It turns out that only two lines of pixels on the board are ever lit at one time. Now, let's suppose the pixels are full on white. So each pixel has 3 LED, each pulling 45 mA. So the current would be:
2*64*3*45 = 17,280 mAHoly smokes. Adafruit suggests a 4 amp supply for a 32 pixel wide matrix. Their calculation for that is 32 * 120 = 3,840 mA -- so a 4 amp supply would be just right for a 32x32 matrix where only one line is ever lit at a time. They are clearly using 120mA from 40 mA per LED (maybe they know something about the panel that I don't). My panel being 64x64 would use 4 times what theirs would, so 15,360 mA.
Now cranking the panel full white could certainly be viewed as an unlikely use case. However, supplying 10 amps for my 64x64 panel does not sound at all unreasonable. I have a boat anchor 8A linear supply picked out for this project, along with a 10A switching supply on order. I plan to use a little 2.5A "wall wart" out of my junk box for initial testing.
It is worth pondering also that my display will only have 2 lines out of 64 lit up at any one time. On one hand I am curious just how bright the display will be with each LED on only 1/32 of the time. On the other hand, if two lines can draw in excess of 10 amps, imagine if it was possible to light up the entire panel all at once. That could draw 32*10 = 320 amps!
Consider also big panels that are used in auditoriums and the sides of buildings. Where I go to church there is a display 2432 wide by 1024 tall. This would be 38 wide and 16 tall if made from 64x64 panels. This is 608 panels all told. If the system is designed to allocate 10 amps of current to each panel, that requires a 6080 amp power supply!
I'll also note that my 2.5mm pitch panel is 6.25 by 6.25 inches. If such panels were used for the church display, it would be:
237.5 inches wide by 104 inches tall 19.8 by 8.7 feet
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