Slightly different color of two WS2812C LED strips

[symbianas]

Hi, I am doing ambient lighting for the car with WS2812C LED strips. I have installed 2 strips and connected them both directly to same power supply of 3.3V 5A(max) and also connected them both directly to same programmable data pin. Their lengths are different though, one is 20cm (~25 LED's) and another 90cm (120 LED's).
The issue I am having is that color is slightly different on 2 strips. What could be the issue that LED strips have slightly different color?

Is it because led strips are different length?
Is it because I must increase voltage to 5V (manufacturer specified voltage (but I want led strips to be dimmer so powered them both from 3.3V))?
Any other ideas what is causing this issue and how could I make the color exactly the same on both strips?

LED strip: https://www.aliexpress.com/item/1005004455070681.html

[tooki]

I don’t think the issue is variation between strips.

They’re not designed to run on 3.3V, and the problem is that 3.3V is already perilously close to the forward voltage of blue LEDs (usually around 3.2V). So any voltage drop whatsoever and the blue LEDs start to dim a bit, then as voltage drops further, green also starts to dim. So you end up with first a color shift towards yellow, then to red. I’m guessing the far end of the longer strip is looking yellower than the near end, and yellower than the other strip, right?

Now, I have run addressable LEDs on undervoltage on purpose before (running a strip directly off the voltage of a LiIon cell, so 3.0-4.2V in that case), but just as indicators, so color shift doesn’t matter.

Your strips are fairly long, so there’s going to be significant voltage drop along their lengths. You can improve this a lot by using thick wires to feed power into the strip at regular intervals (for example, every 20-30cm). Remember that the maximum current for each LED is about 60mA (3x20mA), so your ~150 LEDs have a total maximum current of 9A. Even dimmed to half brightness, that’s nearly 5A, and without the thick power wires as I describe, you’ll have very noticeable voltage drop along the long strip. (The copper of LED strips is very thin and thus exhibits lots of voltage drop at high current.)

[tooki]

Also, one thing you can do if need be — and this may not be a bad idea — is to run the LEDs at 5V, but still use 3.3V data. That way you can still use your 3.3V microcontroller but let the LEDs have their preferred voltage. (Technically speaking, the data input needs to be 0.7x the LED supply voltage, so 3.5V for 5V supply. But I’ve done it many times with 3.3V without issue. You could always reduce the LED supply to 4.5V to ensure it’s within tolerances.)

If you’re using a DC-DC converter with an adjustable output, you might be able to adjust it up to 3.5 or 3.6V, which many 3.3V microcontrollers will still tolerate and might be enough to prevent the color shift. Or then just use a small 3.3V regulator (or even just a regular diode) to bring the 3.6V down to a safe voltage for the MCU.

Regardless, the thick wires are needed, though!

[Benta]

I think the issue is different and lies within LED manufacturing (in fact, I know this).
The key word is "binning", which characterizes LEDs according to brightness and hue/colour. Each LED production run is different, and so is available binning.
Now, you can specify a certain binning for your product, but this comes at a significant price premium: the manufacturers don't like this.
What they will guarantee is that one production batch will all be from the same bin.
And that's exactly why the WS2812C was made: it takes the binning out of the equation, leaving it up to the customer or the end user to tune the correct colour/hue.

When that's said: do not operate devices outside their specifications (especially in automotive apps where temperature range can be extreme); and 3.3 V is certainly out of spec.

[tooki]

Yes, binning is real, but it’s almost 100% NOT the cause of OP’s issue! Running them at significant undervoltage, at high current, without wire to prevent further voltage drop, is a FAR more likely explanation.

[Benta]

at high current,

What do you mean? The devices have internal LED current control.

[tooki]

at high current,

What do you mean? The devices have internal LED current control.

Running the strips at high current, causing significant voltage drop across the thin copper foil traces.

[symbianas]

They’re not designed to run on 3.3V, and the problem is that 3.3V is already perilously close to the forward voltage of blue LEDs (usually around 3.2V). So any voltage drop whatsoever and the blue LEDs start to dim a bit, then as voltage drops further, green also starts to dim. So you end up with first a color shift towards yellow, then to red. I’m guessing the far end of the longer strip is looking yellower than the near end, and yellower than the other strip, right?

Yes, one side of the longer strip color seems to be the same as shorter strip.

Your strips are fairly long, so there’s going to be significant voltage drop along their lengths. You can improve this a lot by using thick wires to feed power into the strip at regular intervals (for example, every 20-30cm). Remember that the maximum current for each LED is about 60mA (3x20mA), so your ~150 LEDs have a total maximum current of 9A. Even dimmed to half brightness, that’s nearly 5A, and without the thick power wires as I describe, you’ll have very noticeable voltage drop along the long strip. (The copper of LED strips is very thin and thus exhibits lots of voltage drop at high current.)

Well its a mess to have more wires... Also it would require quite thorough job to solder them and hide... I would love to avoid that...
Would just replacing existing thin wires at the end of the long led strip to thicker ones help anything? I guess not as the issue is led strip itself?
Would going to 5V help?

[tooki]

Thanks for responding, it sounds like my suspicions were correct!

The problem will be less pronounced at 5V for sure. But the real solution is to power it at regular intervals.

The thickness of the supply wires isn’t the problem, as the voltage drop is due to the thinness of the conductors in the strip, which present a fairly high resistance. Mechanically convenient, but electrically atrocious.

This exact problem is the reason that most non-addressable LED strips operate at 12V, with more and more of them using 24V or higher. Voltage drop is a function of current through a resistance, so for the same power (watts, i.e. volts x amps), a higher voltage will need correspondingly less current, so less voltage drop. And percentage-wise, that voltage drop is far smaller still!

I’ve heard rumor of 12V addressable LEDs existing, but they are rare for sure. So for now we are stuck with the 5V ones and the need to supply them properly. I really don’t think you have an alternative but to run wires.

[drvtech]

12V tape does exist and is readily available. The part number you're looking for is WS2815