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My LED Christmas light strings do not have a series limiting resistor, how is this achieved? They are 70 LED strings with two 35 bulb sections. I have carefully checked and there are no provisions in the plug or inline with the wire. I measured resistance from sockets back to plug and measured no impedance. What am I missing? Can the the LEDS be made (doped) so the internal resistance be high enough that as a group they provide sufficient impedance?

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  • \$\begingroup\$ These lights typically have a power supply for them ...that in all probability will be a CC supply so does not need any series resistance in the LEDs.. \$\endgroup\$ – Jack Creasey Oct 20 '18 at 16:05
  • \$\begingroup\$ These are 120 Vac plug in lights, no separate power supply. I have used infrared thermometer and saw no heat rise in sockets or plug. \$\endgroup\$ – D. Elttub Oct 21 '18 at 21:07
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If you are willing to watch "Big Clive's" vlog, he uses the "easiest" method of finding the resistors, a thermal imaging camera.

In the 220V set he found 3 resistors in 3 of the LED bases, the rest had no resistor. Each resistor was 510R for a total of 1530\$\Omega\$. He also says he found a bridge rectifier molded into the bit where the wires split.

enter image description here

That was for an apparently Polish made set. It's certainly possible that other sets use variations on this theme but you get the idea. Bridge rectifier plus a few hidden resistors distributed out among the LEDs (to spread out the heat dissipation, otherwise they could use just one resistor).

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