Powering two red LEDs via USB port or USB battery pack
I want to power two red LED's from a USB charger or USB battery pack. I want both LED's to light up at the same time from the same USB cable. Can anyone advise what resistor/s I need to fit in the chain and where, please?


The LEDs should be powered from the USB power leads - giving about 5V DC.
Both LEDs have a Vf (forward voltage drop) of about 2.1 volt typical  (datasheet figure 2 page 5) at 20 mA current.  

The series resistors shown in the diagram drop the difference in voltage between the 5V supply and the LEd voltage drop.
Here Vresistor = Vsupply - Vled = 5-2.1 = 2.9 V.
This is approximate but an OK starting point.  

The resistors shown are sized to give about 10 mA - you can reduce them to drwa less current or increase them to draw less.
Here R = V/I = (5V-2.1V)/0.010 A = 290 Ohms.
330 Ohms is a common standard value of resistance.

Note that at the reduced current Vf will be slightly lower. 







simulate this circuit &ndash; Schematic created using CircuitLab