With the rapid development of led. Nowadays, the traditional light source of white LED light source has the advantages of long life, solid lighting, damage, high luminous efficiency, mercury-free environmental protection, earthquake resistance, etc. It will become the third generation light source in the future and will bring another revolution in the field of lighting. The use of LEDs in the field of illumination requires a driver power supply suitable for LEDs. In this paper, a high-power driver circuit for white LEDs is designed using PWM switch control. 1 LED electrical characteristics and drive requirements 1. 1 Electrical characteristics of LED The I-V characteristics of the white LEDs are similar to those of the conventional diodes, except that the turn-on voltage is different, and the LED turn-on voltage of the different materials is generally between 1.5 and 3. 0 V. When in the forward working area, the operating current IF is exponentially related to the applied voltage: Where IS is the reverse saturation current; VF is the applied voltage across the diode; q is the electron charge; k is the Boltzmann constant; T is the thermodynamic temperature. The DC current that the LED can work stably for a long time is called the rated operating current. At this time, the LED voltage drop is called the rated voltage. 1 W white LED with a rated operating current of 350 mA and a rated voltage of 3.3 V. The maximum value of the product of the forward voltage and the current flowing through the LED is allowed to be its ultimate power consumption. When the actual power consumption exceeds this value, the LED's light-emitting characteristics become worse, which may cause structural damage to the LED. 1. 2 LED driver requirements According to the I-V characteristics of the LED, when the voltage applied across the LED fluctuates slightly, it will cause a drastic change in current. At this time, it is easy to make the current too large, and the input power exceeds its limit power consumption, thus making the LED unrecoverable. Damage. When the working current value of the LED is different, the luminous intensity is also different. If constant voltage driving is used, the LED array should be connected in parallel, but due to the parameter error between the individual LEDs, the current of each branch will be different, resulting in an array. The luminous intensity is not uniform, so the driving circuit of the LED generally selects the constant current driving mode, and the corresponding LED array is also connected in series. The driving current is generally set to 70% ~ 85% of the rated current of the LED to protect the LED and prolong the service life. The purpose is also to make the luminous intensity of each LED uniform. In the LED driver circuit design, the following basic indicators need to be considered: (1) Improve the conversion efficiency of the driver circuit and reduce the power consumption in the circuit. (2) Improve the reliability of the circuit, withstand high voltage, and have overcurrent detection function. (3) The circuit is as compact as possible, with a small circuit size and low manufacturing cost. 2 PWM mode switching circuit design 2. 1 PWM principle PWM is pulse width modulation, which uses pulse to control the switching time of the switching circuit to control the average voltage or current output from the circuit to achieve the output power of the control circuit. The basic working principle of PWM switching regulator or constant current is that under the condition of input voltage, system parameters and external load change, the control circuit performs closed-loop feedback through the difference between the controlled signal and the reference signal at a fixed operating frequency to adjust the main The pulse width of the circuit switching device is turned on, so that the output voltage or current of the switching power supply is stable. Since the power consumption of the control device is small and the circuit operating in the switching state is high, the power efficiency can generally be 80% to 90%. This type of circuit has perfect protection measures and is a highly reliable power supply. The PWM switching circuit consists of four parts, namely input rectification filtering, PWM control, switching device and output filtering. The LED driver circuit block diagram based on the PWM mode switching circuit design is shown in Figure 1. Figure 1 LED drive circuit block diagram of PWM mode switch circuit design Most of the common PWM switch control signal generation parts are integrated, and the design of the PWM switching power supply is more streamlined. The following describes the typical PWM mode switch drive circuit designed for high-power LEDs using the chip HV9910B.
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A cable tester is an electronic device used to verify the electrical connections in a cable or other wired assembly. Generally a cable tester consists of:
1.A source of electric current,
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2.A volt meter,
3.A switching matrix used to connect the current source and the volt meter to all of the contact points in a cable.
In addition to these parts a cable tester may also have a microcontroller and a display to automate the testing process and show the testing results.
