing. But, as a rule, in such cases not all of them able to guarantee stability of light parameters (illuminance, luminous flux) of LED devices. In order to confirm this assumption, we will provide the investigation concerning the changes of current and illumination, luminous flux of LED devices with different typesof drivers depending on the temperature. Let's have a look at the temperature dependence of current and luminous flux of LED lamps on the basis of linear drivers (Figure 1). The investigated devices were placed in the thermostat, the temperature varied from +15 \({}^{\circ}\)C to +60 \({}^{\circ}\)C in increments of 5 \({}^{\circ}\)C. The investigations are carried out at AC mains voltage of 198 V, 220 V or 242 V.  
At the same time, the values of the consumed alternating current \(I_{inp}\), ambient temperature \(T_{c}\), LED voltage \(U_{d}\), LED current \(I_{d}\) and illuminance \(E\) were measured. Block diagram of measurements is shown in Figure 2.  
The main experimental installation is shown in Figure 2. The basis of the setup thermostable chamber \"TERMOSTAT\", in which EL LED lamp is installed. Chamber cooling is based on natural conventions. Electronic thermometer VK2 (type TRM-10) was used for independent temperature control. The current EL LED lamp and its voltage were controlled by an ammeter A1 and voltmeter V1, respectively. Voltmeters V2 and V3, together with 1 Ohm resistor R1, were determined the voltage and current flowing through the diodes of the LED lamp. For more accurate measurement of the illuminance, the walls of the thermally stabilized chamber was painted black. The illuminance was measured with the luxmeter, its sensitive element BL1 was isolated from external light. Led lamps LedEX E27B with power 3 W and 6 W were investigated. Their basic electrical scheme is shown in Figure 1. The general view of the lamps without the plastic diffuser is shown in Figure 3.  
Figure 1: Schematic diagram of LED lamp with linear driver. Figure 3: LedEX E27B LED lamps with 3 W (**a**) and 6 W power (**b**). Figure 2: Block diagram of measurements. LedEX E27B lamp (3 W) contains six SMD 2835 LEDs (nominal voltage 18 V, current 30 mA, power 0.5 W, color temperature 4000 K) [14; 15] (Figure 3a). LEDs were connected gradually. Experimental dependencies of the relative current values \(I_{inp}/I_{inp25}\) of this LED lamp on the environment temperature \(T_{c}\) at supply voltages of 198 V, 220 V and 242 V, where \(I_{inp25}\) is the lamp current value at +25 \({}^{\circ}\)C temperature and 220 V supply voltage is shown in Figure 4. The value of input current in the scheme of Figure 1 can be considered as equal to the current through LEDs. Illuminance \(E\) from LED light sources was measured by luxmeter. But, the application of illuminance relative values \(E/E_{25}\), where \(E_{25}\) is illuminance at +25 \({}^{\circ}\)C temperature, makes it possible to confirm that the dependence will be the same for relative values of luminous flux \(F/F_{25}\) of the lamp or spotlight (\(E=F/A\), where \(F\) is luminous flux, \(A\) is the area of the illuminated surface).