In article <> (M. G. Devour) writes:


>   'Meantime...  Is there technology for dimming flourescent tubes? What? Where? 
>   How much? How is it done? Is it possible to slow start them?

>   Thanks in advance.

Here is some info I pulled off of one of these groups on diming fluorescents:

--- sam

Dimming fluorescents 1:

(From: Peter Miller (
I've tinkered a bit... the trick is to keep the filaments at the ends of
the tubes warm. You will NOT be able to dim down to zero - probably about
25% at best. 

Here is a possible circuit:

                         +--+  Choke
    AC Live/hot o--------|  |--^^^^^^^^--+--------------+
                         +--+            |              |
                                         |              |
                   +---------------------+              |
                   |                     |              |
                   +-+   +---------------+  +-------+   |
                      )|(                   |       |   |
        Filament      )|(                   |      -|---|-
      Transformer     )| +------------------+     |  ^^^  | Tube
      2 * 6.3V 2A     )|                          |       |
                      )|                          |       |
                      )| +------------------+     |  ^^^  |
                      )|(                   |      -|---|-
                      )|(                   |       |   |  
                   +-+   +---------------+  +--------   |
                   |                     |              |
AC Neutral/cold o--+---------------------+--------------+

The lamp must be in a earthed / grounded reflector fitting. The metal
end caps of the tube must be connected to the reflector. The dimmer MUST
be a 'hard fired' dimmer capable of operating an inductive load. The
choke is a standard type for the tube in use. Play with an inexpensive
everyday tube before using expensive aquaria ones. with a 240V supply a
4 ft 40W tube operates ok. The main difficulty with this circuit is
in getting the tube to start - starting is greatly helped by a grounded
reflector fitting and connecting the metal end caps of the tube to the 
reflector (Dont ask - it works!). The transformer can be a standard 
valve filament transformer - use a separate transformer for each end
of the tube if you are unsure of the insulation between the secondaries 
of any transformer that you buy. As the tube draws less current the 
voltage across it rises, turning up the heat in the filaments. At start
up, maximum voltage is across the lamp and so the filaments are fully
on. All dimming ballasts/chokes use some scheme to add extra heat to
the filaments at dim running. An undimmed tube draws enough current to
keep the filaments warm by itself. There is no glow starter or other
starting device in the circuit, so the lamps tend to come on smoothly
with no flickering. Shorter tubes are easier to start.  New slimline 
tubes are a real pain to start.

Dimming fluorescents 2:

(From: Bruce G Bostwick (

This applies to rapid start fixtures.

If the fixture says "RAPID START" somewhere on it or on the package it came
in, the internal schematic will be roughly as follows:

                      || +-------+---------o
AC line H o---------+ ||(   +----+            to both pins on
                     )||(  (  <-6.3VAC        one end
                     )||(   +--------------o
                     )|| +=---- 2kVAC* -------------------------+
                     )||(                                       |
                     )||(   +--------------o                    |
                     )||(  (  <-6.3VAC        to both pins on   |
AC line N o---------+ ||(   +----+            the other end     |
                      || +-------+---------o                  +-+
Most of the 48" 40-watt "shop light" type of bulbs use this.  *The high
voltage winding of the secondary is on a branch magnetic circuit that
limits the output current to the mercury discharge.  Open circuit voltage
will be in the kilovolt range, while the voltage across a lighted tube
will be somewhat less than that and _exceedingly_ non-sinusoidal.

If you're using the big bulbs (F96T12's for example) the ballast will only
have the high voltage winding and the cathodes are heated by ionic
bombardment from the mercury arc.  These take a bit longer to light up
when the power is turned on.

If you want instantaneous on/off control, I'd suggest using 4-footers and
linking up two ballasts in such a way that the cathode heaters are driven
from one which is always on, and the arc is driven from the other which is
turned on and off as you desire.  They won't last too long that way, but
it will work better for show effects.  The cathodes could be driven from
a pair of low-voltage filament transformers, but be sure to isolate them
well -- or you could use a ballast with a blown HV secondary ...

Another suggestion: Use solid-state relays to drive the ballast primaries.
These are fairly cheap and provide clean current-zero-crossing switching
even with very reactive loads (I've used them for such! ;-) and provide
a neat and rugged way to connect the lights to logic controls such as your
computer -- great for light sequencing.

Date: 01 May 1996 21:12:55 GMT

Original Subject: Re: Another dimmer Q: flourescents?

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