Pixel Ports and Fuses

[BrianZ]

A bit difficult to answer without knowing more about the physical layout of your display. Yes and no. Yes you can inject at the end of a string from a 15 Amp fuse block but you need to add amperage protection (sub-fusing) to insure the string does not burn. So yes while the amperage for the entire string will or can be increased, no because anywhere within the string amperage needs limited to approximately 5 Amp or less. A question is, where is the 15 Amp fuse? Rather obvious I think that a 15 Amp fuse immediately prior to an injection point is not beneficial.
An example may help. I have a 350 pixel prop about 40 feet from the controller and supply. The electronics are in a garage. The prop has a single power injection cable fused at 15 Amp near the supply. The controller to the prop is a smaller 22 AWG 2 conductor V- and data (no V+). I sometimes like to wake the neighborhood so I designed it for 80% White which calculates to about 16 Amp. To equalize the string voltage drop I inject at the beginning, middle (about pixel 150), and the end. If something happens the string could consume all 15 amp in a localized area and burn the string. To prevent that I sub-fuse, that is each injection point has it own 5 Amp weatherproof fuse. I could have ran 3 power injection cables but that is more cable management; not sure but I think a single larger AWG was a bit more economical as well.

I understand and thank you.

 

[BrianZ]

TerryK makes a good point.

That won't work. Ignoring the problems and assuming that you're fusing the positives, if you wanted to do a 15 amp injection point at the end of the string when the beginning is powered through a 5 amp fuse, you'd need to cut the positive closer to the beginning to divide up the current accordingly.

There are three main problems here:

• TerryK explained one problem which is that a 15 amp fuse might be too large to blow when there's a problem.
• Ignoring the first part of the string, you're powering 15 amps' worth of pixels (approximately 300 pixels) at one end. This will definitely result in voltage drop and incorrect colours. This will happen regardless of the power supply's capacity.
• 15 amps is more current than the 18 AWG (and typically thinner) wire between the pixels can safely handle.

In other words, if you have a run of pixels that's long enough to draw 15 amps, you need more injection points.

Look at this pic, this what confuses me. He is using 5v PSU for pixel and injecting, but doesn’t show cuts in the V+ anywhere. Is it because it’s a 5v PSU? Other videos I’ve watched shows the same when using the same PSU to do both. Now, when using a separate PSU, I’ve seen the V+ wire cut mid-way. What am I missing?

 

[i13]

In your first screenshot, he could cut the positive halfway between each injection point and it would continue to work correctly. What he has drawn will work but I'd recommend cutting the positive halfway between each injection point if you are fusing. If you fuse without cutting the positive, the fuses might share the load when there's a fault so they're less effective. It would be essential to cut the positive between the injection points if multiple power supplies were involved. Multiple power supplies will fight each other.

In the second screenshot, what he has drawn should work but I was there when he gave that talk and I remember him saying that it is not ideal. It would be better for the positives to be cut halfway between each injection point instead of immediately next to them. I agree with this.

 

[BrianZ]

In your first screenshot, he could cut the positive halfway between each injection point and it would continue to work correctly. What he has drawn will work but I'd recommend cutting the positive halfway between each injection point if you are fusing. If you fuse without cutting the positive, the fuses might share the load when there's a fault so they're less effective. It would be essential to cut the positive between the injection points if multiple power supplies were involved. Multiple power supplies will fight each other.

In the second screenshot, what he has drawn should work but I was there when he gave that talk and I remember him saying that it is not ideal. It would be better for the positives to be cut halfway between each injection point instead of immediately next to them. I agree with this.

Ok great, I understand. He did say in the video however, “the Americans like cutting the V+ when using one PSU to inject, but he doesn’t.” Because there was a noticeable difference in the pixel color. I guess I’ll try both soon. But definitely agree when using multiple PSU’s.

 

[i13]

I believe he was referring to cutting the positive next to the injection point. He didn't like doing that because it caused a slight colour change. The last pixel on the previous injection point gets the lowest voltage and it is right next to the first pixel on the next injection point so the colour difference is noticeable. He shouldn't have this problem if the positive is cut halfway between the injection points.

 

[BrianZ]

I believe he was referring to cutting the positive next to the injection point. He didn't like doing that because it caused a slight colour change. The last pixel on the previous injection point gets the lowest voltage and it is right next to the first pixel on the next injection point so the colour difference is noticeable. He shouldn't have this problem if the positive is cut halfway between the injection points.

OOOOOKKK, now I gotcha and makes sense. Thanks for all the info. Soon, it’ll be time to get my hands dirty and really learn.

 

[TerryK]

In the case of PI cables in your display, yes. Fuses are used for other reasons as well.

David, I've been saying that fuses are protection for more than just cable since I joined ACL; keep being contradicted. I realize it's a bit of a tomato/to-mah-toe situation but I fuse to protect the prop and buildings. I could care less if the pixels, string wire, or grass torches (as if the grass would ignite in Ohio in the winter anyhow). Some of the Boscoyo and Gilbert Engineering coro is pricy.

 

[TerryK]

First item I noticed is the lack of fuses. Separation distances would be next. The example I gave is more similar to the first than the second. I as of yet do not cut the positive because I want the string current to attempt equalization throughout the string. It also needs said that (and I realize) there is/are built in problems with such a design philosophy. For the time being, this works for me.

Regarding cutting the positive in the middle or near power injection, I've always felt that it does not matter a lot due to the color and illumination changes one usually sees in a display. Couple this with moving patterns and then determine just how well could one see a difference between two neighbor pixels.