Dimmer + CFL Test Results This is not an area that anyone seems to have looked at closely, so some tests were run to find out exactly
does happen if a
CFL is connected to a
dimmer. The results were a complete surprise. The assumption is that the
CFL probably won't work at all, but most do (although they don't dim). What you can't see is the
RMS and peak
current drawn from the mains!
The symbol on the left means "no dimmers", but may or may not be understood by users - most of whom are normal householders with little or no technical know-how. I don't think it's
clear enough, and it certainly doesn't make the
point as strongly as it should.
Warning! ... If one decides to test what happens if a
CFL is used with a
dimmer, at some settings (with possibly most CFLs) it may actually appear to work perfectly normally. One could easily be excused to imagining that there is no problem, as long as the
dimmer is set to the maximum and left there. There's no visual clue, with normal light output and no nasty noises. Certainly, the lamp can't be dimmed, but that may not
seem a major concern. I have seen this done - the
dimmer knob was taped to hold it at the maximum setting.
Don't do it! While it may appear to work normally, the
current drawn by a typical
CFL used this way increases up to 5-fold, to the
point where it is potentially very dangerous. The
current spikes are very narrow, but can exceed 8A with an 18W
CFL. The
RMS current drawn can be as high as 0.5A - over 5 times that drawn with no
dimmer in the
circuit (and that's with
dimmer set to maximum!).
Where the
CFL has a fusible
resistor at the mains input, this is present to limit the maximum (peak)
current, and prevent internal
short-circuit failures from blowing the main
circuit breaker or
fuse. Fusible resistors do not react (
fuse) with excessive dissipation, so if the lamp is used with a
dimmer (even if set to maximum), there is a very real chance that the fusible
resistor (and/or other parts) will overheat due to the massively increased
current, possibly leading to a (hopefully) small fire. The fusible
resistor value can vary widely. Some have a very low resistance, so the chance of serious overheating is small. Others can use values ranging from 10 ohms up to 22 ohms. Some don't use one at all, but you don't know from the outside.
This is also a potential issue with electronic timers, motion sensors and home automation systems as discussed below. One thing is of great concern in all cases - either the lamp will have a very short life (assuming it doesn't choose to catch on fire), or the
dimmer or other switching
circuit will be severely damaged - or both !
While many
CFL packages do state that they should not be used with dimmers, some don't, and others use a rather cryptic symbol (shown above) that users may or may not understand. While we still have a choice there isn't a major problem, because people will use
incandescent lamps where they have dimmers (after all, that's why the
dimmer is there). Once the choice is taken away, people no longer have a choice. Those in rented premises can't remove dimmers without the owner's approval, and those who own their home (or have permission) will usually have to get an electrician to remove the
dimmer and its wiring and blank off the hole. Many will find that the lamp seems to work fine, so will leave it there. The consequences are potentially very dire, if seemingly somewhat remote at first glance.
At anywhere between 3 to 5 times the normal
current, the chance of a fire may seem pretty small, but even if only
one house burns down or is badly damaged as a direct result, what if it's yours? Will your insurance even
cover it ("
You caused the fire yourself by using a CFL with a dimmer")? What if someone dies? This isn't idle speculation - several CFLs have been tested, and the same problem has shown up with all of them. The chance may be 1 in 1,000,000 but with several million CFLs being forced upon people following a ban, we have far too many opportunities for a disaster.
Tests I ran showed that the operating (
RMS)
current could easily increase from a normal
current of 90mA up to 300mA, with peak currents as high as 3A measured. Other tests run [
10] showed higher currents because a different
dimmer was used, namely a standard wall-plate
dimmer, as used in most households. The one I used is a
unit I built many years ago and is designed for heavy loads. These measurements (tabulated below) also showed
current spikes of around 4.4 amps worst case, reduced to 2.2 amps with the
dimmer on full (peak currents are not shown). The
RMS current measured 0.42 amps at 75% and 0.24 amps at 100%
dimmer setting - this equates to 110 VA and 59 VA respectively.
CFL Power Draws
13w (Nominal
Current Draw (
RMS) 83mA) (Dim @ 75% 450mA) (Dim @ 100% 245mA)
11w (Nominal
Current Draw (
RMS) 80mA) (Dim @ 75% 420mA) (Dim @ 100% 240mA)
8w (Nominal
Current Draw (
RMS) 80mA) (Dim @ 75% 330mA) (Dim @ 100% 190mA)
5w (Nominal
Current Draw (
RMS) 40mA) (Dim @ 75% 260mA) (Dim @ 100% 200mA)
These test results are from real CFLs, connected to a
dimmer set to 75% and 100%. Why test at 75%? Because it
will happen - people (especially children) will fiddle with the
dimmer, and they may be highly amused by the
CFL becoming a flashing lamp at some settings (although not all do). If the
dimmer is in
circuit, a setting of 75% looks alright, in that most CFLs don't flicker or flash, and have more or less normal light output, so it could easily remain like that for some time. Even if the
dimmer is glued, taped or nailed at the maximum setting (not that I recommend the latter
), the
current is still much, much higher than it should be. At the very least, lamp life will be reduced, at the worst ... ?
Just look at the
current drawn! The average increase is 5 times, which means that 25 times more heat is generated in any
current limiting
resistor in the lamp's
ballast circuit. It is inevitable that this will cause a failure, and probable that the
circuit board will be badly charred or set on fire. While there is no guarantee that the lamp will catch on fire,
there is likewise no guarantee that it won't. The
waveform of a
CFL with a
dimmer in
circuit is shown below, along with the normal waveform for comparison.
If the fusible
resistor is rated at 1W (fairly typical) and has a value of 15 ohms (also not uncommon), it will normally dissipate about 100mW - a perfectly safe
power level. In the worst case shown above, the same
resistor with 450mA through it will dissipate 3W, so it will get
extremely hot. Certainly hot enough to cause failure in adjacent components, hot enough to melt the solder holding it into the
PCB, and very likely hot enough to cause the
PCB to catch on fire. I've seen boards that have caught alight because of overheated resistors enough times to know that there is a real possibility of the same thing happening in a
CFL drawing 5 times its normal
current.
To reiterate ... never use a CFL with a dimmer in the circuit, even if it is set (and kept) at the maximum setting. Doing so places you at risk of fire, and at the very least will dramatically shorten the life of the lamp and the dimmer. Remember that these figures were all measured using a normal dimmer and with a variety of different CFLs - nothing is guessed, surmised or imagined - this is real data !
Although you probably won't find information this detailed anywhere else on the Net (although there is a brief mention of just this topic
here and
here), that's because almost no-one has done the tests (although many people
have experienced burn-outs, melt-downs and even fires).
If tests have been done, the results have not been publicised. Anyone with the skills and test equipment can verify the results, and I encourage those who are able to do so. Your results will almost certainly be slightly different because of differing mains
voltage and lamp types, but the general trend will be the same. These results are compiled from tests run independently by two people, using different lamps and test gear, but with very similar results. Again, a total lack of any form of comprehensive mandatory standards means that no-one knows which lamps will just die quietly and which ones may exit in a blaze of glory (see
below for suggested standards).
