I would like to mount 10 or 20 white LED on a thin rod, about 2 m long - and around 3 mm in diameter.

I will need to accept various compromises for conflicting requirements, but I try to understand the basic feasibility for now, so let's just ignore tradeoffs.

An important requirement is, that the mass of the LEDs is as low as possible - it's important to keep the total mass of the rod low.

I would start with the smallest standard SMD white LEDs - are there obvious candidates for?

The total or individual intensity of the LEDs is a minor conocern.

But I think the mass of copper wires to deliver enough current using a low voltage may be a problem. Embeding conductors in the rod itself would be at least tricky - but a possible option.
But, in principle, one could use a higher voltage, with less current, and convert it back at each LED - but that would need SMD packages containing the converter and the LED, to not double the part count. Are there LED modules that can be driven by somethink like 50 V - and preferably small?

In general, is there something like chip on board (COB) LEDs I could make use of, without a machine that bonds the die directly on the actual device?

It's ok if it's hard to implement, because I build only one piece.

Other wild ideas that come to mind:
Does a standard SMD LED have some part that is not needed here and can be removed?
Is it sensible to use SMD LEDs with almost no heatsink, at somewhat reduced power?

So, do you see alternative approatches, or problems in mine?

If I can not easily use a higher voltage, it looks like I should use 12 V,
and basically need a normal LED stick, minus the stick.

  • \$\begingroup\$ The brightness of LEDs is proportional to current. The only way that higher voltage would allow for smaller wires is if you had a step down regulator next to your LEDs. Then, you would have high voltage/low current down your rod and high current, low voltage at the tip. \$\endgroup\$ – scld May 21 '15 at 14:14
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    \$\begingroup\$ If brightness is not a concern, perhaps you would consider making the rod a specialized light pipe with a single bright LED at the center. \$\endgroup\$ – lm317 May 21 '15 at 14:14
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    \$\begingroup\$ There is little actual information about maximum weight, dimensions, heat requirements etc. so it is impossible to determine if something is good enough to suggest. \$\endgroup\$ – PlasmaHH May 21 '15 at 14:16
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    \$\begingroup\$ Standard 5mm or 3mm LEDs don't need heatsinks and nor should an SMT LED unless you are opting for a particular bright version which requires a fair amount of current. \$\endgroup\$ – Dean May 21 '15 at 14:25
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    \$\begingroup\$ @VolkerSiegel No. You are NOT listing constraints as good as you can. People should not have to ask What about this? What about this? What about this? What about this? What about this? and have you say No No No No No - you should just tell =us what size etc is the max suitabl;e. Eg THESE LEDS are about 1.75mm x 2mm x 0.85mm tall. They SOUND ideal. If not you should say why in general terms rather than forcing people to play guessing games. \$\endgroup\$ – Russell McMahon May 21 '15 at 15:45

As a one off project, this sounds like a simple enough thing to do either with off the shelf led strips (single white 3258 smd with 15 or 30 count per meter), attach to (or replace) the rod with some rigid led bars, or use some of the super thin led strings (fairy lights). All of these are bare minimum smd leds, thin wire or copper tape, and resistor.

At typically 20mA per led, even if they are all in parallel, 20 leds at 3.5V and 20mA that is only 1.4 Watts. Conductor size is mostly electrically unimportant here.

The rest is making it look pretty.

  • \$\begingroup\$ The mechanical properties of the original rod are important, but two coper tape strips on each half of the perimeter is a very good idea, in terms of not changing mechanics, in terms of conducting current, and conductin heat away! \$\endgroup\$ – Volker Siegel May 21 '15 at 14:38
  • \$\begingroup\$ See amazon.com/10ft-LEDs-Fairy-String-Lights/dp/B00GWCZBXG for the fairy lights. They use ultra small leds, so light output is a trade off, but you could wrap it around the rod without issue. \$\endgroup\$ – Passerby May 21 '15 at 16:14

Just wire the LEDs in series. They're available down to 0201, at which point you'll want a magnifier to work with them. If I were attempting this, I would:

  • choose 0402 or 0603 LEDs
  • lay them face down on the sticky side of Kapton tape. This stops them escaping, but won't melt when near a soldering iron.
  • take 10cm pieces of enameled copper wire and tin them at each end
  • solder one end to each anode
  • line them up and solder the other end to cathodes
  • take the resulting (absurdly fragile!) necklace-like string of LEDs and glue it to the rod. Possibly by lining them up on the table face-down, applying a bead of adhesive along the rod, and lowering it onto the LEDs.

If you have e.g. 10 red LEDs with 2V forward voltage, apply 20V @ 20ma or whatever the rated forward current is.

  • \$\begingroup\$ Thanks for the tiny LED package! For this case the 5630 seems to be just perfect - 3 mm wide like the rod - so it's roughly ths design: Sections of 3 LEDs and a resistor, sections driven by 12 V - here are good pictures of a LED bar of this kind: aliexpress.com/store/product/…. I would better not distribute the 3 LEDs of a section on rod length, but place them directly adjacent, so I do not need extra wires inside the sections. \$\endgroup\$ – Volker Siegel May 21 '15 at 15:08
  • \$\begingroup\$ Those bars are nifty: I believe they're a thin layer of copper-coated Teflon bonded to aluminium to make a PCB onto which the LEDs are soldered. If you want all your LEDs in a bunch then it's well worth considering making a small PCB, especially if you want to make more than one of these. \$\endgroup\$ – pjc50 May 21 '15 at 15:41
  • \$\begingroup\$ Ah - just seen the flexibility requirement. That rules out a PCB and you may have long term problems with solder joints if you're not careful. \$\endgroup\$ – pjc50 May 21 '15 at 15:43
  • \$\begingroup\$ I think the copper directly on the surface is almost optimal in terms os resisting flexing, and all soldering would be on the rigid LED packages. In the worst case, it needs somewhat flexible glue to hadle the local curvature from flexing. \$\endgroup\$ – Volker Siegel May 21 '15 at 15:47

Your rod is circular, the SMT LEDs are flat. You could fasten them by glue and have thin guage copper wire interconnecting the LEDs but it isn't going to look pretty, unless you intend to cut recesses into the rod with a flat surface so the LEDs are recessed into the material.

If you go for high efficiency LEDs (to achieve the same brightness with lower current), you can drive with say a few milliamps, and make the wire thinner and therfore lighter in weight.

Solutions with higher voltages and use of DC-DC converters as a way to reduce current and therefore the weight of the conductors will invariably not be profitable because of the extra weight of the converters.

  • \$\begingroup\$ The voltage/current issue is solved by using copper foil on the rod, and the shape is not critical, gluing is just right. Achtually, I should try a rod of sqare profile. \$\endgroup\$ – Volker Siegel May 21 '15 at 14:50
  • \$\begingroup\$ Square bar stock would make this simpler. You could get it milled at either end to support a screw/threaded mount if needed too. \$\endgroup\$ – Passerby May 21 '15 at 16:18

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