Difference between revisions of "Driver"

Only by correctly combining the power supply, driver, and LED will a LED function properly. Many Cree LED's require 3.3 volts to light up. If a flashlight is powered by a single AA battery, then the voltage must be amplified. However if a light uses 2 3-volt CR123 batteries with 6 volts, the voltage must be reduced. Power regulation supplies a constant voltage to the LED so that the brightness of the flashlight is constant for most its period of use (many circuits stop working once the battery gets low and brightness will slowly fall off, which is better than the light just turning itself off without warning).

[[File:Nanjg-106-driver.jpg|thumb|300px|17mm linear regulator driver with 4 7135 chips and microprocessor]]If you apply the correct voltage, you don't need a driver at all. Lithium-ion batteries conveniently provide a decent voltage without any further modification. This is quite efficient since all of the power from the battery is delivered to the LED. However a fresh li-ion battery starts at 4.2 volts fully charged and will be 3.6 volts when it needs to be recharged. As power is applied to the LED, it will draw a certain amount of current from the battery. This current increases with the voltage. When the voltage is higher, the LED will be very bright and draw a great deal of current. As voltage decreases the light will dim and draw less current. If a constant brightness is desired, a driver that offers some kind of regulation is required. Also, many LED's are being overdriven by 4.2 volts of a fresh li-ion battery.

Though people call this type of driver a linear regulator, linear regulators regulate the voltage. However these drivers have a number of AMC7135 current regulating chips installed. Each chip allows 350mA of current through and when wired in parallel, the current adds, so 700mA from 2 chips, 1050mA from 3 chips, and 1400mA from 4 (some drivers have higher binned regulator chips that allow 380mA through). Wiring 2 boards with 4 chips each gives 2800mA which is ideal for P7, MC-E, and XM-L LED's. Constrained to a particular current, the LED will settle in at its [[Terminology#Vf|Vf]] for that current and the rest of the voltage from the battery will be converted to heat by the regulator chips. Because any voltage beyond the required amount is wasted as heat, supplying a higher voltage is less efficient and the board becomes more efficient as the input voltage approaches the output voltage. As the voltage supplied drops below the Vf of the LED plus some overhead for the chips, the driver drops out of regulation and goes into direct drive and the light will start to dim. These drivers can also have modes by using a microcontroller utilizing [[Terminology#PWM|PWM]] for lower modes and flashing modes.