# How should I design a power bus for a wearable project?

I am thinking about building a(nother) wearable LED jacket project. The basis will be 60 LED/m WS2812B strips, around 2048 pixels, for a peak possible current draw of just under 125A. In reality I would never run full-white patterns, so count on a peak draw of maybe a quarter of this.

That's still a lot of current and I'm trying to think of a good way to distribute it to the ends of the strips. In my experience with other projects, each strip needs to be directly connected to power and the power bus needs to be very low resistance so voltage drop doesn't cause "reddening" of the strips that are connected at the end furthest from the battery.

I'm having difficulty thinking of a design that is a) high current capacity, b) flexible c) "neat" enough that I don't have exposed wires snaking around and d) field-repairable.

Here is a rough schematic so you get an idea of the orientation of the strips (red and black for +ve and -ve, blue is the signal wire which "snakes" up and down the strips):

• Why are you using general purpose lighting LED strips for a wearable project? Do you realize that is 2kW (!!!) worth of LEDs on your person? I would highly recommend finding lower power LED strips or purpose made LED matrices for this. Those are usually 1-5mA per LED and instantly solve any kind of power distribution (or generation) issue you may have. Apr 7 '14 at 10:54
• Just considering what sort of battery you'd need to be able to provide ~ 30 A continuous for any reasonable amount of time should give you pause. And let's say 30 A at 5-12 V DC (the page seems contradictory about the voltage); how thick wiring do you need to keep the voltage drop in the wiring itself at an acceptable level even at 12 V, let alone 5 V?
– user
Apr 7 '14 at 11:19
• If you have to seriously contemplate using bus bars in a wearables project, something has gone horribly wrong. The heat alone tossed off of 2kW of lighting will cook you alive. Apr 7 '14 at 11:25
• The strips are 5V, each LED maxes out at 60mA and there will be a maximum of 2048 of them so I count 2048 * 5 * 0.06 = 614.4W. And again, this is an upper bound, I don't anticipate running patterns that push more than 1/4 of that. Which is still around 150W in a wearable which is a lot, but not 2kW. I'm nuts, but I'm not that nuts :-D Apr 7 '14 at 12:06
• Also, if you have a link to lower-powered strips of LEDs with integrated controller chips, I'd love to see it. The only ones I'm aware of are 5050 LEDs, at max 20mA per colour channel. Apr 7 '14 at 13:25

Drastic edit of my answer, as I didn't fully understand the LED strips.

I would think the best option is very wide, tinned copper ground strap. They have these with very flexibly braid, used for grounding hinged interfaces. You should be able to solder individual strip power wires to them and just have to keep them isolated vertically. Tying the battery at the center of the strip would minimize the voltage difference among all strands.

• I've edited to say "peak" power draw will be in the order of a quarter of that maximum theoretical draw, most patterns will be sparse and less than 100% saturated colours so much less than even that. Note also the specified "WS2812B" strips have integrated "WS2812B" controller chips (the clue is in the name) and are already microprocessor controlled. Apr 7 '14 at 13:22
• Thanks for the edit. I had considered something along these lines, but how do I prevent the +ve and -ve straps from shorting together, as they'll be adjacent? (It's hard/impossible to heatshrink around a tee-junction) Also, soldering directly to the strap doesn't satisfy my "field repairable" criterion. Apr 7 '14 at 14:48
• Hem the straps inside the garment at sufficient vertical height to be isolated. If you solder with a butane iron, that seems field repairable to me. Only the portion of the +ve and -ve wires that are at the strap are exposed for soldering. Maybe a velco cover over where you field repair the joints?
– Joe
Apr 7 '14 at 14:56
• An update, I've gone with this strategy and it seems to be working very well. Very low (0.3V) drop over the length of the ground strap when operating at around 100W (using 30 LEDs/m strips ends up giving me 640 LEDs in total.) Jul 31 '14 at 7:59

Actually, the upper bound of your estimation is too much high! Low power LED strips are available in the market, with 80 mW or even 50 mW power consumption (per LED). Higher power LEDs are usually used for brighter applications. So I have a few suggestions for your project:

1. Search the following websites to find low-power LED strips:

http://www.ecolocityled.com

http://www.ledworldlighting.com/ledstrip.html

http://www.ledlightszone.co.uk/24-led-tape

http://www.maplin.co.uk/c/components/leds-and-displays/led-strips

https://www.superbrightleds.com/cat/flexible-strips-and-bars

2. A standard LED strip has 8 mm of width, and contains usually 60 LEDs per meter. Assuming a jacket's length to be 50 cm, you can have 30 LEDs in an 8-mm column. Assuming the jacket's front width to be about 40 cm, you can have at most 50 columns of LED strips (and it will be very tight, and rather uncomfortable). So it means that the maximum pixel resolution you can get is 30x50 = 1500 LEDs, and not 2048; unless you continue the LED strips to both right and left (and even back) parts of the jacket. To get that 2048 pixel resolution, you need almost 70 columns, which require 560 mm width. If you add spaces between the LED strips, you will need even more width of jacket (e.g. 700 mm). So I strongly recommend you to lower that resolution to e.g. 1024 pixels.

3. You should probably consider the heat produced by these LED strips. 1024 x 80 mW ~= 80 Watts. This is really a big number for wearable applications. To clarify the issue, let me know: how long can you hold a classic 100 Watt light bulb in your hand!? Probably no more than a few seconds. So as you see, it would be really difficult to wear a jacket that produces that much heat. So I strongly recommend you to search as much as you can, and choose ultra-low-power LED strips. (The maximum acceptable would be 50 mW per LED.)

4. To supply the power to these LED strips, I recommend you to use standard smartphone/tablet Li-Ion batteries. They are usually in the range of 3000 ~ 6000 mAh. If you need to light the jacket for more hours, or if you need more power, you can use extended battery packs, which can supply 12,000 mAh of power, or even more. You can embed this battery in e.g. the pocket of the jacket, or wear it as a belt. Note that if one battery can't supply the minimum voltage or current you desire, you can combine them in parallel (for higher voltages) or series (for higher currents). The following websites provide nice batteries:

http://www.batteries4less.com

http://www.att.com/shop/wireless/accessories/batteries.html

5. Try as much googling as you can, to find both ultra-low-power LED strips, and efficient batteries to supply them.

I would be glad to see the final design of your project. So do not hesitate to post here a picture of your final design. Besides, search for the similar wearable projects to see what others do, and how.

Good Luck!

• There are no individually addressable LED strips that max out at less that 60mA per LED. There just aren't (of course you don't have to give them all that power.) My estimate of 2048 pixels was including front and back 32 x 32 on each side. I may go as low as 32 x 18, but the order of magnitude numbers are still indicative. The plan is to use multiple tablet batteries (four, six?) in series to supply enough current. Apr 7 '14 at 20:12
• Er, in parallel I mean. Jul 31 '14 at 8:00