Re: Projection lamps

[P3D Michael Kersenbrock <michaelk@xxxxxxxxxxx>]

> I don't know about tungsten leaving the filament more with DC than AC, but
> halogens are designed to reduce the loss of tungsten and that is part of why
> they are much brighter. I have been told that they are a perfect choice if

It wasn't a matter of tungsten leaving the filament, but moving from one end
of it toward the other.  Metal migration is one of those funny things.  Ever
seen "wire wrapped" circuits?  They're supposed to be very reliable and were
designed to be able to last at least 40 years (assuming one is not using 
teflon insulation which cold-flows).  One of the reasons for reliablity of
connections made by mechanical non-soldered connections is that at the point
of contact, metal migrates across the boundary and makes the joint "solid"
over time. 

But like I said, current day bulbs may die of other things first.  :-)

> one is planning to run them on DC. Ideally, a DC source for these lights
> should do more than rectify the AC current. It needs some level of filtering
> to reduce the ripple to a reasonably flat current.

Quite right.  With only rectification, absolutely nothing was gained, and if
it was only half-wave rectified, any remaining flickering not thermal-mass "filtered"
would  be dramatically increased (lower duty cycle) and of lower frequency as 
compared to an AC source (going from 120 Hz to 60Hz would probably be a more critical
change than say 20Khz to 10Khz).


Mike K.


------------------------------