On Tuesday, December 17, 2019 at 9:52:43 PM UTC, Roger Hayter wrote:
The Natural Philosopher wrote:

On 17/12/2019 20:20, Dave Liquorice wrote:
On Tue, 17 Dec 2019 07:46:39 -0000, Brian Gaff \(Sofa 2\) wrote:

If you will recall back in the days of filament bulbs all wired in
series across the mains,

Not a lot different now, have chains of LEDS across a supply of a few
tens of volts.

Of course the danger here was that if a bulb blew short circuit then
you could get a wonderful catastrophic failure ...

Tree light bulbs are designed to fail short circuit. I've never known
a set run away blowing bulbs or the fuse bulb. Failing short makes
finding the duff one fairly easy, it's the one that isn't lit. Fail
open and you have to test each bulb individually.

All the lights I had failed open
The ones i remember all but one of the bulbs had internal wire resistors
in parallel with the filaments so that they failed at about twice the
reslstance of a normal bulb. This had little effect on brightness
until quite a few bulbs had failed. But one the bulbs was a designated
fuse bulb which failed open circuit as the filament (a thicker one?)
carried the whole current. These were usually painted white rather than
coloured and were not as bright when lit as the others.


We've got a couple of sets of these but IIRC the other bulbs could still fail and it was a bugger tracing the faulty one.

I do remember one time going to Homebase to try and find some spare bulbs, optimistic fool that I am, didn't get any of course but they were already selling off the Christmas decorations at a massive discount so I got two boxes of 100 lights for five quid.

--
Halmyre