# Calculating current draw of simple LED / Resistor circuit

Discussion in 'Embedded Systems and Microcontrollers' started by stb, Apr 24, 2016.

1. ### stb Thread Starter New Member

Apr 11, 2016
8
0
Hi

Would the following calculation be correct, it seems to work nicely with an incredibly low current if it is!

3.3v supply (Raspberry Pi), 4.82K resistor (at least that's the reading from my multimeter - max setting on a 5K pot), 3mm Red LED.

Voltage read accross the resistor is 1.600v.

So the current accross the resistor is:

V=IR, 1.6 = I x 4820, I = 1.6 / 4820 = 0.332 mA ?

And that is the total current through the whole circuit (because that doesn't change through a circuit) ?

Seems too low to be true when the normal quote is to run at 10mA. I know it's not full brightness but it seems more than bright enough.

Many Thanks

2. ### ErnieM AAC Fanatic!

Apr 24, 2011
7,435
1,625
Try again, I = V / R

3. ### stb Thread Starter New Member

Apr 11, 2016
8
0
I probably should have shown a bit more of my workings, but isn't that what I have?

I = V / R = 1.6 / 4820 = 0.000332 = .332 mA

Or am I messing up decimal places and 'm's?

Thanks

4. ### OBW0549 Well-Known Member

Mar 2, 2015
1,389
932
You didn't mess anything up, 0.332 mA is right. As for "too low to be true", most modern LEDs are pretty darned efficient and I'm not surprised that that current is adequate.

stb likes this.
5. ### stb Thread Starter New Member

Apr 11, 2016
8
0
Thanks so much OBW0549. They really are amazingly efficient.