LED Cyr Wheel: power draw
In one of my last posts, I discussed power draw. In theory, 360 LEDs on each side of the wheel (720 total) might draw a ton of power (20+amps) based on the specification for the LEDs.
UPDATE: Scratch what I have below. I wasn’t measuring full on, but only half! I have two 67 length strips (about 2/5ths of my wheel on one side), and it draw 5.56amps. So yeah, about 0.04149 amps per LED, and I will have: ~330 per side, ~660 total: 27amps.
I’m going to probably have to software limit full brightness to never reach this high, as it will kill battery life, and maybe melt wires if I’m not careful. I’m starting to think about running an 18 gauge 2-wire through the wheel for power delivery. This also means I need to (again) re-think connectors. Molex Mini-Fit Jr are rated for 9amps (or 13amps with better terminals!). I think this would work, as current will be split from the battery to each half of the wheel; meaning the connector on a given side of the battery has to dish out at most half the current needed so a 13 amp max terminal. Damn..now I have to order these terminals. I’m getting the 18g and 16g ones (they say the wire needs to be 16ga to carry 13 amps).
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So, I bought a 40amp 5v power supply (they are cheap) to cover my bases. I hooked up my 6meters of LEDs that I have and ran them at full white. They drew 2.72 amps. Totally way less than what it was supposed to. This is only half the wheel, so let’s say the entire wheel draws 5.5amps. That’s not much! Then I realized the LEDs were dim on the second half of the 6meters. Oh doh, they have loss when run in series. So, I could easily power each end and tested that; 4.32amp draw at full white. Significantly more power use!! I’ll probably power each segment of the 5 piece wheel separately (roughly 1.25m), which will ensure brightness, but also cause more power draw. My rainbow pattern is drawing 3.48amps at most; definitely less than full white, but still quite a bit of power.
So, I need at least two decent size batteries to make this work. Ideally 2400-3000mah. Looks like the thing that will fit in the wheel are 1400mAh turnigy nano-tech cells. I can have two on top of each other in parallel:
1400*2*7.2*.95(efficiency of converter)=19.152whrs. Let’s say 9amp total draw at 5v: 0.42hrs, or 25 minute calculated run time. Not much! But spare LiPO cells will be easy to put in, and actual run time may be better.
A monkey wrench thrown into the equation is that my connectors I bought are rated at 5amps. I spent another $20 bucks at mouser ordering 4 wire Molex MiniFit Jr connectors — they can carry 9 amps of current; in reality, I shouldn’t see more than half the total current through the wire, because the battery will be centered in the wheel and deliver power to each half of the wheel. If I have trouble with getting the full voltage to the other half, I may have to put the second battery for one half the wheel on the other side…but I’m hoping to not do that, as it would involve having two power switches (or more wiring — in which case, I should just wire it with thicker gauge wire).
I experimented with current loss over 14’6″ of 22 AWG wire; the 5.06v at the start reads 4.67v at the end, and this affects the LED brightness. I’m hoping to use 20AWG gauge wire, but I can only do it if I don’t get that large a voltage loss for half the wheel circumference (less than 10′ for me).
