Seasonal, extremely naive, electrical/wiring question

[Another John]

We have some Christmas lights in the form of little brass lamps. They're about 20 years old by now. Even I like them, but -- and this is the point -- they are my wife's "favourites".

Two bulbs have stopped working: no chance of replacing them.

There are 20 lights. The wire (presumably Live) comes out of the 240v plug (no current adapter) passes through each bulb, then goes back into the plug (presumably neutral).

Yesterday I pulled out a bulb to see if I had any that would fit (I haven't). As is often the case, the bulb disintegrated, and won't go back in.

The lights now don't work at all ("of course", you are saying).

If I connected the two bits of the wire that go into/come out of that bulb holder, thus bypassing it, would that be a satisfactory solution? Or will the house blow up?

God rest ye merry, gentlemen

John

[Nick Odell]

My guess, as a user of Christmas lights but not an electrician, is that the voltage passing through the remaining lights will rise and the strain won't burn the house down but will pop more bulbs. Could you compensate by inserting another bulb or two elsewhere in the loop - such as near the mains plug if you don't want them on display? And if you over-compensate, the remaining bulbs might glow a little less brightly but might last a little longer.

Nick

[GB]

I assume this is wired in parallel, and the bulbs are 240v? I'm not sure how you can check that. Are there two wires going into each bulb holder and two going out? The thing sounds a bit dodgy to me.

[Nick Odell]

I'm guessing they are wired in series - 12v bulbs.

Nick

[Andy Burns]

unlikely, if the set has 20 lamps, they'll be 12V ones in series

[Martin Brown]

If they are metal and with mains wiring that is more than a little alarming. I hope they are plated plastic.

There might be, but you have to find some modern bulbs intended for the right voltage and wattage and about the right shape and desolder the old dead bulb from its fitting and solder its replacement in position. Taking a modern one apart is usually easy as they are mostly plastic insert with the wires folded over. It is a slightly tricky operation but entirely possible if you are any good at using a soldering iron.

20 lights in series means a nominal 12v across each lamp. You need to know what current or wattage as well. Shorting the missing bulb will work for a while but will stress the remaining bulbs shortening their life and potentially causing a fire if the set is so old that it doesn't have a sacrificial fuse bulb.

I have never known a bulb disintegrate unless I stood on it. Keep the part that screws or bayonets into the bulb socket and try and find an appropriate bulb to put in there. It is fidly but doable. 12v bulbs come in all shapes and sizes but you may have to live with a white one.

Were they the modern ones where a single failed bulb self shorts when it finds 250 volts across it allowing the rest to continue to work or an old school set where any failure takes the entire string out.

Potentially burn down and the remaining bulbs will go pop more quickly.

Definitely do not leave it unattended if you do what you propose and make sure that the mains joint is very well insulated. You don't want young or old fingers touching live mains. Maybe time for new lights!

[Tim Lamb]

I was once pleasantly surprised to find that light bulbs fitted to '70's vintage rail transport also fitted the work lamp holder on my centre lathe:-)

[NY]

The description "The wire (presumably Live) comes out of the 240v plug (no current adapter) passes through each bulb, then goes back into the plug (presumably neutral)" makes it sound as if the bulbs are wired in series. However the fact that two failed bulbs did not stop the rest of the lights working implies that they must be wired in parallel - and yet when a third bulb failed (disintegrated) that *did* stop the rest working. Weird set of symptoms that appear to contradict each other.

By the way, although series-wired lights put a low voltage across each bulb (eg 6V, 12V, 24V depending on number of bulbs) when all the bulbs are working normally, that's not the case if you remove one bulb and happen to touch the bulb holder (tip or sleeve) and earth or both parts of the bulb holder. In that case, your body resistance is much higher than the resistance of the rest of the bulbs in series, so you get almost full mains voltage.

Probably the safest type of lights are ones with a low voltage transformer and the bulbs (or LEDs) wired in parallel: safe voltage and also resilient to failed bulbs.

[The Natural Philosopher]

Almost ceratnly wired in series.

"Yesterday I pulled out a bulb to see if I had any that would fit (I haven't). As is often the case, the bulb disintegrated, and won't go back in.

"*The lights now don't work at all* ("of course", you are saying)."

[Martin Brown]

Not really. The old modern bulbs in series lights are cunningly designed so that whilst the filament is good they have their nominal working voltage across them and all is well. If a filament fails then the failed lamp finds itself with full mains voltage across it. This is enough to breakdown the special glass with mobile carriers and make it become conductive. The remaining good lamps light up again with the failed bulb shorted out. They now have 240/(N-1) current flowing. The same will happen the next time as well except this time it is 240/(N-2).

Eventually 240/(N-n) exceeds the fuse bulb rating and the whole set goes out (or possibly the failing lamp explodes).

In days of yore the first bulb to die took the entire string out and you had to painstakingly find it by binary chopping or worse still testing each individual bulb.

This is particularly important if he messes around with the wiring there must be no bare conductors exposed at mains potential.

Although more clumsy cabling.

[Andy Burns]

The lamps are designed to fail as a short circuit, this puts more stress on the remaining lamps, but when one lamp disintegrated while being removed, it has presented as an open circuit.

[charles]

No, some bulbs had a resistor in parallel with the filament. This would cope with an open circuit filament, but not with a missing bulb.

[Robin]

as a lad in the '60s if we'd no spare bulbs I used to stick a pin through the wires to bypass each bulb in turn until they lit: diagnosis and cure in one as I'd just leave the pin in place. But I did learn to turn off the mains while inserting the pin ;)

[Malcolm Race]

Would it be possible to replace the wiring and bulb holders with an LED string, retaining the shades?

Malcolm

[The Natural Philosopher]

we used metallic cigarette paper round the dead bulb base

[Brian Gaff]

It will overrun the rest of the bulbs. You really need to find out the wattage and voltage. The voltage is easy. 12V, so then you have to find out what the current is, and unless its labelled anything suitable will have to be guess work. Too high a current and it will probably not work and too low and it will probably blow.

It might, sadly be time to say goodbye to an old friend as if two have blown all the others will be on their last gasp as well. Some very dangerous ones used to blow short and had a fuse bulb to stop the whole thing shorting out the mains.

Brian

[Tricky Dicky]

Although virtually all Christmas light sets now are LED I did notice two of our garden centres still had a small stock of replacement bulbs for older filament sets. I would give any in your locality a visit before you start hacking up your set of lights you never know you may strike lucky.

Richard

[Robin]

not readily available in a roll-your-own household

[Martin Brown]

Likewise with back street electrical retailers if there are any left in your neck of the woods. 12v lamps are relatively common so you might get something intended as an automotive dashboard indicator lamp that suits.

I have a small stash of lamps of various sizes for mending Xmas lights.

[The Natural Philosopher]

Bacofoil?