I have several long strings of LED Christmas lights. Each bulb socket is obviously polarized, with one contact being much lower than the other and the LED bulbs only fit in one way. But the plug at the end of the strand is not polarized, I can plug it in to the outlet in either orientation.

I can't figure out how this works, it seems to me that they should only work in one orientation. The lights are cheap and there is no obvious additional electronics in them.

One thing I noticed is that, if I pull a bulb, all of the bulbs in the string of lights and any attached strings of lights go out, not just the ones behind that bulb. I don't know if that has anything to do with it.

  • 2
    \$\begingroup\$ We are talking about AC power connected to a string of series diodes, right? If so I would recommend to use a simulation tool to figure this little riddle out. \$\endgroup\$ Nov 29, 2020 at 15:16
  • \$\begingroup\$ There is usually a small power supply to regulate current to the diodes. The power supply doesn't care about polarity. Rarely diodes can be driven off of AC directly, in which case they also won't care about polarity (but will be on only 50% of the time). \$\endgroup\$ Nov 29, 2020 at 15:17
  • \$\begingroup\$ Ah right, that makes sense, thanks! I'm pretty new to this. \$\endgroup\$
    – Chris.B
    Nov 29, 2020 at 15:19

3 Answers 3


Most LED Christmas lights I've seen blink at 60Hz, because they're literally just one long series string of diodes driven directly from AC. For example, a stack of 30 LEDs dropping an average of 4V each will drop an average total of 120V, which is how they can work directly from 120VAC line current. There's usually an inline resistor to limit the current through the LEDs, particularly important considering the peak voltage for 120VAC is 170VDC, and LEDs won't self current limit like incandescent bulbs. Manufacturers like this string approach because there's no "power supply", it's just the lights doing their own AC to DC rectification, but one burned out LED will take out the entire string.

Here's a possible circuit diagram, including the tiny fuses usually inside the plug, and the AC daisy-chain wires. The LEDs, being diodes, will only rectify half the AC waveform anyway, so this will still work if neutral is on the top or bottom wire here. Fuse and resistor values are notional, and depend on the number and type of the LEDs. Circuit diagram of direct AC-driven string of LEDs.
Public domain source: https://crcit.net/c/2c440d3599704872a35f9246bf409960

Many other circuits are possible, including multiple strings, or higher efficiency buck converters to do the AC to DC conversion (for a power factor higher than 0.5!).

(The very rapid line speed blinking can cause eyestrain and headaches for some people, especially in the dark and viewed by peripheral vision. Your country's line frequency and voltage may differ.)

Background reading:


The ac portion of the led string is not polarized. Typically a full wave diode bridge, which is polarity independent, will be part of the string to convert ac to dc and a regulator circuit after that to regulate it down to a stable dc voltage. Leds by their nature are directional which is why their are polarized as you say. Depending on the number of lights, the strings are subdivided into sections so pulling one bulb would only affect one section. There is also a pass through connection between sections for the AC connection.


That kind of LED Christmass lights have some amount of LEDs connected in serial, diode and resistor. Current pulsed,it is half sine wave. Consider it as DC power supply. So, polarity of plug does not matter.


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