# Number of Power Supplies

#### deblen

##### Full time elf
Hi,

The rule of thumb for 12 pixels is to Power Inject approx every 100 pixels.

A 350 watt 12v power supply roughly can power 480 pixels at 100%.

If I had say 5,000 pixels, at 12 volt, I would need 5,000 dived by 480 which equals 10.5 power supplies (350 watt).

I would have to Power Inject, every 100 pixels, to have full white at 100%

Do I then have to add extra 350 watt 12 volt power supplies to do allow for the Power Injection very every 100 pixels,

I understand I can reduce the brightness, and wire gauge can make a difference, but for a rule of thumb how do I do the maths

Am I interpreting it the wrong way.

The reason for the question is to work out approx how many power supplies I would need over a given amount of pixels

Thank Deblen

#### merryoncherry

##### Full time elf
Generous elf
This is not a good rule of thumb, unless you like to waste time and money overbuilding. You'll probably end up with a lot more power supplies, enclosures, wires, etc., if you follow this guideline, which means more time and money.
Measure your pixels first. Also decide if you're really going to run at 100%.

I measured mine, I got that 5A 12V will only power 100 regulator pixels, which are quite bright. I don't have that many regulator pixels (just inside coro props, plus some premade icicles and stakes). Most are resistor.

For resistor pixels, and the distance to the controller does matter, you might see 2.5A for 100 (which means half the supplies of your rule of thumb)... but as they get further out, 100% white uses less juice... maybe 3.5A for 200 pixels. If you go to 300, they'll still run, but be a touch dim and blue/green. This is full white. Your show is unlikely to spend long doing full white, if it is imperfect people won't notice. Even if you do inject in more places, these pixels won't use 5A for 100, 3A tops.

There are other things, like strips (mine are 1-2A/m, but I do not / cannot run them 100%, YMMV). C9-style bulbs also behave differently. The ones on the end dim and yellow noticeably if you run much more than 100 at full white, but they can't pull all 5A through the wire... if you tie back to more power you can run 200 nicely on 5A. Rectangle modules - based on my application I get better results injecting every 30... any amount of voltage drop dims them and they're inside coro props and any loss is quite visible.

Or, if you don't run at 100%, things get less demanding, of course.

#### CargoLights

##### New elf
What merryoncherry said is good advice, but to answer your question directly, no, you do not need additional power supplies for power injection. You can just run power injection wires from the same power supplies that run your controllers up to the point where you are close to maxing out your power supplies (don't forget to add fuses). Most people suggest you not go over 80% capacity for power supplies, but you can push that, especially if you don't plan to run your lights at 100%. Personally, I build so that 100% brightness would be right at capacity, then run my lights at 30%. If something strange ever happens and my lights all come on full white at 100%, in theory it should be okay, but I'd rather not try it.

There is something else that you should consider when decreasing brightness. You are also decreasing the color depth available when you do so. Honestly, most houses would look fine at 10% brightness, but then you lose a significant number of color possibilities. Most people find the sweet spot to be somewhere in the 30-50% range. Other people insist on running full brightness so they have every possible color available. Like everything else in this hobby, it's all about what you want it to be. Just make sure you have the information to make an informed decision.

#### merryoncherry

##### Full time elf
Generous elf
All true. While I run most of my stuff at 100% (unless it either isn't stable, or looks too bright next to the other things) this is just "because I can", and I like to leave it to the software to control.

That said, because of the way your eyes see color, and the way the most common LEDs work, the bottom 50% of the power levels have ~80% of the useful shades. said another way, the 50% pixel value uses about half the current but looks a lot more than half as bright.

#### algerdes

##### Al Gerdes
True story - During our first year using pixels (oh, so long ago), SEVERAL visitors stopped to show their appreciation for the display... but to also ask if there was a way to "turn down the volume" (brightness). Either that, or post that sun glasses would be required to view the display.

To say the least, we learned a LOT about using lower intensity and the affects it had on colors.

We run, on average, at 30% - which on the larger, clusters-of-pixels props is almost too much.
Your mileage may vary.

#### Croydon Lights

##### New elf
Generous elf
I ran mine at 100% the first year but when the neighbor across the road open his front blind and back one I could light up his back yard so I dim it down to 30% and he is happy and I still have a great show
Also with the power injection check out this video by Bill Porter

#### djgra79

##### My name is Graham & I love flashing lights!
Global moderator
Generous elf
Everyone has pretty much covered it, short answer is no you dont need extra PSU for power injection.
And whilst you have calculated usage at 100% despite not planning to run at 100% this is a good idea in case there is a fault or your software somehow resets your brightness settings back to 100%. You want to ensure your equipment (PSU, cables etc) can all handle 100% load if required.
But the reality is if you're running lower brightness then you can get more number of pixels per PSU, there is just a small risk of damaging something should you exceed this at 100%.

#### deblen

##### Full time elf
Hi,

Thanks to all that replied, each replied focused on what I should be looking at or doing to keep my show safe.

I am now clear to my question of how many pixels per power supplies, with power injection. Hypothetically using a 350 watt 12v power supply roughly can power 480 pixels at 100%, Power Injecting at every 100 still is only 1 PSU.

Once again, thanks to all,

Deblen

#### merryoncherry

##### Full time elf
Generous elf
What kind of pixels do you have that use .73W each?!

#### uncledan

##### Full time elf
Generous elf
Your math seems right to me. 350w power supply is 350/12=29.16 amps. High number for most 12v nodes is 60ma to be safe. 29.16/.060 = 486 nodes at 100% intensity white. Most 12v nodes don't draw anything near 60ma white 100% intensity

#### merryoncherry

##### Full time elf
Generous elf

.72W sounds too much like one of the incorrect numbers on a sus HC tag... (These measure at .25W, making both numbers wrong.)

#### Skymaster

##### Crazy elf
Generous elf
Keep in mind various factors will introduce measurement differences to what's quoted in specifications.
WS2811 will by definition supply 18.5mA per colour, or 55.5mA on full white per node. That's what the chip is designed to do, as a constant current driver.

Once other components are factored in, such as the crappy wire that interconnects them, long lead-in lines, etc, resistors, regulators, etc for those than run 12V, then you will see further reduction in current utilised.
However, if you calculate based on full white, at 100%, taking into account voltage drops etc you can be completely sure that your system will have no issues.

If you want to save a few bucks and under-build, and hope that your controller never puts out 100%, or you dont sequence full white anywhere, etc, then that's fine too, but be aware that the platform may not be stable. Thin wires = more drop = colour loss, and possible data corruption (especially on 12V systems). Blown fuses, power supply OCP kicking in if your LEDs ask for too much, etc.

When calculating - use the current - in amps - because that doesn't change based on the voltage you use - and also count them "per-pixel" not "per-LED" - because strip will use 55mA for 3 LEDs in series (as a single controlled element), with little wastage in resistors, and a bullet node will also use the same current, but considerably more wastage in resistors.
So for 250 Pixels, that would be 250 * 0.055A = 13.75A total. Which then, you can work out to be either a 68.75W PSU for 5V, or 165W PSU for 12V.

Another thing to keep in mind is to look at the power supply data sheet and find out it's "actual" power/current rating. The model number does NOT always reflect the power supply capacity.
For example- Meanwell LRS350 - will only supply 350W at voltages of 12V or higher. It puts out 60A at 5V, which is only 300W. Other manufacturers may use different numbering techniques.

#### merryoncherry

##### Full time elf
Generous elf

I would be interested in your theory of operation of the "constant current" chip that gets over 18mA through an LED with a forward voltage drop of >3V in series with an >800 ohm resistor (blue/green), or >2V and >900 ohm (red), with a 12V supply (max).

(Modern wimpy resistor pixel design pictured. 5V and regulator pixels differ, and some use more current.)

#### Skymaster

##### Crazy elf
Generous elf
Sure, your particular batch may be different - and that plays into exactly this sentence -
Once other components are factored in, such as the crappy wire that interconnects them, long lead-in lines, etc, resistors, regulators, etc for those than run 12V, then you will see further reduction in current utilised.
If a particular manufacturer decides to put in unknown components (to the purchaser) and reduce the effectiveness of the constant current design, then that's on them. But each manufacturer is different. And you won't know what you're getting until you already have it, and can rip it apart like you did, or effectively measure it, in the particular use-case/operation.

The point here is that when designing, you plan for the maximum draw, based on the manufacturer published specifications. You know that it will never exceed that. And it takes into account all variations of 2811 based pixels.
And then if you wish to 'redesign' based on the specific measured metrics of the particular nodes you have, then that's all well and good. But these can, and will change. And to use them as a basis for 'all designs' would be incorrect, and what might work in the scenario of these particular pixels, may be completely incorrect for other ones, which you even elude to here:
(Modern wimpy resistor pixel design pictured. 5V and regulator pixels differ, and some use more current.)

#### merryoncherry

##### Full time elf
Generous elf
This is turning silly, because an oversimple "x pixels per PS" rule of thumb is silly. I have a "30W" flood sitting here with Ray Wu pigtails on it, so I suppose the safe number of pixels per power supply to tell people must be 11?

1200 12V Resistor pixels* / 350W PSU are completely safe. The wimpy resistor design is quite pervasive in the market for new pixels. Regulators retain a smaller market percentage, especially in interesting configurations like stakes and icicles, and amongst die-hard fans.

*Tested 12V resistors from:
Paul Zhang - eTop, RGB man, Gilbert Engineering, Alling (Amazon), 2022 vintage
HolidayCoro - 2021 and 2022 vintages, bullet and node
Mattos - 2022 vintage

This 1200/PSU still ends up being quite conservative. If you put the pixels in a single string, the wire resistance will keep the theoretical max current down to a level safe for the PSU. You put 600 resistor pixels in a row, it is not going to draw over 5A, they'll dim out if asked to do 100%. Additionally, I would point out that any controller with 5 5A fuses per 12V 350W PSU is going to be safe, or you'll blow fuses and know you've overdone it.

This can make a big difference. If you want a 10k pixel show, and you believe that incorrect tag/data sheet, you'll buy 20 power supplies and associated cables/enclosures when you really only need 10, if that. I already made this mistake, no need for anyone else to repeat it .