Homebrew PCBs

Hi John,

Generally, gas discharge tubes cannot be operated in parallel.  They
need a high voltage to start, but settle down to a low voltage ca. 90
volts (I guess) once they are conducting.   With ordinary inductor
ballasts I have seen two 20W short tubes, each with their own starter,
wired in series with a 40W ballast.  This is for 240V.  Maybe that would
work with a high frequency electronic ballast.

I chose to use a 500 watt floodlamp from the hardware store.  The short
linear quartz halogen lamps give off a lot of near UV, which is what I
want for exposing Riston negative photoresist.

At a distance of about 45cm I have exposure times of 2 to 3 minutes.

This is a much narrower angle source of light than having a bunch of
fluorescent tubes only 10cm away, so for a given phototool and possible
distance from the phototool to the top and bottom of the Riston, I get a
much sharper image (shadow).

The only potential concern I can think of is heating of the phototool,
PCB and whatever means is used to hold them together.  I have a fan
blowing on them to keep them cool.

There's no warm-up time for a QI light.  Tubular fluorescent lamps are
likely to have some kind of warm-up time, making it difficult to predict
their light output unless they have just been running and are fully
warmed up.  High pressure mercury vapor lamps have much longer warm-up
times, and so do the compact fluorescent lamps which are now widely
used.  All these fluorescent lamps - tubular, 400W (or other wattage)
high pressure mercury vapor lamps, and compact fluorescent lamps - come
in "black light" versions which I think put out the ideal wavelength for
exposing Riston.


http://www2.dupont.com/Imaging_Materials/en_US/tech_info/datasheets/index.html

I haven't looked at all the types, but the types I have looked at are
specified to respond best to 350 to 380nm light.  This datasheet for a
400W high pressure mercury vapour fluorescent H500-BL:

  http://www.eyelighting.com/tb/Mercury/MVR/EQS-N-52-78-69917.pdf

shows most of the output at about 370nm.

However, I think their warm-up times are a problem.

QI lamps will vary their near UV output somewhat according to their
operating voltage, but unless the mains voltage varies widely, I would
be surprised if this variation was enough to cause trouble with Riston.


 - Robin             http://www.firstpr.com.au/pcb-diy/