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#74119 01/14/07 05:53 PM
Joined: Dec 2005
Posts: 869
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Cheap lamps.

Try a known brand and see if it lasts longer.

Also a 130 Volts lamp will last longer too as already suggested


The product of rotation, excitation and flux produces electricty.
#74120 01/14/07 08:01 PM
Joined: Jun 2004
Posts: 1,273
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Member
Try 120 times a second...

The voltage goes to zero twice in each cycle.


Tesla
#74121 01/15/07 12:23 AM
Joined: Aug 2005
Posts: 214
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Member
*smacks self in head* I've seen at least half a dozen bulbs seperated from the base, that were dangling from the wires, one of which was after I hit it with my head! (oonly 6' clearence in basement, I'm 6'2, not a great job)

Will *sticks foot in mouth again*

#74122 01/15/07 02:41 AM
Joined: Jul 2002
Posts: 8,443
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Tesla,
Quote
Try 120 times a second...

And how do you work that one out?.
Are you telling me that with a frequency of 60Hz, the real cycles per second are 120?.

#74123 01/15/07 06:54 AM
Joined: Jul 2006
Posts: 15
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Alan, iwire

Quote
Nice theory, er...except that a bulb is switching off and on 60 times a second anyway, in harmony with any arc.

The keyword here is thermal inertia. A filament takes a bit longer than 1/120s to cool down significantly. That's why with bulbs you usually do not have the stroboscopic effect (that's the one that makes some car wheels on TV stand still or go backwards).

A dimmer prolongs lamp life by operating the filament at lower temperatures (= less light, color is yellow/orange). The filament does not care about the horrible waveform feeding it. I've used small bulbs as load devices for all kinds of waveforms, even true power spikes, they don't care as long as you don't exceed the rated power dissipation (the integral of voltage times current over time). Not to say that these bulbs didn't suffer from being "switched" on and off by low-periodicity spikes...

#74124 01/15/07 07:05 AM
Joined: May 2005
Posts: 247
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starting at zero, the sine wave goes up to ~200v, back to zero, down to ~-200 and back up to zero. that is one complete cycle. The average voltage is 120v. 60hz is 60 complete cycles, or 120 zero crossings per second.

The way that solid state dimmers work is that the SCR (or triac) delays the turn on point for
some time after the zero crossing, reducing the average voltage, at the expense of a clean sine wave. better dimmers add a inductor to clean up the sine wave.

#74125 01/15/07 08:17 AM
Joined: Mar 2005
Posts: 1,803
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Let me try and reason my argument. If a piece of metal, say a bulb filament, be heated to a high temperature by means of an electric current; when the current ceases to flow the cooling rate of the metal must be a function of the dimensions, alloy and surface condition of the filament, in the radiative, convective and conductive conditions appertaining. The heating-up rate is constrained by slightly different conditions, ie the material properties as outlined above, plus the filament's resistance/temperature curve, the voltage applied over time, the original temperature and the ongoing heat losses as the temperature rises. As far as I can see these are the only conditions which appertain, and must embrace "thermal inertia". To cause premature failure of a filament would require a change in at least one of the conditions, and the only base variable is the voltage, which I think, intuitively, needs to increase.
$64000 Question: Does an arc produce higher voltages in a circuit?


Wood work but can't!
#74126 01/15/07 10:13 AM
Joined: Jul 2006
Posts: 15
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Alan,

You're absolutely correct about the inertia thing, I use this word as a "lump sum" of all the various parameters that you described.

The change you are looking for that leads to filament failure is mostly in the mechanical dimensions of the filament. Over time, part of it vaporizes and the imperfections in it make it weaker until it burns out.
Take the time and check out this internet page, found googling for +light +bulb +failure: http://invsee.asu.edu/nmodules/lightbulbmod/burnout.html
Have a look at the pictures of the aged filaments - you can see the "corrosion".

Frequently switching a bulb on and off will accelerate this process, arcing is nothing else than a fast, more or less random switch that additionally switches to on - usually not at zero crossing but at substantial voltages (that can gap the bridge between the two contact surfaces) so the inrush current is even higher than usual, accelerating the evaporation process even further.

And, to conclude: In a lighting circuit, arcing should not produce high voltage spikes. Please PM me the $64,000 check [Linked Image] and thanks for the interesting discussion!

#74127 01/15/07 11:12 AM
Joined: Feb 2002
Posts: 114
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Member
Here's a non-technical possibility. A few tenants may be replacing their burned out bulbs with good ones from public areas of the building.

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