Another PI Thread

[Pavle]

Every time I think I've got a decent understanding on handling power injection, I get some doubts, investigate that and return to square one and I'm confused once more.

Watching @lithgowlights present at the ACL mini in 2017 Here he suggests not to cut the +V as you will then have two pixels right next to each other potentially with different voltages and noticeable colour differences.

From the 101 there is this image:



Here there is no cutting of the wires and the only reason I can conclude is it's from one power supply and more importantly, no fuse between power supply and injection points.

Given the above, how should this be wired?



In the above, each injection cable comes from a distro board with fusing for each of the +V

This is something I am really not getting clearly and I want to move forward with wiring up my display, but this is critical that I clear this up before proceeding any further.

 

[i13]

I watched lithgowlights's 2017 presentation in person. He was suggesting that you don't cut the positive wire immediately next to the injection points for that reason. It is still okay to cut the positive wire halfway between each injection point because the pixels there will be the same distance from their nearest injection point. It is important to note that power can travel in both directions from an injection point. It behaves differently to data which is directional.

First diagram that you posted: You're correct that the setup will work without cutting the positives when you're only using one power supply. The critical thing is that the positive should be cut between each power supply. If there was a fuse on each of the positive injection wires, it would be good practice to cut the positive halfway between each injection point. This would stop the multiple fuses from sharing the load if there was a short.

Second diagram that you posted: There is effectively one cut positive. It is between the pixel controller and pixel 1. Note that the positive output from the controller is not being used. This is to prevent the positives from the two power supplies from being connected to each other. This setup will work but it does have the problem that I mentioned above where the multiple fuses could share the load if there is a short. I would recommend cutting the positive halfway between each injection point but it technically will still work if wired as shown in the diagram.

Another point that I'll add in is that you can have different pixel voltages running end-to-end if you cut the positive wire between them. Similarly in your second diagram, you could have the controller and pixels running at different voltages because you have not connected the positive between them.

Bottom line: Multiple power supplies will not cooperate if you connect their positive outputs together through the pixels. This applies regardless of whether they are the same voltage or not.