Power Calculations/Injections

So, I’ve expanded the number of (WS2811) LEDs on my Christmas Tree to 800. One more (50 LED) string to hang. (I know! Airplanes will be attempting to land here.) PSU is 12V, 30A, and I have Power Injection at beginning and end of the string. Struggled a bit to get everything to light. Unchecked ‘Enable automatic brightness limiter’ (default setting 850 mA), and voila. The tree looks great.
The sudden burst of brightness and color got me wondering about power management. I’ve found several sources that attempt to explain the related calculations, but they usually refer to a online calculator. So far, I am unable to match the ‘calculated’ power requirements with my manual calculations. I’d really like to gain some confidence with power requirement calculations before I set my house on fire or burn up my LED strings.
For example, my WS2811 LEDs come 50 per (12ft) string (18AWG, I think). Each string requires 30W.
I currently have 16 strings on my tree (with one more to go). Does this mean I need (17 * 30) * 1.2 = 612 Watt PSU? (1.2 Factor to provide 20% excess for estimated PSU.) Online calculators seem to estimate the power requirement much lower. Appreciate anyone can point me to a good (semi-detailed) explanation of these calculations.
The WLED Power Calculator (on github) estimates LED power consumption for this setup at 133.33 W.

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Your calculations all look reasonable.
Doing your requirements based on Watts of power is easier (IMO) over all and adding the 20% overage is good planning.
You do need to convert back to Amps for the wire sizes when doing injection calculations and that’s part of the issue with the various calculators out there.

The key question is:
Where does your figure of 30W/50 LEDs come from?
Experience has shown that posted specs on strings from mfg. are often ballpark at best.
The safest course is to actually measure the draw of a string at 100% White to get a real reading of what’s needed. You can setup a smaller segment of perhaps 20 LEDs for a comparison set of numbers.

That will give you an accurate reading of worst case current/LED.
You can plug that number into the online calculators to get an idea of the total current required and their recommended injection points.

I’d go 1 step further in doing the drop calculations:
Figure out how many LEDs you can light at 100% White before the string itself shows significant drop.
Going back to your current test, once you measured the actual draw for 50 LEDs @ 100% White you can measure the voltage drop at the end of string under the same conditions. Just measure the start and end voltages, that will give you the real worst case drop.

You may be surprised that the drop is higher than you think.
One issue that can be in your favour, is that 12V strings may be more immune to the effects of drop than 5V, it depends on what type you have. Some have builtin regulators for each pixel while some are just resistor based. The regulator versions give better brightness reliability and tolerate higher voltage drops.

My experience has been that 100 LEDs is the farthest I can expect to get consistent brightness on uninjected strings at 12V. I do injections at “midpoints” rather than ends, so each injection line has to handle the total current of 200 LEDs (100 to the “right” and 100 to “left” of the injection point).

Once you know how many LEDs you can drive before injection, the calculations become easier as its all about getting the current needed for 200 (in my case) LEDs through an injection wire. You use a wire drop calculator (I like: Wire Drop Calc ) to pick the size of wire for the length you need.

The full-on string calculators do something like this with a variety of builtin assumptions, I prefer to measure the real world and see what’s actually happening.

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Thanks divsys,
Points well taken. Was hoping for mathematical treatment to establish boundaries, but appreciate the recommendation for ‘real world’ measurement. Expecting a string of LEDs today & will do some testing to try and establish power requirements.

The other (main) reason I push doing your own measurements is that the general state of mfg. specs on these things is so lousy.
Best know what’s happening in the real world, rather than be surprised.

Please excuse my general lack of electrical knowledge. But…
Your original response seems to focus on ‘voltage drops’ and evaluation of ‘power injection’ requirements. I have measured the voltage at several points, and do not note a significant drop. It is my understanding that if there is a significant ‘voltage drop’, LED behavior typically goes a bit ‘wonky’. (i.e., LEDs experiencing low voltage either fail to light, dim significantly, or exhibit other unexpected behavior). I am observing none of that.
I had intended to focus my original question on Automatic Brightness Limiter (ABL) adjustments. As I understand it, ABL limits brightness by limiting current (mA), and is used primarily to assure current draw does not exceed the capacity of the PSU. If I turn off ABL, lights are really bright. If I turn on ABL, I can adjust brightness by changing mA setting (from 1500 - 3500mA). Is this a reasonably ‘harmless’ range to work in?
So, with my rather feeble level of electrical knowledge, if my (12V) PSU is rated for 30A, it should be able to handle ABL = 3500mA. Or, am I comparing apples to oranges?

So the TLDR answer 1st, 30A=30000mA, (count the zeroes, 4 of them) your supply should be well capable of what you (think you’re) ask of it.

Longer details:
The ABL doesn’t actually limit anything in the sense it doesn’t measure current that’s actually being drawn by your LEDs. All it does is use the current per LED you tell it and then watches how many LEDs your effect/preset displays and multiplies out the mA/LED to come up with a number it thinks is being used. If that number is more than the max you’ve told it your supply can provide, it drops the brightness until the number is lower.

That’s “software as a safety feature” - which is no safety at all and often leads to confusion about what’s really happening with your setup.

If you’ve actually measured little to no voltage drop at points on your string (at all LEDs 100% White). Then your power injection is fine, and you’re good to go. Set the ABL to 30000mA (that’s the capacity of your PS) and leave it at that.

The real protection for your setup comes from using appropriate power wiring (lack of voltage drops is a good sign), fusing, and a good PS. So far it sounds like you’re in reasonably good shape.

Just want to point out that I highly doubt you will be running your
Christmas tree at 100% brightness so your current draw should be much less than your ‘max’ value. I would bet that running at a brightness of 20 to 30 % is more than enough. If this tree is going to be inside your home PLEASE add proper fuses.

What kind of tree are we talking about? A real tree? A pixel tree made up of pixel strips?

On my 1200 pixel mega tree I have it at like 15-20% brightness and am running 400 12v pixels without injection without issue.

Actually that 30a power supply has 1,765 12v pixels on it and I don’t think I saw it draw more than like 15a with them at 15-20% brightness. Granted it’s not sitting there on 100% brightness and white…

If you plan your power for the worst and create your displays for the best, you’ll never have an ooops… moment with power.

No such thing as a foolproof system - fools’s are too ingenious (me included).

Thanks loads for the helpful insights. I’m feeling better and better about understanding my components. Thanks also for your patience. Well, my last string of LEDs has arrived. I’ll be getting out my meter and checking some ‘real world’ values.

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