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LED Strip - Power supply


#1

Dear friends,

I beg for your knowledge of electricity in order to sort out the following problem.

What I have:

So, when I use a 4.5V 0.5A 6W wall adapter from my old Panasonic CD player I am able to make the whole strip lit with no problems.
However, my final project needs about 600 LEDs and I’ve decided to buy a much larger power supply. Here’s basically it:
https://goo.gl/dqBiiQ

As you see it’s 5V 40A 200W. Since I don’t have 600 LEDs right now I’ve decided to test it with the same LED strip as above. As a result only 2 or 3 LEDs lit with some random colors. When I switch the power supply back to the smallest one - everything is fine again.

Do you know the reason for this mystery?


#2

@ DeathBloom, Did you try switchin the power on and then connecting the neo pixels?
From AdaFruit:[quote]Some — even reputable, well-regarded brands — can produce a large voltage spike when initially switched on, instantly destroying your NeoPixels![/quote]

Also, if you’re running the neo pixels at 5V, are you using a logic level shifter to convert the µC voltage to 5V levels? If you were using 4.5V before without a level shifter, then you were doing it wrong in the first place. If you can, try getting a voltage regulator to convert the 5V down to ~3.3V (don’t forget the heat sink on that regulator).


#3

So my first instinct is to talk about signal integrity and my second is to talk about voltage level shifters for signals. Honestly though, if you’re asking these kinds of questions when you are about to scale up, you have many things to worry about. Power is certainly going to be your #1 concern (as you need a heap of power to light all WS2813’s at full intensity white, if your demand requires it), but there are other aspects like speed you can send the data to the chain(s) will also be pushing things. If you aren’t comfortable with the power change to 5v power supply, you’ll have other power challenges you won’t be able to deal with as you scale up


#4

Something else to be aware of. Most of these strips have a plug on either end, This misleads people into thinking they can just keep slapping one led strip onto the end of the next, and make one super long strip.
Many people miss this point that you cannot keep adding strips, because the power and ground traces on the strip cannot handle that kind of current. If using say 3, 15 meter strips back to back, the 3’rd strip will have dimmed leds compared to the first 2.

Just something else to know.


#5

Another thing to consider is that often larger current output supplies often require a minimum load to maintain good regulation on the output. I could not find anything in the website about this but something to consider.

Have you checked the actual voltage of the supply when you use it on the smaller LED strip? It may be higher than 5V.


#6

Hi! Thanks all for the replies! Here’re my comments:

[quote=“Mr. John Smith”]
@ DeathBloom, Did you try switchin the power on and then connecting the neo pixels?[/quote]

No, the schema was assembled and I’ve just switched on the power. I will try this out, thanks for the hint. However, none of NeoPixels were destroyed as I’ve then checked all of them again with the initial smaller capacity power supply.

According to the datasheet it can run 3.5-5.3V. So it looks like both power supplies are good.


I believe there’s something in WS2813 that takes care of this.

I think I am ok with it, just the thing that I can’t understand why my larger power supply doesn’t work as expected. I will get a multi-meter soon and check out the real voltage and current strength.

This is true. For my project I will need around 4 meters of the strip. I’ve calculated and came up with the plan that I need to run it in parallel every 2 meters, so basically only 2 power connections. The data signal though will run serially.

I’ve read about it yesterday somewhere. Nice catch. Do you think a resistor will be a good / enough load? Any ideas on Ohms?

Yeah, this one I am about to check soon… The typical case when you are a programmer and not electrician: you usually believe on whatever is written in specs and don’t check with multi-meters as you don’t have one :smiley:


#7

@ DeathBloom -

[quote] According to the datasheet it can run 3.5-5.3V. So it looks like both power supplies are good.


I believe there’s something in WS2813 that takes care of this.[/quote]

No, I’m talking about logic levels. For a voltage to be considered logic level high, it has to be at least two thirds (2/3) of the supply voltage (aka VCC). If you’re running at 4.5 volts then logic level high starts at 3 volts. If you’re running at 5 volts then it’s 3.3 volts. Now since your µC is running at 3.3 volts, then it’s 2.2 volts. Now since there will definitely be a voltage drop between the pin of the µC and the first LED, if you’re running at 5 volts, then that first chip is most likely never going to identify the µC’s less than 3.3v logic output as logic level high.

This can explain why it works at 4.5 volts and not at 5 volts.


#8

Ah… you are totally right… and it’s also written on Adafruit:

I’ve did some calculations using the WS2813 datasheet where:
Vih = 0.7 * Vdd = 0.7 * 5.3 = 3.71
Vil = 0.3 * Vdd = 0.3 * 5.3 = 1.59

and it looks like with 3.3V logic I am missing the HIGH segment, though LOW is ok. I suppose there’s no 5.5v MOSI pin in Fez Panda2 so I will really need a logic level shifter.

And ongoing question: If I have that shifter in place do I need to place a resistor in between the data output of my controller and data input of the LED strip as it’s said here:

[quote]Place a 300 to 500 Ohm resistor between the Arduino data output pin and the input to the first NeoPixel. The resistor should be at the end of the wire closest to the NeoPixel(s), not the microcontroller.
[/quote]
Is it to protect the NeoPixel from a voltage spike of a controller? If I have a shifter I suppose that resistor still should be place between the shifter and LED strip, correct?

I was also checking the output voltage of power supply and it is ok - 5.2 V. I’ve accidentally touched the data input of the LED strip and it all turned white - so it proves the Power Supply is ok.


#9

@ DeathBloom - If the power supply is driving at 5.2V then logic level high is 3.46 Volts! I think the resistor is mainly to ensure that there isn’t too much current draw from the µC (which has its own set of problems). Since you’re doing the logic level one way, you might just need a NPN transistor to drive the LEDs (but only if you can’t get a logic level shiftier).

Connect Base to µC with 500Ω resistor in series.
Connect 1KΩ between the base and GND.
Connect the Collector to VCC via a 1KΩ resistor.
Connect the Emitter to the LED, and the Emitter to GND via a 1KΩ resistor.

Your resistor values will vary based on supply voltage.


#10

@ Mr. John Smith,

thanks for the reply!

Do you think this schema is what I need?

http://dspview.com/download/file.php?id=385&sid=c2db657f14de96a2c8ffc0cc5159fe37

I probably can, however, I am unable to find any schema with it in action. I am really lost in all of its pin-outs and data-sheet doesn’t make it easier to understand. Do you have any examples maybe?
Or is it only about connecting:

  • Digital output of a controller to A1
  • input of the LED strip to B1
  • 3.3V from the controller to VL
  • 5V from the controller to VH

#11

you want something like this.

Have to be mindful these days using transistors instead of fets. With the speed of data buses growing faster and faster these days, it should be the norm to just go with a fet as they have better slew rates than transistors, and now with so many logic level fets out there there is no need for the gate resistor. less parts, and simplicity.


#12

Checking the logic level shifters and getting confused more and more…

Some of them can convert TTL to CMOS, some of them are CMOS to CMOS only.

WS2813 seems to be CMOS. Do you know whether MOSI1 on Fez Panda2 is TTL or CMOS?


#13

deleted


#14

@ Brett - :smiley:

@ DeathBloom, OK err, µCs are all CMOS. I’m fairly certain that the LEDs are CMOS also. Use CMOS to CMOS.


#15

microcontrollers IOs are compliant with CMOS and TTL - depending on the chip :slight_smile:

OK, so the upshot is you need to check your specific product. LPC2387 (used in Panda2) = TTL (no mention of CMOS in datasheet). STM32F411 family (used in Panda3) says “All I/Os are CMOS and TTL compliant”


#16

Damn, you deleted this just as I was about to reply to it :frowning:

Many of the newer ones are actually CMOS but as per the STM32 they state the following in their datasheets. :whistle:

All I/Os are CMOS and TTL compliant (no software configuration required). Their characteristics cover more than the strict CMOS-technology or TTL parameters.


#17

Thanks for the replies very much, guys! CMOS to CMOS it will be.
I’ll keep you posted on the initial issue resolution.


#18

@ VersaModule

Could you please take a look at this schema?

http://dspview.com/download/file.php?id=385&sid=c2db657f14de96a2c8ffc0cc5159fe37

it uses FET, so I believe it should suit me really well as I only need 1 in/out in one direction.


#19

so lets really talk details here. How are you powering the Panda2? I think we’ve all been assuming you’re powering it via the same power supply, but I can’t see you actually say that. Please be specific - how do you power up both the Panda and the LED strip. Also, have you got that multimeter yet, and have you found what the zero-load voltage is on the 5v? I suspect this is NOT a regulated power supply, and that’s a bad thing…


#20

@ Brett - He said the power supply delivers 5.2V.