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Here's the story:

I was reading Instructables one day, and I had a BRAINBLAST. If I were to add ambient LED lighting to my room, a USB phone charger would be a relatively convenient power source! So I got to work.

However, while using online resistor calculators to determine the resistor suitable for this project, I noticed that they did not require the source current. This got me pondering.

So the question is: Does the source current matter?

2 USB phone chargers
Charger 1:     5V 1000mA
Charger 2:     5V 800mA
Resistor: 1.5ohm 2W Metal Film Resistor
LEDs in parallel
LED:     3.5V 25mA

Help would be greatly appreciated!    :)

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## Discussions

I'm assuming that when you say source current you're referring to the 1000mA and 800mA for Charger 1 and Charger 2 respectively.  These are the maximum rated current that each charger can supply.  The actual current drawn from the supply is dependent on the circuit as stated above.  Where the rated current values come into play is when choosing the power supply for the project.  In this case (from your description I'm assuming you have a current limiting resistor then your parallel array of LEDs) the 1.5 ohm resistor will draw 1A of current ( (5V - 3.5V)/1.5ohm ).  So the 1000mA supply should be sufficient; the 800mA supply will not.

To answer your question, yes the source current matters, but not to determine the value of the current limiting resistor.

.  Ah. In a series circuit, current is the same everywhere. So if you have 25mA flowing through the LEDs, there will be 25mA flowing through the resistor and 25mA through the source.

> while using online resistor calculators to determine the resistor suitable for this project, I noticed that they did not require the source voltage.
.  You must be looking at the wrong calculator(s). Try this LED calculator for single LEDs.

I am sure that one of our LED experts will chime in.  My comments are those of a physicist, not a practical electronics builder :-)  If I'm wrong, I hope one of the experts will explain what I'm not understanding.

The source voltage has to matter, as the purpose of the resistor is to drop the voltage from the source to what the LED can use (too much voltage will burn it out).  The voltage drop across the resistor is determined by the current drawn by the LED (in your case 25 mA).  So if you know the source voltage, you can calculate the resistor needed as R = dV/I, where dV = Vsource - VLED.