Why do all the lights in a string burn out more quickly when one lamp goes out?
Or more precisely, how does the shunt across the filament work?




Each bulb is rated at 0.5 W, 0.2 A @ 2.5 V.

The measured cold resistance is 2 to 3Ω. While on, the bulb takes about 140 mA @ 2.5 V, which makes it effectively 17 Ω.
The measured cold shunt resistance is about 0.4 Ω.

With 50 bulbs in a string, that makes the total resistance 850 Ω. This would draw about 140 mA from 120 VAC line voltage.
A string with 100 bulbs would have two strings of 50 bulbs in parallel. With 240 VAC voltage, there are 100 bulbs in series.

With a cold shunt resistance of 0.4 Ω, how does the filament ever turn on?

The shunt wire has a conductive coating. When wrapped around the filament support wires, the shunt exhibits a higher resistance than the filament. When the filament blows, the current causes the shunt to heat up which melts the coating on the shunt wire, reducing its resistance. The shunt acts as a fuse in reverse. It behaves as an almost open circuit and becomes a short circuit when it blows, similar to the operation of an MOV (metal oxide resistor).

When a bulb blows, the series current increases which shortens the life time of all the other bulbs. The weakest one goes next, causing a cascading event until all the bulbs blow, or one of the shunt blows.

Bottom line: replace a blown bulb soon when it blows. Check the entire string for blown bulbs before hanging on the tree. You can buy the bare bulbs in quantity. Check the voltage and wattage rating. Save and reuse the green plastic bases of the bulbs. Each string and make is different.

(Edit: My wife just bought new strings of 100 lights. The price was a quarter the price of 100 replacement bulbs. Go figure!)

 

Those gun-shaped fixers work well enough to be worth it. It's a piezo device that, when fired, helps the shunt of any failed filaments to do their job and complete the circuit. You can often see the arc when they fuse. Once the string lights again, you can replace the unlit bulbs asap. If the gun fails to re-light a string, I use a multimeter to determine where voltage is lost. One probe to ground. You should see full voltage anywhere along the string and if it falls more than volt or two, it means the preceding bulb is a problem. It's tricky to keep track of where you are and which leg of the string you're working on - longer strings have multiple legs. Don't have your eggnog until you're done.

I've mostly switched to LED strings and don't miss fixing the old strings despite becoming quite proficient at it over the years. The downside is that the LED strings are harder to diagnose. The LED rarely fails but corrosion of the leads is fairly common. That's pretty easy to see if you happen to pull the right LED but until you do, you're fairly blind and it's one LED at a time.

 

The way the shunt works is that it has an insulation layer that is able to withstand the low voltage across the filament of the bulb when it is drawing current. When a bulb burns out, the full mains voltage is across the insulation, and it breaks down, and the resulting arc melts enough of the insulation so that that the two leads that go to the filament are short circuited. The result is that the voltage across each bulb increases a bit, as well as the current. The power increase in each remaining bulb raises the temperature a bit, and eventually a second bulb fails. The process repeats, bypassing the second bulb, and increasing the power and temperature of the string a bit more.

This is the reason that it is quite important to replace failed bulbs as soon as they fail.

An alternative would be to power the string of lights with a constant current regulator. That would prevent the current increase with each bulb failure.

The price of replacement bulbs is high because it costs the same to market and sell a pack of five bulbs as it costs to market and sell a string of 100 bulbs.

 

MrChips;
Thanks for explaining one of the Christmas’ mysteries. About Santa I already knew the truth.
Never understood how a burnt out bulb allowed the remaining series bulbs to light up.