I have noticed that about half of the LED Christmas light strings have a "lump" about 1.5 inches long and 1/2 inch diameter in series with the lights, which have polarized sockets. I assume that it must either be a diode or a resistor or perhaps both. What puzzles me is what limits current and polarizes the AC into pulsating DC in the light strings that do not have any visible "lump"? Perhaps there are resistors built into each individual lamp, but the lamps that I have taken loose have the LED leads going directly out the bottom of the socket. And, I haven't seen any sign of any resistors built into the sockets themselves.

Anyone know for sure what's going on???