What's going on here ? - Page 1
Forum section: electronics / general

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 08, 2015, 03:40:09 pm
content: Attached is an image, of a smallish globe light bulb. The image shows 5 radial LEDs, in a glass bulb. The LEDs are probably wired in series. Are they using 5x 24V LEDs, or is there a power converter tucked into the base ?The spec sheet for the bulb claims 0.4W @ 120 VAC. I can make the 0.4W work out for standard flat-top LEDs, but I'm only up to 17-18 VDC. They can't be dropping the excess voltage across a resistor, as that would sharply increase the watts.Any LED failing would likely kill the entire bulb, which at $3.50 is pricey.

author: mij59
author_rank: Frequent Contributor
timestamp: February 08, 2015, 03:44:16 pm
content: Electronupdate has some nice tear down of led bulb's .www.youtube.com/user/electronupdate?feature=watch

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2015, 05:52:51 pm
content: It most likely uses a capacitive ballast limit the current.LEDs more often than not fail short circuit, rather than open circuit so it's unlikely a single failure would kill the bulb.It's not overpriced: $3.50 is the typical market value for this short of bulb.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 08, 2015, 06:14:37 pm
content: I did not so much think it was overpriced, as I was trying to understand the electrical function, and how they got from 120 VAC down to the LED voltages, and still maintained that 0.4 watt claim. I'm trying to mentally reverse engineer the product, and understand the power flows (and not incidentally understand the BoM costs).I was able to locate one seller on Aliexpress who offers the likely LED component. In quantity they go for about 2-cents/pc. Now I'm trying to decipher the power circuitry. I was able to locate a Capacitor-Coupled, Switched Shunt, (CCSS) Regulator (Supertex SR10) which seems a likely candidate. Quantity pricing for those is ~39-cents/each in quantity at DigiKey. An equivalent item in Shenzhen may beat that price.If the bulb is glass, the bulb and base are commodity items (and have been for years). The construction of that lamp appears to be a new LED circuit inserted in place of the old tungsten filament, which appears to be an approach that differs from most of the bulbs being disassembled in those videos.

author: IanB
author_rank: Super Contributor
timestamp: February 08, 2015, 06:57:41 pm
content: It's not necessary to have an IC regulator in such a lamp. A simple capacitor dropper with fusible resistor and bridge rectifier would work fine. It's even possible to wire up LEDs in anti-parallel so they do their own rectification. Flicker from this is minimized if the LEDs are mounted close together.Example: put two LED's back to back in anti-parallel, and put in series with a mains rated 220 nF capacitor. Power from the 120 V mains and see what happens.

author: Zero999
author_rank: Super Contributor
timestamp: February 08, 2015, 10:23:12 pm
content: They're quite common. I created a thread about these awhile ago, except it was about the type which use LED dies mounted on thin strips to mimic the filament of an incandescent light.https://www.eevblog.com/forum/reviews/led-filament-lamp-teardown/msg558243/#msg558243The driver circuit is normally fairly simple, as there's isn't enough room in the base for anything complex.

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 09, 2015, 02:19:28 am
content: Quote from: Hero999 on February 08, 2015, 10:23:12 pmIs the cap supposed to be rated for 120 VAC ? I looked at digikey earlier. The variety of caps rated 120 VAC (or better) seemed to be small and expensive, typically Film Capacitors.

author: David_AVD
author_rank: Super Contributor
timestamp: February 09, 2015, 02:25:41 am
content: Big Clivehas done teardowns on a number of mains powered LED lamps.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 09, 2015, 03:02:00 am
content: Quote from: cosmicray on February 09, 2015, 02:19:28 amThe cap will need to be X2 "mains rated."Here are some resources for this type of circuit...Wiki article:Capacitive Power SupplyMicrochip App. Note:AN954Post on this forum:Capacitive Dropper PSU's

author: cosmicray
author_rank: Frequent Contributor
timestamp: February 09, 2015, 02:34:07 pm
content: Quote from: TerminalJack505 on February 09, 2015, 03:02:00 amIn the interest of self education, I ran the examples in the AN954 app note manually. It appears that the first example was incorrectly calculated. Using 110 VAC, I get a current 9.956 mA, round to 10.0. To get the 10.4 mA figure, I had to use original 115 VAC instead. Not that I go around back-checking 10 year old calculations, but it is nice to see how something crunches, and to be able to get the same result.Overall, this is a wonderful education in how to do things without a transformer involved.