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Prop-Maker FeatherWing limiting resistors for 3W RGB LED hav
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Prop-Maker FeatherWing limiting resistors for 3W RGB LED hav

by georgehawkins on Mon Jan 13, 2020 10:27 am

Why does the Prop-Maker FeatherWing seem to expect a different supply voltage (3.6V) for red than for green and blue (4.2V)?

Shouldn't red use a 4.87ohm limiting resistor if blue and green are using 1.75ohm resistors?

Details:

In the 3W RGB common anode LED the forward voltage is 2.5V for red and 3.6V for green and blue (according to the datasheet).

The forward current is 350mA for all three colors.

On the schematic for the Prop-Maker FeatherWing you can see that the limiting resistor is 3ohm for red and 1.75ohm for green and blue.

Given the standard limiting resistor equation R = (Vs - Vf) / i we can calculate the supply voltage implied by these values.

So rearranging to solve for Vs we get Vs = iR + Vf and this results in:

  • Red = 0.35 * 3 + 2.5 = 3.6V
  • Green/blue = 0.35 * 1.75 + 3.6 = 4.2V

I was expecting the implied supply voltage to be the same for both cases - and I was expecting it to be about 3.7V, i.e the nominal voltage of a single cell LiPo, or 4.2V (to take into account the initial fully charged voltage).

So if 1.75ohm has been chosen for green/blue in order to handle 4.2V, shouldn't the limiting resistor for red also be for the same supply voltage?

I.e. (4.2 - 2.5) / 0.35 = 4.86ohm, e.g. something like the Yageo 1206 4.87ohm resistor.

If I look at this forum answer by Rick from Adafruit Support as to what limiting resistors you should use when the supply voltage is 5V the math all works out as expected:

On red, use a 7.5ohm 1W resistor
On green, use a 4ohm 1/2W resistor
On blue, use a 4ohm 1/2W resistor


If I plug these values into Vs = iR + Vf then I get the same voltage, i.e. 5V, for all three colors. So here we get a consistent implied voltage, so why the apparent inconsistency in the Prop-Maker FeatherWing design?

Image

georgehawkins
 
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Re: Prop-Maker FeatherWing limiting resistors for 3W RGB LED

by georgehawkins on Mon Jan 13, 2020 6:58 pm

So according to my calculations (based on the the datasheet for the 3W RGB common anode LED) the 1.75ohm limiting resistors are correct (assuming a supply voltage of 4.2V, i.e. a fully charged LiPo) for green and blue but the limiting resistor for red should be 4.86ohm rather than the 3ohm shown in the schematic for the Prop-Maker FeatherWing.

If I instead look at the hi-res product photo of the Prop-Maker I see different resistor values (but still not the expected value for red):

Image

Here you can see the SMD codes used on the resistors on the actual manufactured board:

  • red - 3R6 - 3.6ohm
  • green/blue - 1R82 - 1.82ohm

So green/blue is kind of close enough to the 1.75ohm seen in the schematic (though it's still a bit off and, unlike with through-hole, you can usually get very close to what you want so the difference is still odd). But the 3.6ohm for red is far away from both the 3ohm of the schematic and 4.86ohm I calculated should be used from the LED's datasheet.

So what's the implied supply voltage from these resistor values? Let's see:

  • red - 0.35 * 3.6 + 2.5 = 3.8V
  • green/blue - 0.35 * 1.82 + 3.6 = 4.2V

The red value still implies a very different voltage to the green/blue value. What's going on here?.

Maybe the LED datasheet is wrong and these resistor values are derived from tuning them to an actual LED?

georgehawkins
 
Posts: 23
Joined: Wed Jun 12, 2013 12:12 pm

Re: Prop-Maker FeatherWing limiting resistors for 3W RGB LED

by bcochran1 on Mon Jan 13, 2020 9:35 pm

Hi,

I don't have an answer for your question. My purpose is to compliment your excellent presentation of the problem statement. You write clearly and in lucid detail. I'm impressed.

Thanks so much

Bob

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Please be positive and constructive with your questions and comments.