Re: DIY light source for 10X10" enlarger.

"Richard Knoppow" <dickburk@[EMAIL PROTECTED]
> wrote.

> I have no idea of what differences there are in the balasts [for cold 
> light enlarger heads] other than to withstand intermittent operation.

Fluorescent lamps are one variety of gas-discharge lamp.
They share their major electrical characteristic - negative
resistance - with HID/Mercury/Sodium lamps.  Negative
resistance means that as the lamp current increases the
lamp voltage decreases and thus the current, unless
limited, quickly rises to near infinity (i.e., a lightning
bolt discharge).  The ballast's purpose is to limit the
lamp current.

Graphic arts systems use several types of specialty
ballasts.  Many use a combination of these features:

 o Keep light output constant:

    Regulating ballasts wherein the ballast uses a saturating
    core reactor and works, with the power factor capacitor and
    the lamp, as a ferroresonant regulator.

 o Quickly come up to full brightness:

    Ballasts having a low open circuit voltage
    and a low reactance.  This increases the current through
    a cold lamp so that it warms faster.  The lamps are
    low voltage/low pressure designs with heavy duty electrodes
    that can withstand the higher current without erosion.

 o Keep the lamp warm so it comes up to full brightness
   instantly:

    Multi-level ballasts wherein the lamp idles at low power
    in a keep warm state with the shutter closed and then
    switches to high power with an open shutter to make the
    exposure.  Fan speed is reduced when the lamp is idling
    at low power to keep the lamp at a hot operating temperature.

    This feature is usually combined with the ability to run
    the lamp at several power levels.

 o Extend lamp life, especially where lamps cost many hundreds
   of dollars:

    Very low crest factor (Ipeak/Irms) ballasts than keep the
    electrodes from wearing out. Electrode erosion is roughly
    pro****tional to the cube of the current.

I don't know that the Aristo equipment is sophisticated enough
to use any of the above technologies.

Crest factor is main difference in the quality of a fluorescent
ballast.  The ballasts in $10 work luminaries have a high
crest factor, leading to short lamp life and low light output
for power in.  A high quality low crest factor ballast first
raises the voltage with an auto-transformer and then limits
the current with either inductive or capacitive reactance.
Often the some of the ballast reactance is supplied by
air-gapping or shunting the transformer to increase the leakage
inductance; often a capacitor is used for ballast reactance
and leakage inductance is used for power factor correction.
Additionally, the efficiency of the ballasts of high quality
ballasts is higher: thicker copper wire in the reactor/transformer
windings to keep resistive losses low, the prescience of power
factor correction capacitors and the use of low ESR capacitors.

For a garage work bench lamp all of this is of little concern:
the electricity savings of an efficient ballast will never
pay for its higher initial cost.

Aristo lamps are low current, high voltage, low pressure
mercury with a phosphor coating - sort of a hybrid between
a neon and a fluorescent lamp.  The ballast is specific
to such a lamp and is, I am sure, a high quality low-crest
factor design.

If you are designing a cold light head using off-the-shelf
fluorescents you should look for the highest quality ballasts.
Long lamp life equates with stable light output.

The best economical high performance ballast systems, designed
for office/factory use, aren't applicable to cold-light heads.
Office lighting systems usually supply a high (and usually
bizarre - 347 volts anyone?) voltage from a central
step-up transformer with individual inductive ballasts
at each lamp.  If power factor capacitors are used they
are often central with the transformer.

Much more than anyone in r.p.d., including myself, would
(or should) ever want to know.

-- 
Nicholas O. Lindan, Cleveland, Ohio
Darkroom Automation: F-Stop Timers, Enlarging Meters
http://www.darkroomautomation.com/index2.htm
n o lindan at ix dot netcom dot com