#Christmas tree lights not working half series

When one bulb burns out in a string, the rest stay lit because the burned out one ‘melts’ together. I’ve only blown one in about 40 years, so don’t get your hopes up.

When you exceed the recommendations by stringing about 6 strings in series, you can blow this fuse. When you buy a new string, you usually get a blinking bulb, a spare bulb or two, and a spare fuse (little bitty cartridge thing). It’s hidden inside the plug, generally and is a pain to get at. One additional thing: all strings with the UL lab tag (some of them are counterfeit, from the land that puts lead paint on your kid’s toys) have a fuse in the circuit. Since the larger resistor has more voltage, it’s brighter!

(series, duh…) So V=IR or I=V/R, and a larger resistor will have larger voltage for equal current.

#Christmas tree lights not working half series

The voltage drop is proportional to the resistance:īulbs in series have equal current through all of them. A 100 stringer in a 50 string will be dimmer, a 50 stringer in a 100 string will be brighter. Since the 100 stringers have lower resistance bulbs than the 50 stringers, you have to match them up. You’ve probably learned that you have to be careful replacing bulbs – sometimes the new one is brighter/dimmer than the original.

A string of 35 lights is 35 equal resistors, so the Voltage drop in each bulb is 120/35= 3.4 v, aboutĪll the bulbs have roughly equal brightness, so the larger strings will have smaller resistors as bulbs P=IV = I^2R, so to get equal power the current is higher and the resistance is smaller.

A string of 50 lights is 50 equal resistors, so the Voltage drop in each bulb is 120/50 = 2.4 v.

A string of 100 lights is 100 equal resistors, so the Voltage drop in each bulb is 120/100 = 1.2 v (unless it's really two strands of 50 acting like one strand).

The total voltage drop of a string is 120 v, but I’m going to round that off because it doesn’t matter much. Most of the electricity goes to heat – about 90%!

Lights are resistors that heat up enough from the electrical flow to give off light (explained in the Secret Life of Machines).

The bulbs are in series, known as ‘when one goes out, they all go out’ more on this later, because sometimes it’s not quite true.

All the strings plug into standard 120 v AC US electricity.

(all these comments apply to the miniature light strings – the larger ‘night-light’ size work entirely differently – they are in a parallel circuit) I also usually create lengths of wire from the strands as well. I haven't bought light bulbs for physics labs in years! Just grab a string of Christmas lights and a pair of wire cutter/strippers and get to work. After Christmas you can find strings of 100 lights for a couple of bucks or you can find people too lazy to figure out why their lights don't work any more. Why use Christmas lights? They're dirt cheap or maybe even free. Don’t throw away that string of burned out lights when you can do some physics on them! Don’t have any? Ask your students to bring some in.