Led Fillaments: How do they work?

Thread Starter

craftyjam

Joined Jun 8, 2019
4
Hello, I've been thinking about these led filaments I got for cheap on ebay.
Other filaments of these type that I've used have had a very high forward voltage, around mains voltage.
That makes sense because there are so many led's in series inside the fillament.
However, the type of fillament I got from ebay has a forward voltage of aproximately 1.1v after testing.
How can this be?
 

dl324

Joined Mar 30, 2015
10,911
Welcome to AAC!

It doesn't make sense. The advantage of LED filaments is a number of diodes in series allow them to operate at close to line voltage so they only require a simple power supply. And they have a wider light distribution because they're on a clear substrate.
 

Thread Starter

craftyjam

Joined Jun 8, 2019
4
I could link you to the ebay page but it doesn't really have any information on it. All that I know is that when I tested them they started to illuminate at 1.1v, and reach maximum brightness at 3.8v and 18mA.
I know that there are many led's within each fillament as I can see spaces in the lightat 1.1v.
Could it be that all of the led's are in parallel?
Is this just how COB LED's work?
 

jpanhalt

Joined Jan 18, 2008
9,647
A search for LED filament bulbs on eBay returned over 22,000 listings. Maybe there is something in the image you didn't notice? Is it really that much effort to post a link?

At least then we might have a place to start the discussion. Many of the links on eBay provide a choice of version. If that is the case, be sure to indicate which version you got.
 

jpanhalt

Joined Jan 18, 2008
9,647
Thank you. It wasn't clear to me that you were talking about individual filaments, not completed bulbs.

The later are often described as "COG" (circuit on glass) and the substrate is silicon or sapphire (see: Wikipedia and here). However on further digging, it appears there are numerous patents for using flexible substrates ("FPC") to make filaments that are flexible. Here's one patent that describes parallel electrical connections of serial strings of one or more LED's on such flexible substrates: https://patents.justia.com/patent/20190309906 Unfortunately, when I searched Google patents for that and similar patents, the only images I found were not helpful and descriptions were in a language that I cannot read.

Of course, it seems series/parallel electrical connection could also be done to control operation voltage in filaments with rigid substrates.

Is your filament flexible per chance?
 

Thread Starter

craftyjam

Joined Jun 8, 2019
4
Thank you for that information!
No, it would seem that these are of the glass variety.
So in general, searching through patents is a good way to gain a detailed understanding of how things like this work?
 

jpanhalt

Joined Jan 18, 2008
9,647
So in general, searching through patents is a good way to gain a detailed understanding of how things like this work?
Good laugh.

There is distinct skill in writing patents such that everything you want to protect and more is described in sufficient detail to be protected yet not easily duplicated. They are among the most highly paid of the hourly paid attorneys.

Nevertheless, because of the broadness of claims, patents often pop up on searches and have since the days when we didn't have electronic searches. They can give you an idea of how something works, but are often lacking details and instead give ranges. This phrase "parallel arrangements of one or more series connected LED's" is an example of that type of vagueness.
 
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