Florescent lighting?

Is it feasable, or doable to dim an 18 watt Tri phosphor white tube ( 2
foot)?
Looking for about 50%intensity range.

I think they need a voltage across them to maintain ionization, but perhaps
current control?
Any pointers?

On Thu, 31 Mar 2005 14:45:53 GMT, "Tim" <(E-Mail Removed)> wrote:

>Is it feasable, or doable to dim an 18 watt Tri phosphor white tube ( 2
>foot)?
>Looking for about 50%intensity range.

You haven't given us the exact lamp type but almost all fluorescent
lamps are dimmable. The only exceptions I can think of are lamps that
have an integral glow switch starter, and almost all of these are
compact fluorescent lamps. (The reason dimming is a problem when the
lamp has a glow switch attached, is that the lamp voltage rises when
the current is decreased and this can trigger the glow switch for some
lamp types.)

Instant start fluorescent lamps, those with only one pin on each end
or those that have two pins on each end that are shorter together, are
dimmable, but the dimming range is limited since the electrodes cannot
be heated from an external source.

>I think they need a voltage across them to maintain ionization, but perhaps
>current control?

All discharge lamps, including fluorescent lamps, must be operated on
a ballast that controls the lamp current while providing the voltage
that the lamp wants to run at. To repeat - you choose the current. The
lamp chooses the voltage. Just the reverse of the more common voltage
source.

>Any pointers?

Google is your friend. Also, International Rectifier, www.irf.com, has
a number of application notes on ballast design, including circuit
diagrams. Note that all their circuits are designed to use their
ballast control ICs, which are not necessary in other designs.

--
Vic Roberts
http://www.RobertsResearchInc.com
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Thanks for the feedback, I would be happy with being able to dimm from
100% brightness, to about 50% level.
The tube type is Sylvania, 18W/860 daylight with a diameter of about an
inch, and two pins at either end. I was wondering if I can add any other
components to a standard ballast, to vary/limit the current. I would like
to be able to vary the brightness to see what output is right for a custom
made light fitting, and then set the level at that setting pemamnently. I
guess I may need a variable current source, and some understanding of the
operation of the existing ballast, or is there a better, simpler way of
achieving what I need??

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed) oups.com...
> Tim wrote:
>> Is it feasable, or doable to dim an 18 watt Tri phosphor white tube (
> 2
>> foot)?
>> Looking for about 50%intensity range.
>>
>> I think they need a voltage across them to maintain ionization, but
> perhaps
>> current control?
>> Any pointers?
>
> sure, if you actually tell us what you want to do. dim from 0 to 100%
> at will? fixed 50% level? 50-100%? App? Give us a few clues eh.
>
> NT
>

On Fri, 01 Apr 2005 02:37:52 GMT, "Tim" <(E-Mail Removed)> wrote:

>Thanks for the feedback, I would be happy with being able to dimm from
>100% brightness, to about 50% level.
>The tube type is Sylvania, 18W/860 daylight with a diameter of about an
>inch, and two pins at either end. I was wondering if I can add any other
>components to a standard ballast, to vary/limit the current. I would like
>to be able to vary the brightness to see what output is right for a custom
>made light fitting, and then set the level at that setting pemamnently. I
>guess I may need a variable current source, and some understanding of the
>operation of the existing ballast, or is there a better, simpler way of
>achieving what I need??

Whet kind of ballast? If you are using a line frequency EM ballast
then you can dim the lamp to some extent by adding additional
impedance between the lamp and ballast - this was done in the early
days of the energy crunch in the 1970's, before low energy lamps and
ballasts were available. However, if the lamp is rapid start you still
need a way to heat the lamp electrodes. Those early low wattage lamps
with built-in series impedance also had a small 1:1 transformer added
to carry the electrode heating current around the additional series
impedance.

If you have an electronic ballast, the various designs are too
different to suggest an external solution. If you understand
electronics, you could go into the ballast and change the operating
frequency, which will change the lamp current. However, this requires
a substantial bit of knowledge. Messing with an electronic ballast is
quite difficult and risky, for both you and the ballast.

Have you considered purchasing a dimming ballast made for this lamp?
Or a ballast made for a lamp that runs at about 1/2 the current of
your lamp but the same voltage?

--
Vic Roberts
http://www.RobertsResearchInc.com
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In article <4u23e.19680$(E-Mail Removed)>,
"Tim" <(E-Mail Removed)> writes:
> Thanks for the feedback, I would be happy with being able to dimm from
> 100% brightness, to about 50% level.
> The tube type is Sylvania, 18W/860 daylight with a diameter of about an
> inch, and two pins at either end. I was wondering if I can add any other
> components to a standard ballast, to vary/limit the current. I would like
> to be able to vary the brightness to see what output is right for a custom
> made light fitting, and then set the level at that setting pemamnently. I
> guess I may need a variable current source, and some understanding of the
> operation of the existing ballast, or is there a better, simpler way of
> achieving what I need??

You should be able to buy an electronic diming ballast with a
0-10V control voltage. When using just a single ballast, with
some of these you can just connect a small potentiometer to the
dimming input as they contain a low current internal 10V supply
just for this purpose.

--
Andrew Gabriel

In article <(E-Mail Removed). com>,
(E-Mail Removed) says...
>
>Tim wrote:
>
>> Thanks for the feedback, I would be happy with being able to dimm
>from
>> 100% brightness, to about 50% level.
>> The tube type is Sylvania, 18W/860 daylight with a diameter of about
>an
>> inch, and two pins at either end. I was wondering if I can add any
>other
>> components to a standard ballast, to vary/limit the current. I would
>like
>> to be able to vary the brightness to see what output is right for a
>custom
>> made light fitting, and then set the level at that setting
>pemamnently.
>
>ok, situation clear now. If youre using the standard British glow start
>circuit (flashes during start up), an extra impedance in series with
>the choke will reduce lamp run power. But... if you drop the power by
>more than 20% you'll get falling lamp life, and possibly difficulty
>starting.
>
>This is fine for a lashup test, and might even be ok for long term use
>in some cases if youre willing to accept the downsides.
>
>The best series impedances are inductors and capacitors, but caps must
>be calculated with care, since adding the wrong vaalue of cap can cause
>destruction to the equipment by opposing the inductive impedance
>element, resulting in destructive currents and voltages.
>

Get rid of the starter, and use a low voltage transformer to supply the
cathode heaters. Now just dim the lamp, while keeping the cathode heaters at
full power. Or, otherwise get a regular dimming ballast. I'm sure they are
available as the 18w lamps are "their" equivalent to "our" 17w lamps.

On 1 Apr 2005 06:51:59 -0800, (E-Mail Removed) wrote:


>The best series impedances are inductors and capacitors, but caps must
>be calculated with care, since adding the wrong vaalue of cap can cause
>destruction to the equipment by opposing the inductive impedance
>element, resulting in destructive currents and voltages.

Using a capacitor at line frequency is a bad idea unless the
calculations are done carefully. If the capacitive reactance is
smaller than the inductive reactance, the capacitive reactance will
always subtract from the inductive reactance, reducing the 50 Hz or 60
Hz impedance, instead of increasing it, and increasing lamp current
instead of decreasing it. Once the capacitive reactance is greater
than the inductive reactance, the capacitor will dominate, but the
inductive reactance will subtract from the capacitive reactance.
Better to use an inductor, which will always decrease current for a
simple series inductive ballast.

--
Vic Roberts
http://www.RobertsResearchInc.com
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On 1 Apr 2005 15:28:21 -0800, (E-Mail Removed) wrote:

>
>Using an added inductor is probaly the better idea if the OP doesnt
>have the calc abilties that a cap requires. If you do, the cap has the
>advantages of higher energy efficiency and lower cost. It is also
>smaller and lighter. Also, less critically, it produces a capacitive
>power factor, resulting in less pfc in a multi lamp setup, where some
>are inductive and some capacitive.

Also tends to produce terrible lamp CCF at line frequency, though it
might not if placed in series with an inductor with sufficient
inductance to ballast the lamp by itself.

--
Vic Roberts
http://www.RobertsResearchInc.com
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In article <(E-Mail Removed). com>,
(E-Mail Removed) writes:
>
> A CL ballast is not a problem crestwise, which is what the OP would be
> doing. A C only ballast is problem on 110, but not 240

No, a C only ballast very quickly wrecks a lamp on 240V 50Hz -- I've
tried it.

CL is fine -- Thorn Lighting had a patent for many years for their
lead-lag twin tube fittings getting near unity power factor.

--
Andrew Gabriel

On 2 Apr 2005 00:57:56 -0800, (E-Mail Removed) wrote:

>Victor Roberts wrote:
>> On 1 Apr 2005 15:28:21 -0800, (E-Mail Removed) wrote:
>>
>> >
>> >Using an added inductor is probaly the better idea if the OP doesnt
>> >have the calc abilties that a cap requires. If you do, the cap has
>the
>> >advantages of higher energy efficiency and lower cost. It is also
>> >smaller and lighter. Also, less critically, it produces a capacitive
>> >power factor, resulting in less pfc in a multi lamp setup, where
>some
>> >are inductive and some capacitive.
>>
>> Also tends to produce terrible lamp CCF at line frequency, though it
>> might not if placed in series with an inductor with sufficient
>> inductance to ballast the lamp by itself.
>
>A CL ballast is not a problem crestwise, which is what the OP would be
>doing. A C only ballast is problem on 110, but not 240

We're getting repetitive. Typical CL ballasts on 120-volt mains have
terrible CCF. They are used in very cheap "shoplight" fixtures and
lamp life is very short. As stated earlier, if the L in a CL ballast
is a large as it would need to be in an L-alone ballast, then I
suspect the CCF would be OK, but have not done any calculations or
measurements. I can't comment on 240-volt circuits.

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
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