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# Powering infrared Line Laser using USB port Answered

Hi,

I have a line laser which has following specs!!!

http://www.aliexpress.com/store/product/780nm-60mw-5V-IR-Infrared-Laser-Line-Module-120-degree/640033_1594226134.html

5V DC or 3.3V preferred!!
Working current must be < 200mA

How can power this laser using USB cable or USB port

I know USB has 5V DC (max current is 500mA)

I want to know how to convert it so that  i can power the laser??!!! Thansk in advance for help!!

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but USB provides 500mA which is over for the laser i need to convert it to 200mA before connection How can i do tht?

A resistor, which is a different type of electrical component, will limit the current.

REsistance = Voltage / amps
you don't know what size resistor you need but you know the current limit and the voltage of the usb.
5/.2 = 25 ohms of resistance.
So you need around 25 ohms of resistance.
It looks like the closest standardized rated resistor is 22 ohms, but you can find them with high tolerances to compensate for the 3 ohm difference, or you can add small bits of resistance in other ways.
Inductive windings produce resistance because they compact down long lengths of wire into a compact space (thus taking advantage of the natural resistance of the wire) or you could simply design your circuit run two resistors in parallel, although that mucks with the math a bit.

Look up the pin specs or layout of a usb cable/connector. One end is plugged into the computer USB port. Find the +5volts and Ground wire at the end free end of the cable(cut the plug off or get a mating plug to attach to the laser unit) Red to +pos5volts and black to ground.

Working current musr < 200 ma (USB has 500mA )

The device should only draw what it needs but you can put in a limiting resistor to protect the laser diode. Ohms law v=ir to figure out the value.