Re: TDC DC

[P3D John Ohrt <johrt@xxxxxxx>]

P3D Charlie Hotchkiss wrote:
> While I was in there, I added a rectifier to feed direct current to the
> bulbs.  I had the theory that that might increase bulb life.
> Now months have passed, the projector works very well and the same set of
> bulbs is still going.  I don't think they were new and they have seen
> heavy and hard use.  I'm wondering if anyone  else has done this
> conversion and kept better track of the results than I have.

Having had the priviledge of working with precision calibrations
depending on tungsten bulbs,  the following measures are employed to
lengthen life and insure tight ( <1% ) control of illumination.

1. Use DC. (you guessed that!).  This reduces flicker if you use a full
wave rectification system on AC.  Of course, real DC is better!  This
improves life.

2. Use a capacitor in series with the bulb, or other source of slow turn
on to avoid stressing the filament at turn on.  A 30 second time
constant is fine.  This measure prolongs bulb life by avoiding the turn
on surge.  Don't forget to surge limit the capacitor by current limiting
in the power supply or an external series dropping resistor.  (At turn
on, the capacitor is essentially a short circuit, some power supplies
without current limiting will blow the fuse, the safer and more
preferrable form of current limiting compared to frying the power
supply, house wiring, etc.)

3. Alternate the polarity with each power on.  This improves life.

4. Regulate the current or voltage based on sensing at the bulb,
preferably the sense wiring is soldered to the bulb.  Make sure that the
controlled current or voltage is at or just slightly under the bulb's
rating.  This is both a stability and long life improvement.

5. Mount a thermistor/diode on the bulb and wait until it indicates the
bulb temperature is stable.

6. For the paranoid, mount another thermistor on the heat sink/ bulb
base / chassis near the bulb / convenient location whose temperature is
predominately due to the waste heat from the bulb.  Again, wait until it
is stable.

Gotchas.

Some of the above may be overkill for projectors, but it will give you
an idea of measures that make life and quality better.

If using two bulbs in two sources, ensure the bulbs are from the same
batch. Check the illumination with a light meter when the bulb is
powered by a controlled power supply.  If they don't match, keep
selecting bulbs until they do.

Just remember that what kills light bulbs quickly is:

1. The turn on transient.
2. Over rating operation.
3. Voltage/current fluctuations.

Basically, the life of a bulb (if I remember correctly!) is the inverse
of the 5th power of voltage/current.  As you heat a tungsten filament,
the resistance increases, so at turn on there is a surge because the
resistance is lower.  The same thing occurs at any positive going
fluctuation.  

You do want to operate the bulb at or close to the reccommended ratings
if you wish to achieve the color temperatures for which the projector is
compensated.

The above guidlines are also applicable to halogen bulbs.  Note that
halogen bulbs degrade quickly if you operate them below the rated
voltage/current, defeating the purpose of buying a halogen bulb. 
However convenient dimmers are, or when they are necessary to operation,
keep the low illumination levels as short as possible and always allow
the bulb to come up to full temperature before shutting it off. 
Usually, five to ten minutes is all that is required.  It is the
temperature at shut down that is really critical.  

The halogen long life depends on the assumption that the bulb
temperature will be high and that the filament will cool quickly so that
the filament material adherring to the bulb will migrate back to the
filament, thus prolonging life.  The purpose of the halogen is to
facilitate this migration.


Regards,
--
John Ohrt,  Regina, SK, Canada
johrt@xxxxxxx


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