OT: Shortening Christmas Light Strand

C4 and C9 bulb strings are parallel, but mini lights with a 3-wire cord are generally series. If you look at the rating of each bulb, they run
in the 2 to 6 volt range so series is used to divide up the 120v amongst them. To give you added length, they will divide the set into 2 or 3 sections, hence the extra wire(s). A common set size has 100 lights divided into 2 sections of 50 with each bulb rated for 2.5v to give you the voltage drop on each bulb that 'uses up' the available 120v. If you have self-shunting bulbs, each time one burns out, the set gets a little brighter... at least up to a point. Self-shunting bulbs are becoming very common and could be mistaken for a parallel setup since the whole string doesn't go out when a single bulb fails. If you remove the bulb, the string will go out though. The real problem with these mini light sets is when every manufacturer uses different sized bulb holders and different voltage ranges. You can never find the right replacement bulbs again. :| -Scott
Ismaeel Abdur-Rasheed wrote:

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oic :)
thanks for the explanation
gee, you sure know alot about christmas lights! :)
- iz
Scott Aleckson wrote:

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I have observed the same thing with my experimentation. Now for a question that I have never been able to figure out: how does the shunt work???
I've done many experiments, and not been able to figure it out.
I took a bulb and broke the glass, and physically broke the filament. Now it measures infinite resistance. I can see the thin wire shunt wrapped several times between the leads that hold the filament.
I tried raising the voltage on a bulb until it burns out (usually around 7.5-8.0 volts). The shunt does not seem to do anything. The bulb measures infinite resistance, just like the case where I broke the filament.
So - how does the shunt enter the circuit? Does it somehow "melt" or something when the 120V open-circuit voltage is across the shunt? That is the only explaination I can think of, since my experiments were with lower voltages when the filament goes out.
-- David

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Yup, I remember getting to this point now and keying in on the "wire" wrapped around the base of the filament leads. When the filament blows, the voltage across the shunt goes up to the line supply voltage (ca. 165 volts P-P for 120 V rms), causing it to arc over and establish the shunt connection. I couldn't see the coating on the wire shunt, however, so I still wasn't really sure how it was implemented. That link to HowStuffWorks (above) fills the gap, though in this case it's more like how stuff doesn't work.
It seems a little sporty to me to intentially arc a part with line voltage in the case of a malfunction, especially on a circuit that is in intimate contact with an easily combustible object. You are essentially leaving the fuse as the last line of defense.
Brad Hitch

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120V (or 165v P-P) will not arc across that 1/4" gap where the filament is. It would take a much higher voltage to arc that far. I think the mechanism is more that there is a high resistance coating on the shunt, which exposed to a 120V differential conducts enough to heat up and melt, joining the shunt wire directly to the leads.
-- David

cord
run
amongst
you
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find
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Well, exactly, of course it won't arc across the filament gap. I meant that 0.1 mil layer of dielectric coating on the shunt wire. Probably a pretty safe design, actually, but I'm not used to seeing circuits that intentionally arc anywhere unless it's an ignitor. Of course there aren't any MOSFET's in a string of Christmas lights.
Have we strung thus out enough yet? he says, brightening at the prospect of ending this thread!
Brad Hitch

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Brad Hitch wrote:

Besides, the remaining lights are still in series, so there shouldn't be any extreme current surges (beyond the increase of current when line voltage is now applied to the remaining N-1 bulbs) when the shunt bypasses a failed one.
-dave w
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Yup. All of this talk makes me want to measure it. I think I'll enlist my ten-year-old to dig out an old string and replace N-1 bulbs with shunted ones and then we'll hook up the DSO to capture the transient across a current-sensing resistor when we plug in that last good bulb.
I must not have enough to do.
Brad Hitch
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Brad Hitch wrote:

I think long before you get to N-1 shunts and 1 good one, you'll probably hit a cascading failure mode where the bulbs start popping filaments at a faster and faster rate - and at some point one of the shunts is going to open like a little fuse and shut the whole thing down.
-dave w
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There's a 2A fuse in line with each series; that should blow long before you hit that scenario.
And - just to drag the thread out further - it was my understanding that the coating was high resistance and easily melted, so it heats up and melts when the potential across it rises from 2.5V to 120V. I don't think it requires any kind of arc, even across the coating.
-- David

line
bypasses
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You guys are too much. I'll never get the last word.... Come on, Not even a Little Arc? RF ringing is an arc signature, isn't it? We can see if it's there.
Brad Hitch

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Ismaeel Abdur-Rasheed wrote:

Usually the parallel-wired strings are the ones with the larger screw-base bulbs (which operate at 120V) and the series-wired strings have the small push-in low voltage bulbs.
-dave w
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Why not measure the impedence of the loop, shorten, measure again, and add the approriate amount of resistance to compensate?
Philip

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Philip D. wrote:

That should work. But it prompts me to want to issue a reminder: Making changes like this is what often results in a fire.
For starters, the resistance measured with an ohmmeter may read much lower than the actual resistance under operating conditions - ie, the filaments will heat up and exhibit more resistance at operating levels. Hence, the resistor selected based on the ohmmeter reading could be too low, and result in too high of current.
If a resistor is used to replace the bulbs, it should be overrated. If you think a 1/4 watt is suffiicient, use a 1/2 watt resistor for extra safety.
Lastly, the resistor should be soldered in place, and wrapper thoroughly in electrical tape.
No sloppy work allowed here.
My 2 cents. FWIW.
Doug
--
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Heat shrink tubing would be much cleaner...
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