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Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Remember when Christmas light strings exclusively used C7 or C9 lamps, screwed into parallel-wired sockets? Each lamp about 3 watts (sometimes 5), at 120 or 130 V in USA.
A parallel light string can be converted to series by cutting one wire in each segment between sockets. So a string of 25 3-W lamps could become a 25 mA load for 3000 volt power supply, etc.
Time to try this, since I took advantage of seasonal-merchandise closeout sales. Am looking forward to using lights instead of big tubular resistors, as switchable loads to characterize some HV transformers.
Caveat: In a high-voltage series string, if one lamp filament burns out it will probably arc and could start a fire. I am told that in early 20th century electric streetcars, on a 600 volt grid, cabin lighting used strings of five 120-V lamps. Vacuum-filled lamps, generally obsolete even then, were used to prevent the arc problem.
So the lamps should be arranged and supported so operator can easily keep an eye on the whole array, and should not be left unattended while on. I did this many years ago for Christmas decorating. Wired twenty medium-base sockets in series, loaded them with 25-W colored party bulbs, and ran the string from a microwave oven transformer. Never had any trouble with thermal runaway in the fastest-heating lamps, when the string was switched on.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
My Christmas lights are all led as searching for the blown bulbs in series strings drives me nuts. Some indicator filament lamps are still 'vacuum filled' many filament lamps use low pressure inert gas that can arc. Filament lamps make good PTC ballsts, and the low resistance when cold/starting sort of simulates a capacitive startup load.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Ours aren't LED, but are wired in series.
I seem to remember when I was a kid, that the first bulb in the string was a fused link, or something. I(f they didn't work, you just replaced the first bulb, so they must have been wired in series too.
I think some were wired in parallel though, because I remember some bulbs not working in one string, I think.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Here the bulbs are almost always in series, but if one burns out the rest still works, because they contain a device (most likely a NTC or TVS) which still passes current after the bulb burns out.
Registered Member #4081
Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139
My physics textbook says they're wired in series but designed to short when they fail so it can still conduct electricity. Sulaiman are there really ones that arc when blown? I thought that would get far too hot, surely?
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Christmas lights in series as HV load/ballast
