# Basic Question Regarding LEDs and Selecting Resistors (Ohm's Law)

#### mfin

Joined May 16, 2016
4
I have a working schematic for a PCB board that has a few components including a 3V power supply, a 32 QFN, and 1 RGB LED attached to 3 PIOs on the QFN.

The schematic uses the following resistors for each color in the RGB LED.

Red ---- 360 ohms
Green ---- 180 ohms
Blue ---- 160 ohms

The LED light used (found here: http://www.osram-os.com/Graphics/XPic3/00115411_0.pdf) has the following forward voltages:
...................Min........Typ.........Max
Red....-----...1.8V.......2.05V.......2.4V
Green.-----...2.9V.......3.2V........3.7V
Blue....-----...2.9V.......3.2V.........3.7V

Assuming a current of 20mA....I try plugging these values into the Ohm's Law equation like so:

Ex. (for Red)
R = (Vsource - Vdrop)/(I)
R = (3-2.05)/20
R = 47.5 ohms

That 47.5 ohms is way different than the 360 ohms resistor used in the schematic. I have to be doing something wrong...Can anyone explain the correct calculation?

#### mcgyvr

Joined Oct 15, 2009
5,394
technically is R = (3-2.05)/.020
(.020 not 20)
They just aren't driving the LED at its 20mA rating..

Now your test (homework) is to tell us how much current each diode is actually getting given the resistors actually used in the schematic..

#### mfin

Joined May 16, 2016
4
technically is R = (3-2.05)/.020
(.020 not 20)
They just aren't driving the LED at its 20mA rating..

Now your test (homework) is to tell us how much current each diode is actually getting given the resistors actually used in the schematic..
Yes that was a typo thank you. To get 316.67 ohms of resistance, you only get 3mA going through the LED. Isn't that way too little? I have the actual device for the schematic and the LED light is very bright.

Furthermore, the green and blue LEDs have typical forward voltages of 3.2V (which is higher than the 3V power supply). Why are they using such large resistances for these lights?

#### mfin

Joined May 16, 2016
4
The specs also state that the QFN contains 2 regulators: 1 switch-mode regulator which generates the main supply rail from the battery and 1 low-voltage linear regulator. The switch-mode regulator generates 1.35V and the Low-voltage VDD Linear Regulator has an input voltage range of 0.65V to 1.35V and a programmable voltage of 0.65V to 1.20V.

I am unfamiliar with switch-mode regulators and low-voltage VDD Linear Regulators (have only done basic google searches on them). Could these two elements be affecting the supply voltage?

#### hp1729

Joined Nov 23, 2015
2,304
I have a working schematic for a PCB board that has a few components including a 3V power supply, a 32 QFN, and 1 RGB LED attached to 3 PIOs on the QFN.

The schematic uses the following resistors for each color in the RGB LED.

Red ---- 360 ohms
Green ---- 180 ohms
Blue ---- 160 ohms

The LED light used (found here: http://www.osram-os.com/Graphics/XPic3/00115411_0.pdf) has the following forward voltages:
...................Min........Typ.........Max
Red....-----...1.8V.......2.05V.......2.4V
Green.-----...2.9V.......3.2V........3.7V
Blue....-----...2.9V.......3.2V.........3.7V

Assuming a current of 20mA....I try plugging these values into the Ohm's Law equation like so:

Ex. (for Red)
R = (Vsource - Vdrop)/(I)
R = (3-2.05)/20
R = 47.5 ohms

That 47.5 ohms is way different than the 360 ohms resistor used in the schematic. I have to be doing something wrong...Can anyone explain the correct calculation?

In order to get the "white" they wanted with all three on they do not drive them all at 20 mA.
That "20 mA" is not mandatory. Buy one of the LEDs they use, or similar, and play with it.

#### mfin

Joined May 16, 2016
4
In order to get the "white" they wanted with all three on they do not drive them all at 20 mA.
That "20 mA" is not mandatory. Buy one of the LEDs they use, or similar, and play with it.
That makes sense except that the blue and green have the same forward voltages yet they use different resistors for each one. Could the two regulators I mentioned in post #4 (right above yours) be affecting something?

#### mcgyvr

Joined Oct 15, 2009
5,394
post the schematic (including parts list if not present) if you want more help..
Either you are interpreting it wrong or its working as intended/designed to work..