Fairy Lights

Hi all. It's that time of year again, and (as always) the assorted strings of lights which all seemed to be working when we put them away last January, now have a few "dead" bulbs.
First question... The number of bulbs per string (and hence the voltage) varies - but the fittings are identical. So how do I know which spares (also archived in the loft from year to year) fit which string? I've blown several already today, while undertaking an exercise in destructive testing :-(
Second question... when one or two blow, the rest remain on - how do they manage that when they're in series? Is it the magic of Christmas, or is there a technological explanation?
TIA
--
Martin


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I imagine they have a resistor in parallel with the filament in each lamp.
Incidentally I thought the different No of bulbs per string had a different fitting, certainly we've got two different types.
Steve
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shazzbat wrote:

No. The bulb is arrange to turn into a dead short once the filament fails. There is a link inside that has enough insulation not to breakdown when the filament is intact. Once the filament fails, full mains voltage appears across the bulb and the insulation breaks down causing the bulb to become a short circuit and the string of lights continues to work. Once a few bulb have failed in this way the string current becomes too high and the fuse bulb which is simply a bulb without this device built in, then fails protecting the circuit.

There are two factors to consider when swapping bulbs. 1) the voltage related to the number of bulbs per string and 2) the operating current of that string. If the current rating of the replaced bulb is not the same as the other bulbs, the voltage sharing between all of the bulbs is upset even if all bulbs are from set with the same number of bulbs per string.
For very many years, different bulbs have been coded by the plastic base fitted to the bulb and by this means, incorrect bulbs cannot be fitted into sockets.
If the OP has interchangeable bulbs, these possible date back to when they were fitted with LES bases (Lilliputian Edison Screwcap) which should have the voltage and current marked on them.
Bob

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In the lights I've got, I'm fairly sure that the two wires which support the filament are 'springs', and these keep the filament under slight tension.
Attached to one of the supports is a small 'hoop'. [Imagine someone doing the Hoola Hoop, and suddenly the hoop sticks to them!] This encircles the other support - but doesn't quite touch it.
If the filament burns out and breaks, the supports spring apart. The support to which the hoop is NOT attached makes contact with the inside of the hoop, and the bulb becomes a short circuit.
--
Ian

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Thanks to all for your replies.
Re. bulb fittings, they're the push-in type - and definitely interchangeable with (and look identical to) some spares which I succeeded in blowing yesterday.
I take your point, Bob, that the operating current per bulb is also significant.
But am I right in assuming it's not really possible to do any accurate sums on this, since (presumably) the filament resistance remains very low until it's heated up - and glowing prettily ?
But my assumption is that, if it's a higher wattage, it will simply stay cool, and effectively the string will operate as if one bulb had short-circuited.
If it's a lower wattage, then I assume it would have to be considerably lower for it to blow *before* the other bulbs had reached near-enough operating current to limit total current in the string. I've noticed a couple of replacement bulbs which I've fitted are a tad brighter than the rest - but the whole string seems to be working fine (so far...).
As an aside, and in case it's of interest to any G+S fans, at least I discovered the origin of the name "fairy" lights. According to Wiki (...pedia, not leaks...) the fairies on the opening night of Iolanthe wore miniature electric lights on stage...! That was 128 years ago - and possibly pre-dates even the string I'm trying to repair :-)
Thanks again.
--
Martin




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Martin wrote:

I've just chucked a set of fairy lights in the bin after finding it impossible to obtain replacement bulbs. The details of the type of bulb were on a plastic tag around the cable near the controller - but I've been everywhere, including internet sites and the garden-centre where we bought the lights in the first place, and no-one has them. We only bought them 3 or 4 years ago as well. We've bought a set of LED lights this time. Hopefully they will last a bit longer...
--
Kev


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I agree that LEDs are the way to go. I have the same problem as you every year, the glass bulbs are delicate and are expensive to replace. This year I bought 4 sets of LED coloured lights which look the same as the lamps in the old sets which are about 35mm long, not the puny little bare LEDs. The LEDs are in envelopes the same size as the plastic ones but they are plastic. They look clear until lit, they are much brighter, use hardly any elec, and should last far longer. The problem is finding them as they only seem to have appeared this year, however try Amazon- (Amazon.com product link shortened)92265873&sr=1-12
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On 13/12/2010 18:46, AJH wrote:

(Amazon.com product link shortened)92265873&sr=1-12
The problem with plug in style LED lights is that the water gets in and one pole will then corrode quite quickly. By the 3rd Christmas I was searching for the duff joint, so don't expect them to last forever
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