LED Filaments

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tetsujin
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LED Filaments

Post by tetsujin » Mon May 01, 2017 9:38 am

So I can't be the only one who sees some model-making potential in these things right?

Basically, there's a kind of LED light bulb on the market called a "LED Filament" bulb. The idea is that it's meant to emulate the look of an old light bulb, with a visible incandescent filament inside giving off the light.

Where it gets interesting is the fact that these "filaments" are in fact tiny little LED strips. Typical ones are about 30mm long and 2mm diameter. They're a type of LED product known as "chip-on-board" - which I guess means that rather than taking a bunch of surface-mount LEDs and soldering them to a flexible circuit board, they actually skip the middle man, and integrate the (guts of the) LED directly onto the board.

As a result, the strip has a density of about one LED per millimeter. If you look carefully you can make out the individual points of light, but when it's at full glow, it looks pretty uniform. (One side of the filament does tend to be darker than the other, unfortunately...)

The filaments are coated in a phosphorescent material that helps spread the light more uniformly around the strip. With care some of this can be cut away, but it seems all too easy to pick the LEDs of the strip in the process, destroying the strip.

The trick is, all the (roughly 25-30) LEDs on one of these filaments are all in series, which means you need around 70V to power the thing - which probably means getting a voltage booster circuit of some kind to drive it.

I recently built a voltage booster circuit (based on the JT Filament kit I linked above) and have been testing out one of these "filaments", getting a feel for how fragile they are (when I try things like bending the leads at the end, or stripping off some of the phosphor layer) and what the effect is like. Among other things I've found that when the voltage booster is running, the power supply (a pack of AA's at present) becomes too unstable to power a microcontroller. A good-sized capacitor could probably straighten that out, but I'm still looking into better solutions for the voltage boost. (The JT circuit is an oscillator made from a couple high-voltage transistors, combined with an inductor producing the voltage boost. There are dedicated voltage-boost ICs that might do the job, but I haven't picked one up yet.)

A few applications immediately come to mind... Though at the moment I'm trying to get my hands on some longer filaments. So far that has proven kind of difficult. I can find bulbs that are built around longer filaments, but so far, not the filaments themselves (which should be much cheaper than buying and destroying some LED light bulbs to get their LED filaments) - if I simply took two LED filaments and joined them end-to-end, it would result in some gaps in the light pattern. Ideally I'd like a filament around 60-70mm long - perfect (for instance) for a 1/12 scale light saber.
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Re: LED Filaments

Post by Tracy White » Mon May 01, 2017 12:16 pm

tetsujin wrote:perfect (for instance) for a 1/12 scale light saber.
I like the way you think! First obstacle I see to that is the return circuit. How would you complete the circuit without a noticeable line through the light? Is there any sort of conductive clear coat?
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Re: LED Filaments

Post by tetsujin » Mon May 01, 2017 1:39 pm

Tracy White wrote:
tetsujin wrote:perfect (for instance) for a 1/12 scale light saber.
I like the way you think! First obstacle I see to that is the return circuit. How would you complete the circuit without a noticeable line through the light? Is there any sort of conductive clear coat?
Well bear in mind these things don't truly emit a 360 degree arc of light anyway. The chip-on-board has a front side, which emits the most light, and a back side, which has some windows in in to let light through from the emitters on the other side. When the phosphor layer is on, you get almost a full 360 arc of light. The back side looks a little darker than the front, and the phosphor layer helps spread the light out to the sides and such as well. So it won't be perfect, but it should be very effective.

As for the return wire, since the strip has its LEDs in series, the whole thing still uses about as much current as a single LED. Ohm's law says that the effect of resistance (thus, the power loss over a resistor or the length of a thin wire) is proportional to the current through the wire. Since that current isn't much, I could use a very thin wire and not lose much power. If I cut off most of the original phosphorescent layer, then the return wire can be routed very close to the filament's substrate, and then a new layer of diffusing material can be applied on top.
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Post by Zubie » Mon May 01, 2017 1:53 pm

IIRC there are similar products available commercially for clothing effects.

https://www.aliexpress.com/popular/led- ... thing.html
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Post by tetsujin » Mon May 01, 2017 2:14 pm

Zubie wrote:IIRC there are similar products available commercially for clothing effects.

https://www.aliexpress.com/popular/led- ... thing.html
The URL is a bit incorrect, those appear to (almost) all be electroluminescent strips, not LED products.

There's a few exceptions, like This one, which is a transparent medium lit with a LED on one end to produce "side glow". An approach like that certainly would work for a light saber (put a LED in the hilt, frost the saber's clear part) - it's been done, there are pre-made products out there to make it easier to do. The limitation tends to be that you get a lot of light at one end of the saber, and not a whole lot further up. Inverse-square law and all that.

EL strips and EL wire can be very effective in the right conditions, but one of the big problems with them is they're just not very bright. EL strip is commonly used to illuminate wrist watch displays, things like that. So if the ambient light levels are low enough you do get a good glow from them. People have used EL strips for things like lighting up the Millennium Falcon's main engine, for instance - but the effect tends to be a little too mild for a well-lit room.

That said, EL technology does have its advantages: The light tends to be distributed very evenly without the need for a diffusing layer, EL strips are very compact, and it doesn't take much power to operate them. It's also usually very easy to cut EL strips and EL wire to the size and shape you want.

But these LED filaments are made to light up a room. It's a much more potent effect. It's bright enough that I think I should probably find a way to turn it down, you know? :)
Last edited by tetsujin on Mon May 01, 2017 3:07 pm, edited 1 time in total.
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Re: LED Filaments

Post by Tracy White » Mon May 01, 2017 2:32 pm

tetsujin wrote:Well bear in mind these things don't truly emit a 360 degree arc of light anyway.
Which I think would be acceptable for light sabers in most cases. If it's upright, face the darker side towards the figure as most people won't be able to look through the painted plastic. If it's pointed forward, point it down towards the ground.
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Re: LED Filaments

Post by tetsujin » Mon May 01, 2017 2:38 pm

Tracy White wrote:
tetsujin wrote:Well bear in mind these things don't truly emit a 360 degree arc of light anyway.
Which I think would be acceptable for light sabers in most cases. If it's upright, face the darker side towards the figure as most people won't be able to look through the painted plastic. If it's pointed forward, point it down towards the ground.
Exactly. It won't be perfect, but it'll be workable, and depending on how I do the diffusing layer there's the potential to make the effect even better. That's part of why I'm eager to try this. But the filaments I have at present aren't long enough for a 1/12 light saber, so I'm still working that part out.
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Post by naoto » Mon May 01, 2017 4:04 pm

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