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2 injections from the same power wire?

Started by Reys Dad, August 28, 2019, 12:44:34 PM

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Reys Dad

I've got a 5v prop that is 209 pixels. I'm planning to power inject at pixel 77 & pixel 159. I'm trying to keep the number of wires to a minimum so can I get away with using 1 port on the F8 and using 1 power cable to supply power to both injection points or do I have to use 2 ports on the F8 and 2 separate power cables one for each injection point to do this?

TypingOffKey

#1
209 5v pixels at 100% white (assuming 60 milliamps per pixel) would pull about 12.54 amps. Assuming the distance isn't too great where voltage drop starts to play into the equation, 14 AWG wire would be sufficient to carry the total load of 12.54 amps, whether it's injected at 1, 2, 3 or more points. The problem is going to come down to fuses and trace sizes on the F8 board itself. The fuses are 5 amps, so even with 2 injection points perfectly balanced it's possible that you'd exceed the current capability at 100% white for an extended time. Realistically, if you're running less than 100% and not using full white, you'll likely be fine with 2 points, and may even be able to get away with 1 if you turn down your brightness further.

That's all assuming the pixels aren't drawing any current from the Differential receiver itself, and are getting all of their power solely from the F8. If the Diff. receiver is also connected to the same power supply, it will be providing upto 5 amps into the start of the string, so you can use that to reduce the remaining requirement from the power injection. If it were me, I'd send power into the Diff. receiver and do a single injection between pixels 139 and 140. That injection point effectively splits the string into thirds, with the first third being powered from the receiver, the second third being powered (in "reverse" - from the tail end instead of the beginning) from the injection point - pixel 139 down to pixel 70 - and the final pixels 140 to 209 being powered in the "normal" direction from that same point. The current load at the space between pixels 139 and 140 will be 2/3 of the entire string (about 8.36 amps full white, 16 AWG would be fine) but the current will flow very happily upstream and downstream to all of the pixels along the existing pixel wire.

Reys Dad

Thanks for the detailed response, I forgot to mention that I'd be driving the 1st pixel off an F16V3. If I inject at 140 I'll need to splice into one of the strings since it's in the middle of one of the strings but it's certainly doable.

TypingOffKey

Got it! For some reason the first time I read your post as running off an F48, (not F8 power distro) but once I corrected that mistake in my head I was still thinking about differentials  ::)  ... Everything else still applies though, and if you can power inject around the 140 mark and keep away from doing full white at 100% you should be fine with just one injection.

TypingOffKey

Oh, and if you've already got convenient injection / splice points at 77 and 159 pixels, I'd say just use a single port on the F8 wired to both of those points in the pixel string (share a 16 AWG pair of wires if convenient, no need to run 2 pairs of wire) which will still split the load pretty evenly around the right length in the string.

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