Why is there no resistor in a christmas LED string light?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

For what it's worth, I've just recently been disecting a string of 50 LED lights, advertised as "Constant On", meaning that if you remove one of the LED's in the string, the rest remain lit. Here's what I've found so far: The 50-light string consists of 2 25-light strings in parallel. Inside BOTH the plug and the socket is a small pc board containing two 1N4007 diodes. On the plug end, after the fuses, each diode's anode is connected to one of the line wires and the two cathodes are connected together. This point feeds the anode side of each of the two 25-light strings. On the socket end, there are again two 1N4007 diodes, but on this end each diode's cathode is connected to one of the line wires and the two anodes are connected together along with the the cathode end of each of the two 25-light strings. Finally, there are two wires which connect the plug and socket together directly. As for the constant-on feature, it appears to me that in each light socket there is a built-in zener diode in parallel with each LED. If the zener diode voltage is higher than the LED voltage, the zener normally does not come into play. If the LED is removed however, the zener will conduct and the circuit continuity is maintained, keeping all the remaining LED's lit.

Answer 4

Half a series of 100 led Christmas was off. The problem was one of the two resistors. Changed for a 4.9k ohms, 1/2 watt, back to work!

Answer 5

I tried to fix same LED Christmas's light. It has resistor and diode inside. One string 50 LEDs divide to two parts. 25 LED bulbs connected in series and one diode and resistor. Measuring current gave half sine wave, amplitude 60 mA. So the average is 20 mA. Smart inventors just replaced incandescent bulb with LED and added diode and resistor.