Subject: Current limiting diodes...Are exit sign retrofits worth while?

From: [email-masked] (Chris Young)
Date: Thu, 7 Sep 1995 14:15:47 GMT

I recently had a meeting with a facility manager who indicated that his exit
sign system made use of a diode that cuts the current to the incandescent
bulb in half. Obviously this will impact on the cost effectiveness of L.E.D
Exit sign retrofit kits and when compared to Compact fluorescent kits the
retrofit becomes more expensive to operate.
My understanding is these diodes have been in use since the 1950's. 
These diodes apparently extend bulb life to 50,000 hrs.
While I have not checked the light output I assume it is also cut in half.
But, since the exit signs contain two 25 watt bulbs the light output
should be acceptable.

If anyone has installed Exit sign retrofit kits and the payback has not
been achieved this may be a factor to consider. 

I would appreciate in information you may have on this topic and exit
sign lighting sources.

Thanks,
Chris

---

From: [email-masked] (Loren Wilton)
Date: 8 Sep 1995 09:53:30 GMT

In article <[email-masked]>,
>But, since the exit signs contain two 25 watt bulbs the light output

Incandescent lamp life is proportional to the inverse 13th power of the
applied voltage.  This factor must be derated because lamp life is shorter
on DC or pulsed DC, which is what you have here.  If these are halogen bulbs
(unlikely in an exit sign) there would be an even steeper derating curve,
since halogens like to run within 15% or so of rated voltage for the halogen
cycle to work correctly.

Incandescent output is proportional to the 3.5 power of applied voltage.

Thus, running the lamps on half wave will make them last much longer, and
also make much less light.  They will in fact be less energy efficient in
terms of lumens/watt than running them at full voltage.  But they will last
a heck of a lot longer (and run cooler), and that is important for exit 
light service.

This trick has really only been available since the middle 1960s in any
practical form.  The only solid state diodes avaiable before the 1960s 
were seleniums, and they had a raft of problms (not the least of which
were size and voltage drop) that made them impractical for this service.
In the 1960s "top hat" germainium and later silicon diodes came along,
whcih gave the first practical diodes for this service.

Get a copy of the Chicago Minature Lamp Catalog.  It can be had for free,
and has lots of useful lamp design information in the front matter.

You are aware that companies like IDE make LED incandescent replacements,
aren;t you?  They are socketed and have the appropriate dropping resistors
or whatever; you simply stick them in the socket in place of the dead bulb.
You could likely choose these in green or yellow or a combination and
remarket them with suitable packaging.

	Loren