Series vs parallel Christmas lights

Series: one path, small slices

Old mini-light strings wire every bulb in series: the current leaves the outlet, goes through the first bulb, then the next, and so on, around one single loop and back to the plug.

Because the bulbs share one path, they also share the outlet's voltage. Each bulb takes a slice, and with equal bulbs the slices are equal. That's why mini bulbs are tiny, low-voltage lamps: each one only needs a few volts, not the whole outlet.

One break, all out

A bulb lights because current flows through its thin filament. When a filament breaks, it leaves a gap in the wire. In a series string, that gap is a gap in the only path, so the current stops everywhere and every bulb goes dark, even the ones that are fine. Then comes the hunt for the one bad bulb.

Parallel: every bulb its own path

In a parallel string, each bulb connects between two shared wires, like the rungs of a ladder. Every bulb has its own path from one wire to the other, and each gets the full voltage. If one bulb dies, its rung is broken, but the other rungs still carry current, so the rest stay lit. Big C7 and C9 bulb strings are usually wired this way.

The shunt: a hidden bypass

Modern incandescent mini lights are still wired in series, but most bulbs hide a shunt: a small coated wire coil wrapped across the two lead wires, just below the filament. Normally its coating keeps it out of the circuit. When the filament fails, the full string voltage appears across that one bulb, the coating breaks down, and the shunt connects the two leads. The current flows through the shunt and the rest of the string stays lit.

The shunt lives inside the bulb, though. Pull a bulb out and the shunt leaves with it, so the socket is an open gap and the whole string (or that section) goes dark again. Always unplug the string before swapping bulbs.

Try it yourself

Next time you put up a string of incandescent mini lights, look for a section that is dark while the rest glows: longer strings are often made of several series sections joined in parallel, so one bad spot only darkens its own section. With the string unplugged, look at the bulbs in that section for a loose, missing or broken one. Never work on a plugged-in string, and throw away any string with a damaged cord.

Good to know

• Incandescent mini-light strings are wired in series (a typical example is 50 bulbs at about 2.5 V each on a 120 V North American outlet, per patent descriptions), and longer sets join several series sections in parallel.
• Early mini-light strings had no shunts, so one failed bulb darkened the whole string.
• Shunts don't always work, so some strings still go dark when a filament fails.
• When a shunt takes over, each remaining bulb gets slightly more voltage and burns out sooner, so replace dead bulbs promptly.
• Removing a bulb opens the circuit because the shunt leaves with the bulb.
• Large C7/C9 strings are usually wired in parallel. LED strings vary: many are series sections with their own failure behavior.
• Mains voltage is dangerous: unplug before touching bulbs or sockets, never repair damaged cords, and leave wall wiring to a licensed electrician.

Answer to the comment question

Sixty equal bulbs in series share a hundred and twenty volts. How many volts does each bulb get? 2 volts. In series, equal bulbs share the voltage equally: 120 V ÷ 60 bulbs = 2 V per bulb. (This is a practice string with made-up round numbers, not a specific product.)