wiring lights in series

[securespark]

have a really good look (you may need magnifying glass to see)

That's a bit mean, isn't it?

 

[plugwash]

im guessing there is some from of resistive shunt accroos the bulb terminals
in fact thinking about it i may have seen this

so that even if a bulb blows current will still flow and the set will stay lit

 

[Lectrician]

Modern Xmas lights have a shunt resistor in parallel with the fillament.

The instructions always say to replace bulbs quickly, this is because the shunt resistor doesnt drop the full 12v. The other bulbs age quicker with one blown

Modern ELV ones usually are a combination of series and parallel.

 

[breezer]

Modern ELV ones usually are a combination of series and parallel.

to avoid confusion:

the above is tue. but they are sets of 20 (which are in series) in paralell this way you get sets of 200+ lights

 

[ban-all-sheds]

Modern ELV ones usually are a combination of series and parallel.

to avoid confusion:

the above is tue. but they are sets of 20 (which are in series) in paralell this way you get sets of 200+ lights

Mains sets are commonly groups of 80, using 3V lamps, with multiple groups in parallel, so that you buy 80, 160, 240 light sets. And the lamps do fail-closed, not open, or you'd be forever trying to find broken ones by trial and error.

 

[plugwash]

The instructions always say to replace bulbs quickly, this is because the shunt resistor doesnt drop the full 12v. The other bulbs age quicker with one blown .

wouldn't the shunt resistor be dropping more than the bulb it shunts drops when unblown

higher resistance=more volt drop

 

[Lectrician]

Do you know what plug, I think you may be right

 

[securespark]

And all this cos I mentioned Xmas lights as an example of series wiring!

 

[journeyman]

http://electronics.howstuffworks.com/christmas-lights2.htm

 

[plugwash]

interesting

so when the bulb is working the shunt has a high resistance and saps very little power

when the bulb blows the shunt resistance goes down stopping it causuing a significan't volt drop

clever

 

[kendor]

modern mains series christmas lights have all but one bulb (the fuse bulb) designed to fail short

no they don't

have a closer look at one

all christmas tree lamps fail open, that is why they fail, the filament is burnt out and is open

Not true, there are sets that have a small coil of wire around the filament legs that allow the string to keep working even if one or two lamps have blown, although i don't approve of such lights ( the more that blow the brighter the others become as the voltage increases across the ones left)
the fact is they do exist.

 

[ban-all-sheds]

Not true, there are sets that have a small coil of wire around the filament legs that allow the string to keep working even if one or two lamps have blown,

So how do the lamps work? How/where is that coil of wire connected when the filament is intact and the lamp is working normally?

although i don't approve of such lights ( the more that blow the brighter the others become as the voltage increases across the ones left)
the fact is they do exist.

Would you rather the users of such a set had the problem of replacing each of up to 60 or 80 or whatever lamps one-by-one on a trial and error basis until they found the one which had blown?

 

[plugwash]

a howstuffworks link was posted earlier in the thread

the coil of wire is wrapped round the wires leading up to the filament

however it has an coating which gives it a high resistance

when the bulb blows the wire coils gets very hot (because it is dropping a lot of votage at this point) and the enamal melts lowering the resistantance

 

[phil_ballard]

That's interesting ..... how can a coating on something make it have a HIGHER resistance than when the coating is missing? I could understand it the other way around (ie the coating forming a low-R path and the resistance rising as this disappears) but I can't picture how it works this way around?

Or am I just being dim? (no pun intended)

 

[plugwash]

my guess is that the wire has some springyness and pushes through the melted coating