Undoubtedly, the use of high-brightness LED lighting will become a major feature of future cars, thanks to many of the basic advantages of LEDs over traditional incandescent lighting solutions. In addition, the use of LED lighting can also drive changes in automotive design techniques and design styles. However, just like any innovative technology, LEDs still have to overcome many difficulties before they are widely used in automotive lighting.
Key characteristics
1. Reliability and service life
The expected life of the LED is 50,000 hours, while the tungsten halogen lamp is 20,000 hours and the tungsten incandescent lamp is 3,000 hours. Compared with incandescent lamps, LEDs are structurally strong and are not susceptible to vibration. The brightness of the light output during use is not significantly reduced. Lighting solutions based on multiple LEDs also have the "redundancy" benefit, and even if one LED fails, the lighting can continue to be used. Proper use of LEDs (especially the temperature of the LEDs properly controlled) can effectively extend the life expectancy of the LEDs. Conversely, if the temperature is too high, the LED is easily damaged. LED applications also involve many legal definition issues in automotive lighting. Most countries have a clear definition of brake lights or headlights - lights on or off. However, for lamps with multiple LEDs, it is difficult to accurately define whether the lamps have been damaged. Manufacturers and legislatures are defining ways to use LEDs.
2. Efficiency / lumens per watt
Compared to standard incandescent lamps, LEDs consume more light output per unit of electrical energy. However, the advantages of the actual light output of the LED are not significant when compared to halogen lamps. The latest LEDs have excellent lumens per watt, but some values ​​are obtained under optimized conditions and are usually not obtained under the highest output conditions. In general, when the current of the LED increases, the amount of light output does not increase linearly. Therefore, even if the LED outputs x lumens at a current of 0.5 A, it does not output 2x lumens at 1.0 A.
3. Response speed
Take the brake light and the direction indicator tube as an example. If the vehicle has a speed of 125 km/h, that is, 35 m/s, the hot start time of the incandescent lamp is about 250 milliseconds, and the fast-reacting LED can be sent about 8 meters earlier. Brake warnings to effectively avoid car collisions. The same is true for the indicator light.
4. Directionality
Another key feature is the way LEDs are illuminated. Unlike incandescent lamps, LEDs emit light through only one surface, which is good for headlamps and aeronautical light applications, but may not be suitable for other lighting applications.
Method of controlling LED
Current control
A fundamental problem with LEDs is that LEDs are current controlled devices with relatively low voltage drops. The easiest way is to use a resistor to limit the current of the LED, but this method is not suitable for systems with a rated voltage of 12V or 24V, because the actual voltage of the battery is from 6V to 18V or 12V to 36V. Therefore, if it is necessary to maintain brightness, constant current control must be performed.
2. Linear control of current
Linear control refers to the constant current held by the LED through the linear regulator. Linear control is inefficient in some cases. For example, a single 1A (3W) LED with a forward voltage of 3.5V requires the regulator to reduce the rated 12V supply to 8.5V while maintaining 1A, thus using 3W LEDs. A power of 8.5W will be wasted. Linear current control is the least noisy technique, and from the EMC point of view, linear current control is the safest.
3. Switching regulator
Inductive switching constant current technology produces more electronic noise, but it is more efficient. Depending on the number of LEDs used, a buck or down/boost regulator can be used.
4. EMC issues
Radiated and conducted noise must be minimized to keep the noise within acceptable limits. Although the frequency of the PWM method is fixed and relatively easy to filter, since the LED load is relatively stable, the hysteresis controller and the PFM are suitable choices if appropriate measures are taken. The trend in switching regulators is that the frequency will be higher to reduce the size of the inductor/capacitor. This is always the best solution for automotive applications. Keeping the frequency low helps to avoid interference problems.
The “jitter†or “expansion†technique of the fundamental frequency does help to meet similar peak EMC test requirements, but the best approach is to not generate any radiation, which is difficult to achieve with any switching regulator.
Radiant heat, conducted heat and heat management
One of the key issues and biggest challenges faced by users of high-brightness LEDs (especially in the automotive industry) is the self-heating of LEDs. The lumens per watt of LEDs have been greatly improved, but in fact most of the LED's electrical energy is converted into conduction heat. LEDs produce less radiant heat for compartment lighting, but in cold climates, the radiant heat of the headlights effectively melts the snow on the lens. Therefore, thermal management is the key to reliable LED control.
Thermal management mainly refers to reducing current when temperature increases. The advantage of using a high-brightness LED is that the eye does not perceive a change in brightness when the current changes greatly. In general, the current is reduced by 25%, and the brightness of a single LED does not change significantly.
However, LEDs change color with changes in temperature and current, and whether this will affect automotive lighting applications remains to be explored. Whether the spectrum of the LED is suitable for illumination and whether it affects the driver's sense of distance under normal night vision effects may be more important.
Using the PWM method to reduce the brightness ratio instead of the DC control, a larger ratio of light to dark can be obtained, and the color temperature does not change. Therefore, it is better to use the PWM method to reduce the brightness. However, the choice of frequency is also important. It is generally considered that the frequency is 200 Hz, because the human eye does not feel the flicker of 200 Hz light, and the lower frequency ensures that the switching frequency is lower than that of the switching regulator. However, the potential problem of stroboscopic effects in headlamps must be foreseen. A more appropriate method is to use a higher frequency to adjust the brightness of the LED to avoid the "yaw" effect. In addition, the inductor must be carefully selected to avoid audible noise in the car.
The temperature sensing of LEDs is also a problem that needs to be solved. Thermistors are widely used, but care must be taken when using thermistors. The temperature control response should be set to the upper temperature limit for the current that the LED needs to reduce. When the ambient temperature is lowered, simple temperature control can cause the current of the LED to increase. Figure 2 shows the typical response of an LED to ambient temperature.
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A rice cooker or rice steamer is an automated kitchen appliance designed to boil or steam rice. It consists of a heat source, a cooking bowl, and a thermostat. The thermostat measures the temperature of the cooking bowl and controls the heat. Complex rice cookers may have many more sensors and other components, and may be multipurpose. Cooking rice has traditionally required constant attention to ensure the rice was cooked properly, and not burnt. Electric rice cookers automate the process by mechanically or electronically controlling heat and timing, thus freeing up a heating element on the cooking range that had to be otherwise occupied for rice cooking. Although the rice cooker does not necessarily speed up the cooking process, with an electric rice cooker the cook's involvement in cooking rice is reduced to simply measuring the rice, preparing the rice properly and using the correct amount of water. Once the rice cooker is set to cook, the rice will be cooked with no further attention.
Features:
For modern home rice cookers, the smallest single-person model cooks 1 rice cup (180 ml), whereas large models can cook 10 cups. Commercial models can cook 20 or more cups. As a possible source of confusion, model specifications and names may list either cooked or uncooked capacity. Rice roughly doubles in size during cooking; therefore, a 10 cup (uncooked) rice cooker can produce up to 20 cups of cooked rice. The prices vary greatly, depending on the capacity, features, materials used, and the country of origin.
The majority of modern electric rice cookers are equipped with a stay-warm or keep-warm feature, which keeps the rice at an optimal temperature for serving without over-cooking it. Some gas cookers also have electric stay-warm mechanism. However, the usefulness of this feature degrades over time, a microwave may be more energy efficient or better suited to reheat rice that will sit longer than four hours.
Some rice cookers use induction heating, with one or more induction heaters directly warming the pot. This can improve energy efficiency.
Most modern rice cookers use aluminium for the inner cooking bowl. There are some models that use stainless steel instead of aluminium. Various other materials, such as copper, pure carbon, ceramic, and diamond powder coating, may be used for higher heat conductivity or better taste.
The pressure-cooking models can raise the water's boiling point higher, e.g., from 100 °C at 1.0 atm up to about 110 °C at 1.4 atm, which speeds cooking. The pressure-cooking models can also be used in high altitude areas, where the boiling temperature is below 100 Celsius. Pressure cookers are also suitable for cooking brown rice (which contains oils and bran fiber that cook differently from pure white rice starch). Some pressure rice cookers have a varying pressure control mechanism (named the "dual-pressure" method) that creates repeated pressure/release cycles during the cooking.
There also exist mechanisms to collect and return the boiled over liquid to the inner rice bowl.
Many cookers now have microprocessor-controlled cooking cycles, which are often used to adjust for rice and cooking type.
Applications
Rice cookers are typically used for the preparation of plain or lightly seasoned rice. Each rice cooker model may be optimized to cook a certain type of rice best. For example, most Japanese rice cookers are optimized for cooking Japanese rice and may not be the best for other types of rice[citation needed], although cooking time can be lengthened simply by more water.
The typical method of cooking long grain rice is boil-and-strain and/or steaming method. The absorption method used in Japanese rice cookers will produce slightly different texture and taste, usually stickier rice.
Brown rice generally needs longer cooking times than white rice, unless it is broken or flourblasted (which perforates the bran).
Different varieties of rice need different cooking times, depending on their grain size, grain shape, and grain composition. There are three main types of Asian rice: Oryza sativa subsp. indica, i.e., Indian rice (long grain rice, e.g., basmati rice and Thai jasmine rice), O. sativa subsp. javanica, i.e., Java rice (large grain rice) and O. sativa subsp. japonica, i.e., Japanese rice (medium grain rice, e.g., Calrose rice, short grain rice, e.g., most Japanese rice and risotto rice).
African rice, Oryza glaberrima, is an entirely separate species, but can be cooked in the same way. Zizania is not even in the same genus, although it is often called a rice (or "water oats"); it, too, can also be cooked in a rice cooker.
A rice cooker can be used to cook many boiled or steamed granular foods, such as pot barley, bulgar wheat, and dal. Provided the ingredients have similar cooking times, a rice cooker can cook mixtures such as khichdi. Some rice cookers can be used as automated couscoussiers, cooking couscous and a stew simultaneously.
Rice Cooker
Rice Cooker,Drum Rice Cooker,Deluxe Rice Cooker,Straight Rice Cooker
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