LED Christmas Light Curiosity

Looking to learn about how those strings of 30 LEDs are wired to what seems like a direct mains (120VAC@60Hz) connection other than inline fuses. Obviously must be pretty sloppy, my guess is either all in series with each getting 4V peak and using the diode function of the LEDs to just ignore the negative half of the AC waveform, so the lights are actually flickering at 30Hz and each one getting 0V-4V-0V each time. Even if forward voltage is only rated at 2V, the fluctuation takes the actual duty cycle of being over spec to a small percentage. And even the typical reverse voltage of 5V is upheld. So other than the moments between 2V-4V-2V, everything seems to be within specs. Could probably just have added a few LEDs to even do away with that. Tell me if I'm right or wrong, wax poetic about the travesty of the design, all comments welcome.

If they are indeed wired like that they must flicker like hell…

But can’t tell, from the 230V side of the world :stuck_out_tongue:

Houses over here don’t move around much, 30fps for relatively dim decorative lights hanging still aren’t a problem unless you have a habit of driving by at a high speed and affix your visual focal distance.

It would only take a smallish capacitor to reduce the flicker a lot...

regards

Allan

Obviously must be pretty sloppy, my guess is either all in series with each getting 4V peak and using the diode function of the LEDs to just ignore the negative half of the AC waveform, so the lights are actually flickering at 30Hz and each one getting 0V-4V-0V each time.

I dot know how these things are made... There's probably a current limiting resistor like any basic-normal LED circuit (built-into one of the lights). That takes care of the "over voltage". And, possibly there's a regular diode (or two) for reverse-voltage protection.

You're not going to notice the flicker except when you move the lights or possibly when you move your eyes.

30Hz at a duty of something like 10%, even if you sit still it flickers like mad! 100Hz at a reasonable duty is kind of okay when you sit still.

All led Christmas light I know from here use a diode bridge and a capacitive dropper and the better ones have a capacitor on the output to reduce the flicker.

On AC if it's just positive voltage side, is 50% duty cycle regardless of frequency. I think you're overstating your ability to detect flicker, but maybe you're one of those people who say fluorescent lighting gives them aneurysms.

Alright, first of an error from my side. Of course it's still 60Hz, not 30Hz... But:

INTP: On AC if it's just positive voltage side, is 50% duty cycle

No it's not. With 30 in series they only start lighting at around 2V. So that makes 60V. That's only reached @ 11,5% into a halve sine wave @ 120VAC. So for 2 x 11,5% = 23% of a halve sine wave the leds are still off. 77% on. Both without a rectifier the other halve is off as well. Leaves you with a duty cycle of 77% x 50% = 38,5%

And no, I don't see flicker with a fluorescent because that's 100Hz or 120Hz which is quite a bit higher. Also they don't have a sudden turn on voltage but light almost all the way. But with old school tubes I do see shimmer near the ends.

But yeah, I do see flicker with a led + resistor connected to straight to 50Hz AC (which does give you closer to 50% duty). And yeah, I could not work on a 60Hz CRT for long. That's why I had a 120Hz.

Isn't the nominal 120 VAC household voltage an RMS value? True peak would be 1.414 times 120 or nearly 170 Volts.

Yep, that's why the 60V is reached 11,5% in of a halve sin wave (0,115 x pi rad). sin(0,115 x pi) x 120V x sqrt(2) [for peak value) = 60V If you would forget the peak value it would have been a 16,7% :D