12 volt bulbs on 240 volt circuit

[Paulwith]

Hi just seen the bulbs on my outdoor Xmas lights are 12 volt 3 watt on 240 volt circuit. How can they be working? They do!

 

[Carolina]

They're in series or there's a transformer before them

 

[Paulwith]

They're in series or there's a transformer before them

Thanks no transformer, could you explain please

 

[westward10]

They are in series then, 20 lamps at 12v = 240v.

 

[DPG]

Hence why if you remove one bulb, all the rest go out as well

 

[Mike Johnson]

Not a good thing to have 240v in the size of cable used for Christmas lights IMO, tends to indicate they may be very old, I would put them in the bin and get some new low powered LED lights.

 

[Paulwith]

Hence why if you remove one bulb, all the rest go out as well

Yep they do! But some bulbs which don’t light do complete the circuit and allow the remainder to light. Why?

 

[Mike Johnson]

Hence why if you remove one bulb, all the rest go out as well

Always fun on Christmas morning hunting the blown bulb, I think in those days they where all screw in bulbs as well.

 

[Mike Johnson]

Yep they do! But some bulbs which don’t light do complete the circuit and allow the remainder to light. Why?

Because the filament may have burnt out, but the circuit is still being maintained by the bits of filament across the contacts, or parts of the string is three or four wire.

 

[westward10]

Yep they do! But some bulbs which don’t light do complete the circuit and allow the remainder to light. Why?

Some bulbs were made which incorporated a shunt so even if the lamp element failed the shunt still completed the circuit.

 

[littlespark]

Too many “blown” lamps can soon create problems…. If 2 or 3 become bypassed, then the 240 is divided by 17 or 18 instead of 20…. Raising the 3V to a little bit more… which the remaining lamps struggle to handle.

Later strings incorporated a “fuse” bulb, which should be the first to go…. Giving you an indication of a problem.

 

[DPG]

Some bulbs were made which incorporated a shunt so even if the lamp element failed the shunt still completed the circuit.

Very good point. I'd forgotten about that. But unless you fixed it then it was more likely that other bulbs would fail due to receiving slightly higher voltage.

 

[freddo]

The shunt is called an 'anti fuse' it is simply a short length of wire wrapped a few times around the filament support legs. The wire has a very thin layer of insulation that is able to withstand 12V so nothing happens when the filament is lit. When the filament blows 240V appears across the legs, the thin insulation breaks down which then causes the support legs to be bridged, completing the circuit again. Clever but as mentioned above will over drive the other lamps in the string.

Nothing wrong with old filament xmas lights as long as the cables are in good condition! The nicest sets I have here are all mains powered filament, though I haven't decorated for Christmas in a few years, too much other stuff to do.

 

[westward10]

My mother still has a set not used anymore we put up as kids from Woolworths. All the bulbs were little houses and snowmen etc...

 

[freddo]

My mother still has a set not used anymore we put up as kids from Woolworths. All the bulbs were little houses and snowmen etc...

It's nice old sets had things like that, most modern light sets seem to be bare leds or filament lamps. My favourites were always the ones with the spiky shades like these: