Power to a Matrix

[BundyRoy]

I have a 10 strands in a matrix I want to supply power to. I can't power it through the controller as it is fully commited power wise. It is 10 strands of 42 ws2811 nodes. So this is 10x42x0.6W/pixel = 252W total. I have a 350W PSU so that is 72% load. So that should be okay. 252W/12V = 21A total. Now for the wiring up. I have 2.5mm2 cable. This is good for 22A but that is a bit close to the wind. So say I run two cables to the matix. It is a 2.5m long run. So 10.5A per cable.

Resistance is .0066/m = 5*.0066 = 0.033ohms. Voltage drop = 11*.033=.36V. I can live with that. Now my issue is when I get to the matrix I have to join to pigtails that are rated to say 7 amps or less. Can I just solder on two pigtails at the end of the wire and half the amps. So 5.25 amps each. If this is correct then how do I fuse the cable. Do I put a 15 amp fuse back at the PSU. I am worried that the 7 amp pigtail will melt before the 15 amp fuse blows if there happens to be an issue. Do people put smaller 7.5 amp fuses between the large cable and each of the pigtails.

Can this be done or do I need to run enough cables so that I can have a fuse that is rated 7.5amps so that it will blow before the pigtails melt. Any tips on how people normally supply power to these things will be greatly appreciated. I don't need this up by kick off on the weekend but I am planning on getting it going this year before Christmas if possible.

 

[David_AVD]

Maybe use better connectors that are rated for more current?

I use the Deutsch DT series for a lot of my Halloween props.

Altronics sells them as a set with the pins in 2-pin, 3-pin and 4-pin versions.

 

[BundyRoy]

Thanks David. So those are rated for 13A. How do I do the fuse part. Do I just design for the load in each cable to be less than 10A and put a 10A fuse back at the PSU.

 

[David_AVD]

10A fuses at the PSU end should be fine with 10.5A per cable load.

I assume you'll be splitting the matrix panels up into 2 groups power wise.

Make sure there's no connection between the positives of the two banks at the matrix end.

 

[BundyRoy]

Thanks David. Good tip about isolating the positives. I probably wouldn't of made sure they were separate as I would be thinking they are from the same PSU. But I understand they need to be separate to isolate the fuse loads/circuits.

 

[djgra79]

Also note, I assume all your calculations are based on all pixels being on 100% brightness on white.
The reality is this will rarely happen, so your power draw will be considerably lower, but always best to protect the system to its capabilities

 

[BundyRoy]

Thanks djgra79. Calcs are all on full white.I must admit I have been tempted to take shortcuts as I know there is no time when any of my sequences have any amount of white turned on at once. I can't do it though as I know one day it will catch me out and I don't want to be fixing things during peak season.

 

[djgra79]

I'm sure my wiring standards aren't, well, up to standard(!) but I know that they work. Also, despite being all white etc, many people turn down the brightness either at the controller (if an option) or in the sequence at the model level. Sometimes matrixes and megatrees wash out other lights nearby due to the pixel density, plus some effects simply look better defined slightly duller to make the colours pop out a bit more.

 

[BundyRoy]

It's a little off topic but I know what you mean. I am a bit worried this matrix will be a bit bright and overpower other elements but I'm not going to get any testing so it will be what it is for this year. This is the first time we've done a display at this house and the front yard is a bit smaller too which doesn't help with spacing things apart.