Light output of dimmed lamps

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Incandescent bulbs have a minimum starting voltage? :-)
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

Well a minimum voltage at which they will emit visible light.
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Joseph Meehan

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m Ransley wrote:

That and measure the light output.
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Joseph Meehan

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Yes incandesant have a minimum starting voltage as a dimmer reduces voltage output. I just took my X10 dimmer and put a Kill-A Watt on it with 100w incandesant. A 100 watt light bulb wont give visable light till 5 watts are pulled . So at 4 watts you may think its off, but its pulling 4 watts The Kill a watt in a different configuration to measure V dimmed the display unreadably at 37v and the 100watt bulb using 16 watts, figuring reduction in steps of V it figures to 31V at 10 watts.
I always thought all filament bulbs lasted indefinatly dimmed as incandesant do, I have several 240V incandesant bulbs that on average burn 10-15 years 7 days x 12 hrs , but my dimmed kitchen halogens dont last, as stated and now Ive learned, Halogen redeposit and need full voltage to run right.
A triac wall dimmer on minimum setting I did not try, but X10 is probably Triac and at its reading 4% would be waisted without being sure the switch is off.
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Without a V meter test and from what the kill a watt read I guess 10- 22v is the point at which you will see illumination.
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Most 120V incandescent lamps glow dimly visible to a dark-adapted eye in a dark room at 6 volts. This varies from one lamp to another. For example, the lowest wattages that get a gas fill have the heat conductivity of the gas affecting things more, and need more voltage to glow, and brightness varies a little more with voltage than with high wattage lamps and vacuum ones.
With a tric type dimmer, you need a true RMS meter to get an accurate meter reading of the voltage when dimmed.
- Don Klipstein ( snipped-for-privacy@misty.com)
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Without a V meter test and from what the kill a watt read I guess 10- 22v is the point at which you will see illumination.
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On Sat, 24 Dec 2005 08:53:09 -0600, snipped-for-privacy@webtv.net (m Ransley) wrote:

You might be able to see something at a lower voltage, with a small video camera that responds to infrared. Infrared is just below visible light in frequency, and will be emitted by a filament that's not quite hot enough to emit visible light.
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Every black body above absolute zero emits visible light.
Nick
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On 2005-12-24, snipped-for-privacy@ece.villanova.edu wrote:

More precisely, every black body above absolute zero emits light in the visible portion of the spectrum, often at an intensity too low for the human eye to see.
Cheers, Wayne
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Well nick I find lights kind of usless unless they illuminate things I see.
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m Ransley wrote:

Sure Triac once triggerd to conduct, only way to stop current flow is switching the load off. I have quite a few dimmers in the house. I always turn it off when not in use, don't leave it at minimum setting. Tony
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Tony Hwang wrote:

I agree to turn it off when not in use due to leakage and whatever load the trigger circuit creates.
But it's NOT because the triac is always on once triggered. It does indeed stay conducting but FYI it remains on only until the half of the power cycle is completed and the voltage is at zero, so 1/120th of a second.
I think we need a little discussion here of how these dimmers work. The power passes through the triac (which is the essentially the same as a pair of SCR's in parallel facing the opposite with, with combined triggers). The triac, when triggered, will remain on through the remainder of the half power cycle when it switches off and stays off until triggered again.
The trigger in this case is coming from the incoming power line but passes through the potentiometer (small variable resistance...the knob or slider you turn or move). It takes a certain tiny voltage to trigger the triac to conduct.
If you have the knob turned all the way up (zero resistance) you are getting full power line voltage as trigger. As the voltage rises with the sine wave wave form the trigger voltage is reached almost instantly so the triac starts to conduct and the full half power cycle is passed through. Then when the next half cycle happens (opposite polarity) the same thing happens again.
Now let's say you turn the knob back. Greater resistance means you've reduced the trigger voltage...imagine the sine wave but lower overall in height (voltage).
With the lower voltage trigger the voltage necessary to switch on the triac doesn't happen until slightly later in the half power cycle. At this point the triac does turn on (output voltage jumps from near zero to whatever voltage the power line is at that moment, minus the fractional volt drop going through the triac).
Because the voltage stayed turned off for part of the cycle the load isn't getting powered part of the time and thus for a light bulb it will be dimmer.
Turn the knob back farther (even greater resistance) and you delay the triggering even farther.
If you turn it far enough back it won't trigger until the waveform is at it's peak and you are effectively only giving the load half power.
Turn it back farther still and it won't trigger on at all.
There would still be leakage through the triac and also whatever current the trigger circuit passes.
The key here is the switching on and off. You could make a dimmer using transistors to have voltage partially on / partially off but then you're subject to the same Ohms's Law as a pure resistive dimmer (rheostat). Because the triac is always full on or full off Ohm's Law is avoided (except for the tiny voltage drop through the device).
But that is also a drawback because of the funny waveform. That's why these dimmers are not suitable for many things and why some light bulbs will "sing" at some dimming levels.
And also accounts for the only other components you will find if you crack open a dimmer, besides the triac and the potentiometer: Typically a small inductor (coil) and a capacitor. This is to reduce RF since that rapid rise in voltage when a partly dimmed dimmer triggers will create a lot of it.
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And a phase shift cap and a diac in series with the pot.
Nick
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But a pot and triac alone will give you a working dimmer.
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snipped-for-privacy@SPAMBLOCKfilmteknik.com says...

Perhaps, but one that works very poorly. The trigger voltage across the triac (Vac) is a function of the gate voltage, but is rather sensitive. It's far better to use a diac or other negative resistance widget (e.g. neon bulb) to set the timing independent of the anode-cathode voltage.
--
Keith

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Kraus wrote:

Pot and triac alone can sometimes make an unreliable one variable from close to full, down to half of each half cycle going through, and needs a higher wattage pot. That would work by varying what point in the waveform the pot lets through enough current to trigger the triac. They don't do it that way. For one thing, tolerances in trigger current in the triacs would be a big issue.
The usual dimmer also has a capacitor and a diac. The pot (used as a rheostat) adjusts the amount of time it takes the capacitor to charge up enough to cause the diac to become conductive. Then the charge in the capacitor discharges through the diac and the gate of the triac. This arrangement does limit the range of the waveform at which the triac starts conducting to the range where the instantaneous voltage exceeds the breakover voltage of the diac. Most dimmers without a "positive on" do not let lamps reach full brightness.
- Don Klipstein ( snipped-for-privacy@misty.com)
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Ah...I thought the extras were just for RF. Thanks for the elaboration.
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Best place to find the information is the Illuminating Engineering Society Handbook (any edition); but it's not on-line. You can usually find a copy in a good reference library, however.
The light output and power of an incandescent or halogen incandescent bulb are exponential functions of the applied voltage. One point on the curve that I remember is the half-power point. Say you operate a 100 watt bulb on a dimmer at 50 watts, the light output is about 1/3 of full rated. When you operate a bulb on a rectifier (diode) such as a high-low switch in the low position, it operates at 1/2 power. As I recall, the measured voltage is about 85 volts.
Take a look at: http://www.sylvaniaautocatalog.com/new_sylvania/tung_fila_lamps.htm to see the curves. The values there apply to low voltage automotive, but the numbers for 120 volt lamps aren't much different.
Since efficiency drops faster than light output when incandescent bulbs are dimmed, dimming is not a good energy-saving strategy. At best, incandescent bulbs only emit 10% of their power input as light anyway.
Variable autotransformer dimmers were never widely used for residential lighting, but they were indeed used. Residential versions were made by the Superior Electric Co. in the 1950s and 60s. If you happen to see some reruns of the TV show "Frasier", you'll see several of them on the wall of his radio studio. The have rather large control knobs -- about 4 in. in diameter and required a wall box that is about 4 times the size of a standard box.
TKM
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