Powering 3 IR LEDs off a USB supply 5V DC.

Submitted by Olias 2k9 on Wed, 04/21/2021 - 16:40

Im Making a simple light setup for my sons toy, it uses three 5mm LEDs wired in parralel and one resistor in series to that

Rough text sketch

USB 5V 500 ma (max) + _______________________________________________________< X > ____ {3leds +}

USB 5V - ______________________________________________________________________________{3 leds -}

It plugs into a USB port on his pc and just drives the LEDs, no data tranfer involved, just the LEDs

The LEDs are 5mm with a current of 20 ma at 5v DC The resistance that came out of the old unit had its stripes rubbed off and the Ohms rating is somewhere around the 15M ohm so obviously thats dead and useless. Im assuming the resistor is just a current limiter marked as <X> in my diagram above. How do I calculate the resistance required?

I dont know the internal resistance of the diodes yet, they havent been ordered and I need to know the resistance required before ordering.

Any and all information is appreciated if for nothing else than to cheer my son up.


20mA is the maximum load current for the each led. lets choose 10mA each.  In such case the three parallel LEDs will consume current of 10mA x 3 = 30mA. Since you have the 5V voltage source, the resistance will be V/I = 5V/0.03A = That is 167 Ohms roughly. Now, the Power rating of this resistor would be = 0.03 x 0.03 x 167 = 0.1503 Watt.

Any resistance from 150 Ohms to 180 Ohms with 0.25W rating would be great.

  Joined February 12, 2018      696
Monday at 02:11 PM

Try using a logic level mosfet.

  Joined February 12, 2018      696
Monday at 02:11 PM

A transistor isnt needed, apart from that there isnt anything that requires a mosfet transistor. Thanks for the reply though, thats just the info that I was after.


Stay safe and well.

  Joined April 21, 2021      1
Wednesday at 04:26 PM