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.
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 ( firstname.lastname@example.org)
On Sat, 24 Dec 2005 08:53:09 -0600, email@example.com (m Ransley)
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.
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.
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
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
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
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
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
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.
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 ( firstname.lastname@example.org)
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
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