# Smart lightbulb LED runs at 37 volts?

#### rasmasyean

Joined Jan 27, 2024
7
Hi,

I was puzzled by this finding.
I opened the cap of a smart LED light bulb and probed the individual SMD LEDs. It seems to go up to 37 volts at max brightness. This is on the color LEDs as well.
I was thinking of replacing one of the colors because it was flashing. It looks like LEDs are 3V.

Can someone explain this?

Thanks.

#### sparky 1

Joined Nov 3, 2018
757
not easy to say without a schematic of the circuit of a smart light
Vdc=Vac∗.636
58*.636=36.9
37/10=3.7

#### Tonyr1084

Joined Sep 24, 2015
7,899
Vdc=Vac∗.636
Thought that was 0.7072. Am I wrong?
Rectified and filtered AC to DC is 1.414 times the AC value. The inverse of that is 0.707213578500707

#### rasmasyean

Joined Jan 27, 2024
7
Are you saying that one little chip may contain 10 "standard smd LEDs" in series powered up to 3.7V? It's not the same as those "3V replacement 3528 smd LEDs", etc. you'd find on Amazon and eBay?

#### Tonyr1084

Joined Sep 24, 2015
7,899
Are you saying that one little chip may contain 10 "standard smd LEDs" in series powered up to 3.7V? It's not the same as those "3V replacement 3528 smd LEDs", etc. you'd find on Amazon and eBay?
Not clear what you're thinking or where you're going.

Standard 5mm LED's have a forward voltage (Vf) that depends on who made it and what color it may be. There are standard voltages associated with each color but there's no "convention" to exactly what Vf you may have in your hand. The only ways to be sure is to either have the manufacturer's data sheet OR to test the Vf yourself. Me? I start with 5V and target 20mA. Using a 5V source and a 125Ω resistor (gives me 40mA without considering the Vf). Then measure the voltage across the LED. That will give me a pretty good idea what the Vf of an LED is. Suppose you get a reading of 2.1Vf.
You then calculate the numbers:
(5V - 2.1Vf) ÷ 125Ω = 23.2mA. IF 20mA is the desired operating current then I'd use a 150Ω resistor. That would give me 19.3mA. Very close to what I wanted. OR you could use a ammeter (amp meter) and measure the current through the whole circuit and figure out what the voltage is. Just apply Ohm's law.

#### Jon Chandler

Joined Jun 12, 2008
1,050
one little chip may contain 10 "standard smd LEDs" in series
Yes. The LEDs on commercial light bulbs often do contain a number of LED elements in series. If you vary the applied voltage to the point where they just start to glow, you may be able to see the individual elements.

Bigclivedotcom on YouTube has dissected and analyzed dozens of LED bulbs – it's a great channel to watch to learn more.

#### MisterBill2

Joined Jan 23, 2018
18,477
Those long LFD things in an LED bulb ARE strings of LEDs somehow assembled into one long tube. It is indeed a wonderful bit of magic, and it certainly does look good, much better than some of the early bulbs. How many LEDs they get in there is not clear, and probably not the same for each color, although I have not investigated.
As for a scheme to power one of the elements, once you get it to light you will probably find it is more than 20 milliamps. You can get an estimate by using the whole bulb watts, then dividing by the number of those long elements to get the watts per element. And as you measured the volts, you can get an estimate of the current.
But now looking back, the bulb did not have the tubes that simulate the filaments, but rather SMD elements. The same approach would apply, although counting the number of elements may not work out as well, as the different colors use different amounts of power.

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#### rasmasyean

Joined Jan 27, 2024
7
I suppose eventually they will be OLEDs? Like the flexible TV screens but without the pixel electronics.
This way you can buy a "sheet bulb" of whatever size you want to stick somewhere.

#### Tonyr1084

Joined Sep 24, 2015
7,899
Sheet lights. Hmmm. Probably good in the bathroom ? ? ?

Actually I've not heard of this form of lighting. Heard of OLED but lack familiarity with them.

#### rasmasyean

Joined Jan 27, 2024
7
OLED TV screens are manufactured by LG from what I'm aware of. They are relatively new and replacing LED TV's (i.e. LCD with LED backlight). It's as if each pixel is a tiny red, green, and blue LED group inside a "sheet". So therefore, no backlight is needed. As a matter of fact, every black area on the TV emits zero light.

They are real expensive still, but I suppose eventually everyone else will manufacture the technology in various ways. I can imagine you can stick it to the ceiling with some simple power electronics inside the hole / square electrical box thing.

#### rasmasyean

Joined Jan 27, 2024
7

#### Jon Chandler

Joined Jun 12, 2008
1,050
Here's a picture of a modern LED flood lamp. It's rated for 120 VAC, 7 watts, with a claimed current draw of 175mA.

You can see 8 LED assemblies, and the two black squares are a linear current regulator and a bridge rectifier. On the back of the board is a DC smoothing capacitor (NOT a capacitive dropper).

The peak voltage, which will appear on the DC smoothing capacitor, is close to 170 VDC (1.414 × 120VAC minus a couple diode voltage drops.

Each LED assembly has a number of LED elements in series, and the 8 assemblies are in series. It's desirable to have the forward voltage of the entire series string to be close to DC voltage of the rectified supply voltage, which is nearly 170 VDC in this case.

8 LED elements × X LEDs/element × 3 volts < 170 VDC (Vf of a white LED is approximately 3 volts).

If there are 6 LED elements in each assembly, Vf would be appropriately 144 volts, resulting I'm about 20 volts across the current regulator, with some headroom if the actual voltage is slightly lower than 120VAC.

Power dissipation across the regulator would be about 20V × 0.175 amp, around 3.5 watts. Power dissipation of each LED element is around 3V × 0.175 amp = about ½ watt and 3.5 watts for each package.

It's possible that each LED package may not have the same number of LEDs in order to optimize the efficiency of the current regulator.

Joined Feb 7, 2024
124
Hi,

I was puzzled by this finding.
I opened the cap of a smart LED light bulb and probed the individual SMD LEDs. It seems to go up to 37 volts at max brightness. This is on the color LEDs as well.
I was thinking of replacing one of the colors because it was flashing. It looks like LEDs are 3V.

Can someone explain this?

Thanks.
I find in a lot of cases soldering the flashing led fixes this.

#### MisterBill2

Joined Jan 23, 2018
18,477
I find in a lot of cases soldering the flashing led fixes this.
That is very interesting. I have a big bundle of 48-inch LED "tube" lights that have failed and in each case the failure started with flashing. Each tube has, I think three parallel strings of 24 LEDs in series. The slow failure would begin with one segment flashing, and grow to all segments flashing and then dark. I held onto them because some would still light a bit, and it was easy to stick them in a corner.. It could be simple to open and resolder each LED. If that was a fix it would be very interesting indeed.

Joined Feb 7, 2024
124
That is very interesting. I have a big bundle of 48-inch LED "tube" lights that have failed and in each case the failure started with flashing. Each tube has, I think three parallel strings of 24 LEDs in series. The slow failure would begin with one segment flashing, and grow to all segments flashing and then dark. I held onto them because some would still light a bit, and it was easy to stick them in a corner.. It could be simple to open and resolder each LED. If that was a fix it would be very interesting indeed.
I bet they are cold solder joint.
Every 120 light replacement that has failed on me has been cold solder joints. Even the candelabra type I can't take apart nicely.
Which those are crimped on and the crimp came apart.

#### rasmasyean

Joined Jan 27, 2024
7
I tried to heat the pad from the bottom with a heat gun, but it didn't fix it for 2 units. I think my Blue LED's are just bad units, because it failed first in all of the light bulbs of that make. It's also mentioned in the reviews of this bulb. Unless they subcontracted the installment of the Blue LED's separately for some strange reason, it's probably not a cold joint I guess.

In my blubs, there's some white stuff on the circuit board perimeter. It's not uniform's distributed tough. Is that some sort of glue? Or is it heat paste to thermally connect it to the base?

#### rasmasyean

Joined Jan 27, 2024
7
So I took another one and heated it up more, thinking I'd ensure the solder would melt. The white plastic housing started to warp. And...no change with the BLUE LEDs, but the Green LEDs no longer work at all. lol

Are Green LED semiconductors more heat sensitive?

#### MisterBill2

Joined Jan 23, 2018
18,477
I have enough of those tubes to experiment with resoldering the connections. That would be an interesting development indeed. Worth trying though.

Joined Feb 7, 2024
124
I have enough of those tubes to experiment with resoldering the connections. That would be an interesting development indeed. Worth trying though.
I use my hot air pencil from my smd rework station I have. Plus removing what solder that is left since the solder is contaminated. Then I use a no lead paste or solder and solder back the part. Preheating the whole thing wouldn't be advisable since some do have thermal fuses.

#### MisterBill2

Joined Jan 23, 2018
18,477
I would use the LEAD/TIn solder because it is a better product that works better. AND I have no sympathy for the fools who eat circuit boards with lead solder on them.