Abstract: To supply power to the LED, it must be powered by a constant current power supply. After the current is constant, no matter how the temperature changes, the volt-ampere characteristic shifts to the left and the current does not change! The junction temperature will not viciously cycle!
Now everyone is beginning to realize that light decay is the main reason why high-power LED street lights can't work for a long time. It is also beginning to realize that an important way to reduce light decay is to improve its heat dissipation. Despite this, judging from the test results of various street lamps by the Shenzhen Lighting Environmental Management Center, the light decay of most street lamps is still unable to meet the requirements for use. The light decay after 1200 hours of lighting, the best is 8%, the worst is 26%, the average is 14%. According to Cree's test results, when the junction temperature is 105 degrees, 14% of the light decay should be after 6000 hours of operation, and the junction temperature of most street lamps is above 105 degrees.
A siren is a loud noise-making device. Civil defense sirens are mounted in fixed locations and used to warn of natural disasters or attacks. Sirens are used on emergency service vehicles such as ambulances, police cars, and fire trucks. There are two general types: pneumatic and electronic.
Many fire sirens (used for calling the volunteer fire fighters) serve double duty as tornado or civil defense sirens, alerting an entire community of impending danger. Most fire
sirens are either mounted on the roof of a fire station or on a pole
next to the fire station. Fire sirens can also be mounted on or near
government buildings, on tall structures such as water towers,
as well as in systems where several sirens are distributed around a
town for better sound coverage. Most fire sirens are single tone and
mechanically driven by electric motors with a rotor attached to the
shaft. Some newer sirens are electronically driven speakers.
Fire sirens are often called "fire whistles", "fire alarms", or
"fire horns". Although there is no standard signaling of fire sirens,
some utilize codes to inform firefighters of the location of the fire.
Civil defense sirens also used as fire sirens often can produce an
alternating "hi-lo" signal (similar to emergency vehicles in many
European countries) as the fire signal, or a slow wail (typically 3x) as
to not confuse the public with the standard civil defense signals of
alert (steady tone) and attack (fast wavering tone). Fire sirens are
often tested once a day at noon and are also called "noon sirens" or
"noon whistles".
The first emergency vehicles relied on a bell. Then in the 70s,
they switched to a duotone airhorn. Then in the 80s, that was overtaken
by an electronic wail.
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