Pifco Xmas tree lights

I have been called uppon by SWMBO to repair her Pifco Xmas tree lights which she purchased over 5 years ago. (Yes, we have plenty of modern LED versions to
replace them)
1. The instructions say that if one bulb of the 20 fails, then the res will remain illuminated. What's the technology here because they're all wired in series?
2. Is the Fuse Bulb a capacitive unit, because I get a momentary flick of the ohmmeter?
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On 06/10/2021 18:43, gareth evans wrote:

Sticky key ... 50 years ago
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On 06/10/2021 18:47, gareth evans wrote:

The bulbs are designed to fail as a short circuit. So as more fail the others get more overloaded ...
Dave
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writes

Yes. The filament support wires keep it under tension. There's a loop of wire welded to one of the support wires, and it loops around the other support wire (but doesn't touch it). If the filament breaks, the support wires spring apart, and the wire loop shorts one to the other.
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On 06/10/2021 21:54, Ian Jackson wrote:

Interesting.Time to get out the magnifying glass and be curious tomorrow!
73 ES TKS IAN!
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On Wed, 6 Oct 2021 21:54:06 +0100, Ian Jackson

The problem with these Asian $2 a string lights is usually a bad connection between the lamp and the socket so all of that fail safe logic fails.
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On 06/10/2021 18:47, gareth evans wrote:

Each bulb has a resistor in parallel.
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No it does not, the previous description is accurate. If there was a resistor in such a small space it would get very very hot. The fuse bulb has no loop of wire so blows open circuit, but of course many many people used to just shove a normal bulb in which eventually made the fuse in the plug blow or in the case of one we had which plugged into a light socket of the bayonet sort. Blue the fuse on the whole lighting circuit resulting in instant darkness or fried Christmas lights depending on the loading!
Brian
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Yes, well, they were on sale in pound shops only 10 years ago and were frankly death traps. Brian
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On 07/10/2021 08:39, Brian Gaff (Sofa) wrote:

Only if you either have very small fingers or a death wish and poke wires into it whilst switched on. Modern ones for the last couple of decades have all the lamps in insulated plastic plugs.
Old style screw ones had threaded brass at potentially mains voltage. In theory it should blow the fuse bulb in the live line to isolate it but that isn't always the case if plugs are incorrectly wired.
People changing bulbs with the mains still on deserve a Darwin Award.
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Surely not when they were wired is series? In a string of 20 bulbs there would be at most 240/19 volts (I think). Or are you suggesting that this is the case when all the bulbs have gone short circuit?

Always did it that way. Swapping all the bulbs sequentially with a new bulb was the easiest way to find the duff one.
Tim
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On 07/10/2021 09:56, Tim+ wrote:

It is the case if one bulb has gone open circuit for all bulbs 'upstream' of the break..

It was, sadly.
Now all LED....

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On Thu, 07 Oct 2021 08:56:46 +0000, Tim+ wrote:

With a bulb removed, there's no current flowing and so no voltage drop across the remaining bulbs. One side of the empty holder will be at mains voltage and the other side at zero. In the worst case, you've removed the first bulb and it's connected directly to mains live. In the best case you've removed the last one and it's connected through all 19 other bulbs - but cold unlit bulbs have quite a low resistance so this might not be enough to save you.
Also even with the string lit, there might only be 12V across each bulb, but the bulbs at the "live" end still have over 200V to earth.
Mike
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On 07/10/2021 08:39, Brian Gaff (Sofa) wrote:

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On 06/10/2021 18:43, gareth evans wrote:

A special glass that has a low breakdown voltage in the "normal" bulbs. If a filament fails then the glass finds itself with the full 240v ac across it and quickly becomes conducting as a dead short. The remaining N-1 bulbs then remain lit but with N/(N-1) volts across each of them. They burn progressively brighter with each failed bulb in the string.
Things get hairy when 3 or more bulbs fail which is when the fuse bulb should prevent a cascade failure where they are all quickly destroyed.

No. It is has a slightly higher rating and behaves like fuse in series. It should blow before things get out of hand but like all safety devices can sometimes save itself by allowing everything else to fail!
Test each bulb in turn on resistance. Dead ones will be almost zero ohms. Spares are usually available from eBay.
Mains strings of lights are these days considered somewhat dangerous.
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No a lot more basic than that. The fuse bulb will be a blow open one, Normally this has a bit of a white colouring near the base. The rest aware blow short. Normally about 20 all underrun so they can tolerate a couple of duds, then the fuse bulb goes. I to feel that many of the older filament lights were somewhat nicer to look at in their day than the led replacements despite their bit pattern of strobing effects they are often a source of interference. I think in many ways its the safety aspect that spelled the demise of traditional series lights since every bulb has the mains on it and the single insulated wires and the poor workmanship at the wire holder join has resulted in some nasty electrocution events in the past when people put them up while lit and holding onto an earthed radiator at the same time. Brian
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I've still got a set from around 1950 (12 times 20 volt lights, I think). There was also an additional 'flasher bulb' (use optional), with a bi-metal pair of contacts. When used, this created severe 'splat-splat' interference on the radio (especially the long wave). Unfortunately, about half the bulbs are blown - but with the insertion of bits of silver paper where required I can still run them from a variac.

Most modern LED Christmas lights are absolutely awful.

Most of these deaths were probably not actually caused by electrocution, but instead from contact with the floor!
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