On 01/03/2019 16:30, whisky-dave wrote:
On Friday, 1 March 2019 16:02:52 UTC, NY wrote:
"whisky-dave" wrote in message
...

Dimmable fluorescent fittings have been around since the early 1960s.
Nothing new there.

How are fluorescents dimmed?

I waqsnt; sure so looked it up.

https://www.etcconnect.com/Support/A...ures-Work.aspx

I'd always thought that it was impossible,

well I knew it was possible but a bit more difficult than just replacing a light switch with a dimmer switch.

It is basically replacing the light switch with a dimmer (suitable for
inductive loads), but you also need to add a separate pair of isolated
supplies to keep the filaments hot enough to continue operating in
thermionic emission mode when the tube current is too low to do it just
by itself. This will also enable the tube to strike without a starter.

Indeed, I made just such a light as a teenager.

Commercially, Transtar was a well known manufacturer of dimming magnetic
ballasts (and very high quality non-dimming ballasts).

and
was surprised when the fluorescents in my school lecture theatre could be
dimmed. They were probably installed in the early to mid 70s, which gives an
idea of what technology was available then.

One issue with banks of dimming fluorescents is all the tubes had to be
replaced together, otherwise newer and older tubes are significantly
different light output, and different makes can be different too.

Also, you get colour shift and a significant drop in CRI when dimming
fluorescents, as the ratio of light output from the different mercury
line changes, which upsets the colour balance from the arc and the
phosphor (tend to lose more green and end up with purple from the over
balance of red and blue).

Nowadays with microcontroller based electronic ballasts, adding dimming
is trivial, although it's not done by using triac dimmers. 0-10V, DALI,
switch-dim, or photocell to generate constant lux level are common, and
many electronic ballasts support multiple of these.

Indeed, many electronic ballasts support multiple different tubes that
run at different currents, and having identified the tube in use from
the electrical characteristics, effectively use the internal dimmer
logic to limit the current to the max allowed for the identified tube.

--
Andrew