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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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In my experience, and I've disassembled quite a few of these strings, there is indeed a resistor in one of the LEDs. Look for a slightly larger LED receptacle, almost always at the far end of the set. If you extract the LED, there will be two brass contacts and apparently no resistor. The resistor is behind one of these metal contacts. Is a tiny -but not SMD- resistor. In a translucent-wire string that failed, I found one of these tiny resistors in a leg of the last LED, under heat-shrink tubing. Maybe mine are different, but almost every time I find these resistors hidden in the receptacles. They look like these: https://rayshobby.net/wordpress/wp-content/uploads/2011/08/IMG_0669.jpg except I find them in gray or blueish enamel, instead of beige. They're about 1/3 of a rice grain. Hope this helps.

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