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?
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.
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
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
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.
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
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.
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
I take your point, Bob, that the operating current per bulb is also
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
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 :-)
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
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
(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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