WHY are some CFBs incompatable with dimmers?

[Dr F]

Hello everyone,

Many compact fluorescent bulbs (CFBs, CFLs) say that they should not
be used with dimmer switches. Why not? Will the bulbs simply burn out,
or might they cause fires and blown fuses? What causes this
incompatability?

My specific question is this: I have a three-bulb ceiling fixture (3x
75 watt max) on a 270 degree turnpot dimmer. Can I safely put
non-dimmer-compatible CFBs in this fixture? What if I always keep the
dimmer switch at full power, and never at partial power? Should I just
replace the switch?

Thank you,

B.W.

 

[Victor Roberts]

On 15 Mar 2004 14:59:36 -0800, bollweevil@centralpets.com (Dr F)
wrote:

Hello everyone,

Many compact fluorescent bulbs (CFBs, CFLs) say that they should not
be used with dimmer switches. Why not? Will the bulbs simply burn out,
or might they cause fires and blown fuses? What causes this
incompatability?

They use a full wave bridge rectifier followed by a BIG energy storage
capacitor. This type of circuit draws current only near the peak of
the line voltage, resulting in a power factor of 0.5 to 0.6.
Triac-based dimmers do not like this type of load. You could burn out
the dimmer, or perhaps even damage the lamp if it operates in short
pulses with this type of dimmer.

My specific question is this: I have a three-bulb ceiling fixture (3x
75 watt max) on a 270 degree turnpot dimmer. Can I safely put
non-dimmer-compatible CFBs in this fixture? What if I always keep the
dimmer switch at full power, and never at partial power? Should I just
replace the switch?

It might work IF you never moved the dimmer off full power. Some
dimmers even bypass the triac at full power. However, to be safe you
should replace the switch.

--
Vic Roberts
http://www.RobertsResearchInc.com

 

[Victor Roberts]

On Mon, 15 Mar 2004 19:29:47 -0500, Sporkman
<sporkedUNDERLINEagainMUNGE@bigfootDOT.com> wrote:

CFBs are incompatible with dimmers for the same reason that regular
sized fluorescent bulbs are incompatible. They all require a short high
voltage bump (provided by the ballast) to jump the gap between
electrodes. In a CFB the (compact) ballast is in the base but it still
operates on the same principle. If you reduce the supply voltage you
can't be sure the voltage spike will be high enough to jump the gap, nor
can you be sure that the continued voltage will be high enough to
maintain the arc.

Not really. One the lamps are started the high voltage spike, which
is present in only some types of ballast anyway, is not present at
all.

--
Vic Roberts
http://www.RobertsResearchInc.com

 

[Dr F]

Thank you Victor Roberts. Your advice is definately helpful. But I
have to ask:

Is there ANY CHANCE of me starting a fire or blowing a fuse by putting
a CFB on my dimmer switch? I do not care if I blow the switch, because
that will simply give me a motive to replace it I do not want to
start a fire, though,

B.W.

 

[Victor Roberts]

On 16 Mar 2004 01:08:58 -0800, bollweevil@centralpets.com (Dr F)
wrote:

Thank you Victor Roberts. Your advice is definately helpful. But I
have to ask:

Is there ANY CHANCE of me starting a fire or blowing a fuse by putting
a CFB on my dimmer switch? I do not care if I blow the switch, because
that will simply give me a motive to replace it I do not want to
start a fire, though,

B.W.

There is always a chance of starting a fire even when electrical
products are used correctly. However, when you misuse an electrical
product it not only increases the chance that you might have a fire,
but if one does start, you will be responsible since you have used the
product against the manufacturers recommendations.

My recommendation is that CFLs should NOT be used with dimmers unless
they SPECIFICALLY state that they are compatible with dimmers. If the
dimmer compatibility statement is only on the packaging and not on the
product, keep that packaging until the product has worn out and has
been thrown away.

--
Vic Roberts
http://www.RobertsResearchInc.com

 

[Tomi Holger Engdahl]

bollweevil@centralpets.com (Dr F) writes:

Hello everyone,

Many compact fluorescent bulbs (CFBs, CFLs) say that they should not
be used with dimmer switches. Why not?

The electronic power supply used in this kind of bulbs
is built using switched mode power Supply.
This type of power supply uses a full wave bridge rectifier
followed by an energy storage capacitor, and then followed with
the rest of the electronics.

This type of circuit draws current only near the peak of the line voltage.
Triac-based dimmers do not like this type of load.
The troac-based dimmers put out very sharp rising edge to the
voltage waveform when they start to conduct, and the power supply
circuit might not like this (very sharp rising voltage edge going
to a rectifier and capacitor circuit, can lead to very high current
peak taken from current, the rectifier and capacitor deigned for
sinewave power might not like it).

Will the bulbs simply burn out,
or might they cause fires and blown fuses?

You could burn out the dimmer, or damage the lamp if it operates in short
pulses given by triac based dimmer.

In best cases the lamp will operate on some dimemr setting well, and
when you dim it just starts working baddly (flashing, etc.) without
dimming nicely...

What causes this incompatability?

It is related to the features of the both dimmer operation and
bubl electronics design. Maiking them such that they would work
nicely together and dim nicely woudl make the system much more
expensive...

My specific question is this: I have a three-bulb ceiling fixture (3x
75 watt max) on a 270 degree turnpot dimmer. Can I safely put
non-dimmer-compatible CFBs in this fixture?

I would not recommend it.

What if I always keep the dimmer switch at full power,
and never at partial power?

This could work possibly (quite often I think), but there is
no guaratee that it will be problem free. And consider the situation
when you just happen to leave the dimmer to wrong position....
I woudl not risk on this.

Should I just replace the switch?

Replace the switch if you want to use non-dimmer-compatible CFBs.

--
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at
http://www.epanorama.net/

 

[Victor Roberts]

On 16 Mar 2004 15:38:18 +0200, Tomi Holger Engdahl
<then@solarflare.cs.hut.fi> wrote:

[snip]

What causes this incompatability?

It is related to the features of the both dimmer operation and
bubl electronics design. Maiking them such that they would work
nicely together and dim nicely woudl make the system much more
expensive...

Actually there are some CFLs specifically designed to operate with
normal phase-cut incandescent lamp dimmers. They are only about 50%
more expensive than normal CFLs that will not work with dimmers.

--
Vic Roberts
http://www.RobertsResearchInc.com