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Old 16th Jan 2020, 6:24 pm   #505
broadgage
Nonode
 
Join Date: Jan 2010
Location: North Somerset, UK.
Posts: 2,129
Default Re: Vintage Christmas Tree lights

20 volt lamps intended for use in series strings of 12, do not usually have a shunting device that shorts out a failed lamp.
Reputable manufacturers decided that shunting a failed lamp in a string of less than 20 lamps resulted in excessive over voltage on the remaining lamps.

A minority of imported 20 volt lamps DO have shunting devices. If a shunting device has operated prematurely it may be defeated by connecting the lamp to a suitable supply that can deliver a substantial current. A 12 volt lead acid battery will serve. This will burn out any shunt that has operated.

Think twice before operating 12 volt 2.2 watt pilot lamp in series with 20 volt 3 watt Christmas lights. The 12 volt lamps will be under run, perhaps dropping as little as 4 volts and thereby potentially overrunning the 20 volt lamps.

BTW, the electric lamp industry allegedly standardised on series strings of 20 volt lamps so as to ensure continued sales of the relatively expensive 20 volt lamps.
If instead, say 10 lamps each of 24 volts had been used, then people might have used cheap 24 volt pilot lamps or automotive bulbs.

In the USA 15/16 volt lamps were popular, 7 in series on 110 volt circuits and 8 in series on 120 volt circuits, again to deter use of cheap pilot lamps or automotive bulbs.

Ingenious persons still used cheap pilot lamps, altering the number in series accordingly.
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