the wire wrap around the filiment posts

[ferris]

On modern christmas tree lights, there is a tripple wrap wire below the filiment.  My understanding is that this is put there in the case that the filiment blows.  The current can then pass through the wire allowing the remaining bulbs to stay lit.

If it were a simple 'in parallel' circuit then the resistance of the wire would have to be much greater than the filiment to not steal current away.  But then, if it were much greater, it would then steal voltage from the remaining bulbs (fixed current) when if the filiment burns out.

So, what is going on here ?  Is the material of the tripple wrap wire have a temp coefficient affected by the proximity of the filiment, or am i just reading the circuit incorrectly ?

Any help from an EE out there ?

Thanks,

Ferris

[Chris W. Millinship]

The shunt wire wrapped around the lead-in wires has a special insulating coating which has a high resistance. It isn`t a complete insulator, but is nonetheless enough that it becomes inconsequential when the main low voltage filament is there in paralell with it.

When the main filament burns out however, the voltage accross the lead-in wires rises to near that of the incoming supply, since at that moment almost no current flows through the rest of the bulbs in series. With this high voltage present, now the resistance of the coating of the shunt wire becomes more significant. Enough current flows to cause it to heat up and melt at the points where it touches the leads. Once melted through, the leads are essentially shorted together by the shunt wire, bypassing that bulb and lighting the rest again. This all happens extremely quickly when a bulb burns out so it appears almost as if the supply to the rest had never been interrupted.





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[ferris]

That makes more sense.  Does anyone know more about the specs of the wire ?

What is the material of the shunt wire ??   What about the insulator ??  
Is it coated, or a natural oxide of the wire itself?

Is there an electrical reason for the tripple wrap ?  or is it just a mechanical reason to give hold it ?

Where can i buy some of the wire ?

Many thanks again.

J

[James]

The wire is lightly oxidised aluminium.  You produce it from aluminium wire simply by heating it to around 450 celsius in a humid atmosphere.  Usually, the lampmaking machines are just fed with plain aluminium wire and the machine oxidises the wire itself immediately before coiling it in place.

The oxide layer thus formed is a good electrical insulator.  As Chris points out, no current flows through this route when the voltage across the lamp is low.  But when the filament fails and the voltage applied across the lamp rises to the mains voltage, it is sufficient to break down the insulating coating and weld the inner core across the lamp lead wires.

Care has to be taken with the oxide thickness - too much and it won't break down, too little and it short-circuits at lower voltages.  The only reason for wrapping it several times around the lead wires is to guarantee a better contact and make the wire self-supporting.  If only one loop was done, it could come loose and fall off, but several coils help to hold each other in place.

Best regards,

James