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I bought a bunch of different LED string lights for Haloween and Christmas. All of them are arranged as 2 sets in parallel. They are either 50 or 100 LEDs.

I noticed that most of them have a molded resistor built into each parallel circuit (which I believe is to handle any left voltage and limit maximum current).

However a few of them (models) don't seem to have any resistors built anywhere into the circuit. These have 50 LEDs (arranged in 2 parallel circuits). For example this one: https://www.homedepot.com/p/Home-Accents-Holiday-24-ft-100-Light-Cool-White-LED-Dome-Light-String-TY-100LD-W/305026726

There doesn't appear to any resistor anywhere. Just 2 tiny fuses in the power socket and the rest at all LEDs.

Is it okay to have a LED string light running off 110v without any resistors?

A couple of these LEDs have stopped working (they are dead, while the rest connected to the same series circuit are very faintly dim) so I need to buy replacement LEDs, without any resistor would I just assume the each LED would be 110v/25 = 4.4V forward voltage LED? What would happen if I used a 3.2v LED?

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    \$\begingroup\$ I think Big Clive on YouTube did a video on such a resistor less string a while ago and the conclusion was they relied on the resistance of the (crappy) wiring. No wonder those LED's died. \$\endgroup\$ – Unimportant Jan 2 at 20:46
  • \$\begingroup\$ "A couple of these" - how would that work? If one of the LEDs in series is broken, all of them will turn off, wouldn't they? \$\endgroup\$ – Thomas Weller Jan 2 at 20:46
  • \$\begingroup\$ Couple of LED's are dead while other in the same (half) circuit are on very dimly so I'm guessing there's a current leak with the dead LED's. When I remove them (dead LED's) the rest in the same series circuit stop working completely. \$\endgroup\$ – rboy Jan 2 at 21:52
  • \$\begingroup\$ @Unimportant do you have the link? So in those circuits once just replaced the LED's? How do I figure out the voltage required? 3.2 or 4.4 or it doesn't matter? \$\endgroup\$ – rboy Jan 2 at 21:56
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    \$\begingroup\$ @Unimportant Big Clive did a video on a really crappy Polish string and used a FLIR infrared camera to locate the several resistors hidden in a few of the bases. \$\endgroup\$ – Spehro Pefhany Jan 2 at 21:58
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Each LED has it's own internal resistance.

When you consider the typical tolerance on AC lines is < 10%, and the tolerance on LED internal resistance can be up to 50% about the rated forward voltage, how many LEDs do you need to survive a 10% voltage tolerance without resistors and not exceed its current rating?

Well that depends on what nominal current you choose and the statistical variation of internal resistance on all batches. I know the variation can be extremely small (<1%) in a single batch. So the prudent choice is to use that value for the maximum voltage tolerance.

Consider an LED with the transfer function of Vf=2.8 + If*Ri for If rated @ 20mA and Ri = 16 +/-50% worst case i.e. Vf 2.96 to 3.28 or 3 to 3.3V. or 3 (+0.3V/-0) (Some are better and worse than this example)

Now consider;

  • 120V/3.0V = 40 LEDs
  • 120V/3.3V = 36 LEDs

  • The LED tolerance on 40 LEDs is 40 *0.3V = 12.3V

  • The 10% tolerance on 120V is 12V, so it meets the criteria.

Conclusion

the more parts in series, the greater tolerance to applied voltage.

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