Power Topology

This is the area where most folks run into trouble. Power conversion and distribution on a large LED installation is not a thing to be trifled with…done wrong it can lead to component failure, fire and/or injury. Really!
Don’t mess around.
Think about the trunk and branches of a tree: hefty at the core, progressively thinner toward the extremities. Why is that? Well yes, of course, the tree needs to hold itself up. But also…you can’t squeeze a whole tree’s worth of water and nutrients through a narrow branch.

Power distribution is exactly like that.
Wire has resistance. Thicker wires have less resistance; they can transport more current safely. The rest is lost as heat. In extreme cases, a lot of heat…enough to start a fire.

Google around for “wire gauge ampacity.” You’ll turn up charts with recommended limits for various gauges of wire. Stick within these bounds and you’re safe. A couple points to keep in mind:

  • Most charts show different limits for “free air” vs “enclosed” wire (or “chassis” vs “power distribution”). The former is for short runs where ambient air provides thermal relief; latter is generally for long runs through conduit, with a lower safe threshold. For a project like this one, we can consider it chassis wiring…but that’s no reason to push the limits. Allow for some overhead!
  • The charts are almost always for solid-core wire. Stranded wire is nice for its flexibility, but its equivalent cross section is lower — the gaps between strands mean there’s less current-carrying copper, so you need to scale back the numbers a bit.

Being overly dramatic, aren’t you? LEDs are super efficient, they hardly use any power!

Indeed they are. It’s not a matter of efficiency though, but scale…

ALL THE LEDS!


A single LED uses so little power, it can run off a tiny coin cell battery for hours, even days!

One NeoPixel contains three LEDs: one each for red, green and blue, plus a tiny embedded controller chip:
At full brightness, one NeoPixel (producting enough light that it kind of hurts to look directly at it) draws about 60 milliamps of current at 5 Volts. That’s a modest amount of power. Efficient!

Our 2-meter strips each hold 60 NeoPixels. 60 NeoPixels × 60 milliamps = 3,600 milliamps…3.6 Amps! Suddenly, that’s a real amount of current. Yet we haven’t lost efficiency, we’re just dealing with a whole crapton of LEDs!

Multiply that by the number of strips: 3.6A × 24 strips = 86.4 Amps at 5 Volts. By comparison, the power brick for a phone charger might provide 1-2 Amps, tops. 86.4 Amps is a lot. If it makes you feel better, run around screaming it like Doc Brown’s “1.21 Jiggowatts!”

Referring back to those ampacity charts, you’ll see this would require cables with solid copper as thick as a pencil. This is where people run into trouble. The usual electronics project hookup wire and “wall wart” power supply aren’t gonna cut it…a project of this scope requires a change of materials and techniques. Some of these parts we don’t offer at Adafruit; it’s beyond the scope of the hobbyist…you’ll need to turn to industrial suppliers.

Look at this thing:
This is a 5 Volt, 150 Amp DC power supply bought at a local hamfest (the baseball is just there to provide some sense of scale). See those two bolts? Those are the DC output! Bolts! This thing requires cabling like the battery in your car.

It’s okay to use an oversize power supply — one with a higher amperage rating than our LEDs need — the circuit will pull only what it needs, and the supply will run a bit cooler. Just don’t use something with a higher voltage. 5 Volts is it!

If you’re a surplus scrounger, be certain what you’re getting is a DC supply. This is easily overlooked! AC will kill your pixels.

Shopping around sites like Mouser and PowerGate Express, I ultimately settled on this supply:
Instead of bolts, it features three pairs of spade terminals branching directly off the power supply. Since each pair only needs to handle 1/3 the current, this allows for more conventional wiring (albeit still heavy gauge). The three-way split also correlates nicely with the three Fadecandy boards, providing some organization to the system: for each group, data and power branch out to 8 NeoPixel strips. Also, each set could be separately fused — an electrical short in one area won’t bring down the whole system:
The Raspberry Pi taps off one set of bus bars. That’s fine…it uses so little power as to be negligible.

A Chain is as Strong as its Weakest Link…


The extra per-section fuses add a margin of safety…but, due to haste, also created an unexpected bottleneck that would determine the scale of the power system.

Not wanting to wait around for special-order parts, I found some nice inline fuse holders at the local auto parts store. These came pre-molded around 12 gauge stranded wire pigtails…which had a recommended max rating of 22 Amps. 3 × 22 = 66 Amps, about 25% short of our desired 86.4 Amps.

I could have done something stupid at this point. I could have crossed my fingers and “redlined” the system, or skipped the fuses altogether. Or I could be sensible and delay the project to locate some 10-gauge fuse holders. Or…the path ultimately chosen…I simply limit the maximum brightness of the LEDs to 70% in code, so they never exceed 60 Amps total. The Fadecandy software has a setting to handle this!

This is not necessarily a bad thing.

1,440 NeoPixels at full throttle will knock you on your ass. 1,440 NeoPixels at 70% brightness will still knock you on your ass. Honestly, we’re NOT left wanting for photons. Scaling the system down to 60 Amps pays dividends: there are more power supply options at this size (and at lower cost), and 12 gauge (vs 10 gauge) wire is more manageable and saves money. (No point using 10 gauge wire if the fuse holders are 12 gauge…we can’t “get back” that current…it truly is a matter of the weakest link.)

The power supply in the photo above is a Mean Well RSP-320-5, rated for 5 Volts at 60 Amps. I chose this one because it’s slim and has the multiple spade connectors and a decent rep…but there are many options from many manufacturers that’ll do the job just as well. (I also liked that the spade connectors were recessed…safer that way, unlike the one with bolts sticking out, just waiting for a dropped wrench.)

Could I instead use a whole bunch of 5V power bricks working together?

Could I drive framing nails with a whole bunch of upholstery tack hammers working together? It’s a matter of the right tool for the job. Large DC supplies are purpose-built for this kind of load, providing consistent voltage across the system. They’re not that much more expensive.

Besides, where would you plug in all those bricks? It would look terrible!

How about an ATX power supply?

That’s a fine hack for medium-sized projects…ATX supplies can be found as free scrap sometimes, and it just takes a single wire jumper to turn them on. They’re inadequate for really large projects like our full NeoPixel curtain though…even the beefiest ATX supplies top out around 40A total on the 5V lines. And the distribution is rarely even…you may have six Molex connectors on a single cable, two on another…it doesn’t balance out, and you don’t know the safe limits on each wire.

Is this cheap power supply on eBay any good?

Speaking personally, I would never plug in an imported no-name thing that I can’t pick up and carry the fire outside. I might consider a surplus or secondhand unit if it’s a reputable brand normally carried by a distributor like Jameco, Mouser, etc. (like that Powertec hamfest find) and in good condition.

Whatever you decide, I do strongly recommend buying it domestically. I’ve ordered big-ticket items from overseas and had entirely different items arrive…and then return shipping for a refund would cost more than the original price. Buying closer to home costs more, but you won’t get stung if an exchange is needed.

What if the software brightness throttle fails?

Then we’ll know for sure whether the fuses work!

This power supply says it can provide X Amps continuously…but also X+Y for 1 minute or X+Z for 5 seconds. Since my LEDs won’t ALWAYS be full white, could I use a smaller supply this way?

Two good reasons why this is a bad idea:

  1. When a power supply is pushed to its limits (e.g. LEDs exceeding the continuous-rated current), even briefly, the output voltage often sags. We’ll be powering our Fadecandy server off this same supply…and with a power brownout, the computer may lock up. When this happens, the NeoPixels retain their last color…which we’ve already established is more than the power supply can sustain. Brrzap, fizz, pop.
  2. When engineers design a bridge, they don’t just do the math based on trucks filling the span bumper-to-bumper…they take that and perhaps double it, even though it’s physically impossible. This is the engineering overhead. Nature excels at throwing curveballs, and systems are often put to unexpected demands. When success depends on that “X+Y for 1 minute,” you’re not just leaving yourself no overhead…you’re purposefully dipping into the red every time!

Either get a bigger power supply that can sustain the peak estimated LED current, or implement the software brightness throttle (demonstrated on the Fadecandy Server Setup page). If I wanted to be really good about it, I’d dial back the brightness a bit further.

“With Great Power Comes Great Responsibility.”


To reiterate the opening message, please be super extra careful around this stuff.

Ever had a component on a breadboard explode? One little capacitor or an LED or something? It throws shrapnel at your face and it hurts…and that’s just a tiny amount of power in one tiny part!

We’re not in Kansas anymore. Dropping a wrench or a screwdriver across the terminals of a high-current power supply can spot-weld the tool in place. The arc can burn or even blind you. Sparks can cause fires. Please:

  • Avoid working on a live circuit. Unplug power and let any charge bleed off before poking around. (Sometimes you have to, obviously…metering voltages and such…be careful.)
  • As you build up a system, test subassemblies; don’t throw the switch on Las Vegas all at once.
  • Never bypass safety devices such as fuses, interlocks, covers or terminal partitions.
  • Cover power terminals. Insulate wire splices. Nothing conductive should protrude.
  • Remove electrically conductive jewelry, including rings, watches and necklaces.
  • Ampacity is all about cross-sections and contact areas. Don’t skimp on wire gauges. Don’t file down a terminal that doesn’t fit…get the right terminal.
  • Work methodically, check everything. Twice. Even the “obvious.”

Bigger?


It’s possible. You’ll need a USB hub, then add one extra Fadecandy board per 512 pixels. Also a bigger power supply, proportionally larger cables and bus bars and an extra helping of common sense. Very industrial stuff…suppliers for large boats and RVs might have suitable components…such vehicles also rely on low-voltage, high current power systems.

Personally, even the 60A project scares the crap out of me.
Spotted this terrifying bus bar in an electronics/industrial surplus shop. Notice at this scale how everything’s bolted. This might fan out to smaller bus bars, and eventually to conventional wiring at the extremities.

At some point (around 250A on the DC side, I estimate) you run up against the limits of what a standard house or office circuit can safely provide…to push it further is to blow a circuit breaker. Stop and think. When one’s work approaches such a threshold that you’re considering rewiring a building, you really have to ask yourself: is this art, or just MOAR LEDS?
This guide was first published on Aug 11, 2014. It was last updated on Aug 11, 2014. This page (Power Topology) was last updated on Jul 15, 2019.