Just what is in the "shunt" built into series string decorative lamp bulbs which takes over when the filament opens?
Is it something like a PTC thermistor which heats up and melts some powdered metal into a permanent short?
My curious mind wants to know.
Thanks,
Jeff
its a shorting bar of some sort but isnt 100%
I will ask a knowledgable buddy just what and how it works
The shunt is a loop of wire around the base of the bulb filament supports with an insulation so thin that it will break down if the voltage across the bulb goes above 10 - 20 volts. When the filament opens as the bulb burns out, the full 120 V is across the open bulb causing the insulation to fail and restoring continuity. Of course, all the bulbs in the string now have a slightly higher voltage, and they tend to burn out a little quicker. After a few more failures, the voltage across the remaining bulbs is up by 10 -20%, and then it is a runaway situation. That is why it is important to replce burned out bulbs promptly.
Bob Hofmann
Thanks, Bob...
I wouldn't have guessed anything as simple as that, but I suppose in that application it doesn't need to be 100% reliable, so if an occasional shunt failed closed prematurely it'd be no big whoop.
I guess the reason for TWO fuses in the plug must be to protect against the wire loop getting shorted to ground somewhere along it's length...two are needed because the plug isn't polarized. Cause, one fuse would be enough to prevent a wire burning up in those runaway situations you described.
Jeff
If that's all it is then it seems like they ought to be able to make the shunt have the same resistance as a (hot) filament. Or, gosh, BE a 2nd filament.
As it turns out, I don't really know what those things are made of.
I would guess the whole string needs some sort of fuse - eventually all of the shorting devices will do so.
- Don Klipstein ( snipped-for-privacy@misty.com)
f
The fuse in the plug is both for the light string itself, and to protect against too many strings being plugged one after another into previous strings and overloading the wires in the string closest to the power source.
You're a ham, I gather. Did you graduate from college in 1957, if so what school, I graduated U of FL in EE in 1957 and NYU in 1961 with MEE.
MIT '57
My previous ham radio call sign was W6KAH, which I got in 1949.
Happy Holidays,
Jeff
Oops, I already forgot that I also got an MSEE from U. Penn in '58. That school seemed so easy after the one I'd just graduated from that I doubled up my classload, wrote a thesis describing how one could use an power transistor in place of the breaker points on a gas engine (a precursor to the development of solid state ignition systems), and got my degree in nine months.
I wasn't the only one to see it that way, a few of my undergraduate classmate buddies who took the same path also got their Masters in one school year there.
I'm not sure if that grad school experience made me a legit "Ivy Leaguer" or not, but I never gave a rat's ass about that sort of stuff anyway.
BTW, speaking of rats, that "Brass Rat" in my sig line is recognized by other MIT grads for what it is. That's what the class rings we wear are called.
I believe you must be writing about parallel strings with 120 volt bulbs in them. I was asking about series strings, and I don't think you can plug another string into those, can you?
Jeff
No, this applies to both. Many series strings have a third wire, allowing power to be carried to a receptacle on the end to which you can plug in another string. This also allows multiple segments so the whole string doesn't go out at once if there's a fault. Troubleshooting the old 100 light strings with 20 light segments is a lot easier than those with 50 light segments!
eff
I have in my possession several 100 light sets, 2 50-light sets in series, with a male plug for providing power and a female plug at the other end of the 100 lights to plug in another string, and so forth. Eventually there is too much current draw and one or both of the fuses in the first circuit will blow. Frequently there will be a label on such types of sets saying how many strings can be connected in series. Take a look at some of the lights in the stores right now.
Thanks for teaching me that. I suspected that might be the case after thinking about it a bit more after posting my last reply.
Being Jewish, the electric holiday lights I've had hands on experience with have been limited in size, like "electric menorahs" I was at a son's home last weekend as he was putting one of those in a window. It was a flat plastic cutout with a series string of about 20 bulbs threaded around and attached to it.
I noticed the fuses in the plug and also the spare fuses and bulbs which came with it, along with a warning on the instructions to replace "burned out" bulbs promptly.
That's when I started wondering what the "shorting shunt" was made from. So I made my original post starting this thread.
Happy Holidays,
Jeff
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