Voltage drop - does this look right or is something wrong

[janastas]

Hi All,

After wiring up a number of chromaeggs correctly last night I decided to check if all worked well and how much voltage drop I can expect and where I need to power inject.

The setup I'm testing on is

• 5v 350 power supply connected to
  • Rpi28d+ hat and pi3

I'm connecting a single chroma egg (49 pixels) to one of the 28d+ ports just for the purpose of this question
I'm setting the port brightess to 30% and the distant from the port to the first pixel is the length of the two pigtails (around 36inches)

If I run some figures into the voltage drop calculator at http://spikerlights.com/calcpower.aspx I get the following:

pixel count: 49
pixel voltage: 5v
pixe watts: 0.3 watts (questioning whether I should change this)
pixel spacing: 3inch
intensity: 30%
pixel wire size: 24awg

According to the above I'd expect a voltage drop at the end down to around 4.6v?

With my pixel meter I'm finding the voltage is atually 4v at the last pixel.

Can anyone point me in the right direction as to why I'm getting 4v instead of around 4.5? - 4.6v?

Happy to post some photos or answer any questions if that helps.

 

[AAH]

Check the voltage at the power supply and at the start of the string.
Using https://www.da-share.com/calculators/cable-voltage-drop/ I calculate a voltage drop of about 0.3V per conductor or 4.4V at the end. David's calculator doesn't work out a progressive voltage drop. Are the pixels 3" spacing or 3" wire length? 3" spacing will have a wire length of about 4". I don't know if this is allowed for in the spiker voltage calculator.

 

[janastas]

Yep will check voltage at power supply and start of string after I finish work for the day.

The pixels I purchased were the following: https://www.aliexpress.com/item/32761557345.html?spm=a2g0s.12269583.0.0.6d55788dCw9OYN

Not the smartest buy for me in the world but I had to start somewhere with pixel strings (made the mistake of buying led strips and might repurpose them at some stage in the future)

Should have gone for 1000 pce 18awg regulated instead.

 

[AAH]

Yep will check voltage at power supply and start of string after I finish work for the day.

The pixels I purchased were the following: https://www.aliexpress.com/item/32761557345.html?spm=a2g0s.12269583.0.0.6d55788dCw9OYN

Not the smartest buy for me in the world but I had to start somewhere with pixel strings (made the mistake of buying led strips and might repurpose them at some stage in the future)

Should have gone for 1000 pce 18awg regulated instead.

I'd use the ones you bought over 12V regardless of whether it's resistor or regulated every day of the week.

 

[i13]

Can you measure the resistance of the pixel wires to check whether they're actually 24 AWG?

If these are WS2811 pixels then 0.3 watts is too high but correcting this will make the calculated voltage at pixel 49 even higher. WS2811 pixels can't exceed 18.5 mA per colour. Multiplying this by 3 (R, G and B) gives a total of 55.5 mA or 0.0555 amps as the maximum possible current. Some pixels will draw less. Multiplying the current in amps by the 5 volts gives 0.2775 watts per pixel as an upper limit for the possible wattage.

 

[darylc]

24AWG advertised pixels are likely really 26AWG or 28AWG just like 18AWG advertised pixels are really like 22AWG.

 

[janastas]

Check the voltage at the power supply and at the start of the string.
Using https://www.da-share.com/calculators/cable-voltage-drop/ I calculate a voltage drop of about 0.3V per conductor or 4.4V at the end. David's calculator doesn't work out a progressive voltage drop. Are the pixels 3" spacing or 3" wire length? 3" spacing will have a wire length of about 4". I don't know if this is allowed for in the spiker voltage calculator.

Voltage at power supply: 5.00V
Voltage at pigtail connected to controller port: 4.99V
Voltage at end of last pixel in the string with lights off: 4.85V
Voltage at end of last pixel in the string with lights on (all white at 30% brightness): 4.00V

I got roughly the same results above on all three chroma eggs I put together so must be the pixel strings and the gauge of wire?

 

[TerryK]

Use 26 AWG in the calculation to get the result closer. As Daryl indicated and has been discussed in other threads, Chinese wire sizes run small. I physically measured 18 AWG pixels from Paul Zhang and compared to 'typical' chart values the wire size equated to 20/21 AWG.

I also checked the listing. The LEDs are indicated at 0.24 Watt, maximum power at 0.3 Watt, and maximum current at 0.060 Amp. Something is a bit off in the listed specifications.

 

[janastas]

Thanks all for your input.

For the purpose of the three chroma eggs my plan is just to power inject after every prop.

I did this last night and if you just run a standard rgb pattern through each prop the pixel tester indicates around 4.5V the end the of each prop which sounds about right.

Push it to all white and the voltage drops to 4V.

Note this is all at 30% brightness.

I went browsing last night and found the some 5V 18AWG pixel strings with ray wu pigtails already attached.

I'll wait for these to arrive and give them a try (needed more pixels anyway!)

 

[TerryK]

The lighter gauge wire can have a larger effect than one would suspect. Some testing I did with WS2811 18 AWG 4 inch spacing LEDs had the voltage at 3.96 volt on pixel #48 when driven at 100% White. The source for these as I recall was Wired Watts.

 

[AAH]

Where possible I always link the beginning and end of props. That way there is essentially no voltage drop from 1 side of the prop to the other and onto the next 1.

 

[janastas]

Where possible I always link the beginning and end of props. That way there is essentially no voltage drop from 1 side of the prop to the other and onto the next 1.

Yeah was thinking about that after seeing a few youtube videos.
I'll give that a try on the next set of small props I get.

 

[janastas]

Hi All,

After wiring up a number of chromaeggs correctly last night I decided to check if all worked well and how much voltage drop I can expect and where I need to power inject.

If I run some figures into the voltage drop calculator at http://spikerlights.com/calcpower.aspx I get the following:

According to the above I'd expect a voltage drop at the end down to around 4.6v?

With my pixel meter I'm finding the voltage is atually 4v at the last pixel.

Can anyone point me in the right direction as to why I'm getting 4v instead of around 4.5? - 4.6v?

Happy to post some photos or answer any questions if that helps.

So an update on my original post above.

I had some Ray Wu pixels delivered today that already have the Ray Wu pigtails attached and is using 18AWG wire.

Connected up to my controller running at 30% brightness and the voltage meter at the last pixel was reading 4.65V so much much better.

Cranked up the brightness to 100% and the voltage dropped down to just under 4v so definately a big improvement.

I think the lessen learned for me here is to not buy something on sale unless I check the specs in detail first.

I'll keep the chroma eggs as is and just power inject after every prop but going forward will check the specs more closely of pixels I buy and ensure they're at least 18AWG.

Also noticed that the brightness and clarity on these new ones is much better, even the epoxy used to fill the pixel is so much clearer.

The first batch of pixels I bought were very cloudy and you couldn't even see anything.