LED DRIVER

General project help for Adafruit customers

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Novatron
 
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Joined: Fri Jul 03, 2015 10:36 pm

LED DRIVER

Post by Novatron »

Hi
I'm after a small LED driver circuit to drive a 1W high brightness LED from a 3.7 Li -Ion battery and preferably with some form of brightness control.
Do you stock something for that or a suggestion if or how your 12 CH PWM LED driver might be used?
Thanks
Karl

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adafruit_support_mike
 
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Re: LED DRIVER

Post by adafruit_support_mike »

The first thing to do is check the datasheet for your LED. Some 1W devices want supply voltages up to 5v.

The one we have in the shop works down to 3v (https://www.adafruit.com/products/518), but that's a reduced-power mode of only about 600mW.

The Servo Driver can't handle the power necessary to control the LED directly, but you can use it to control a transistor that runs the LED. Most power mosfets don't work very well with 3.3v control signals (they want more like 5v to 10v), but bipolar transistors work below 1v. They're the most popular choice for low-voltage power switching, but are a little harder to control.

The circuit you want would look something like this:
led.jpg
led.jpg (10.06 KiB) Viewed 133 times
The 3.9 ohm resistor will reach a voltage around 0.8v when 200mA flows through it, leaving about 3v for the LED. The 100 ohm resistor on the input would be controlled by the Servo Driver, and provides enough current to make sure the transistor will let 200mA through the LED.

If you need to buy the resistor, you can tweak the value down to about 3.3 ohms, which will raise the LED current slightly.

Ordinary 1/4-Watt resistors will do.. the power dissipated by a 3.3 ohm resistor would be less than 150mW, which is well within its rated limits.

For the transistor, you could like a PN2222: https://www.adafruit.com/products/756 It's designed to control currents in the 100mA range.

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