On Jul 3, 9:31*pm, Andy Champ wrote:
Bill Wright wrote:

Some time ago I reported in this hallowed forum my disapointment at the
apparent inefficiency of some mains voltage halogen downlights I'd fitted in
the dining room. Based on the 12V halogens I have used elsewhere I expected
the room would need eight 25 watt lamps. In fact it needed eight 50W ones.
400W to light a medium sized room!

partly because theyre (mostly) used in downlighters.


I'm now intending to put new lights in the kitchen. Today I lit a 12V 20W
lamp and a 240V 25W lamp side by side. Both lamps were the 36deg type. *The
12V lamp was much brighter. I was running it on 12.00VDC, but the current
drawn was 1.7A which equates to 20.4W, so I guess the brightness would be
the same on a nominal 12V AC supply.

performance on 12v ac is identical to on 12v dc


The 12V lamp had a neutral colour,
whereas the mains lamp was rather warm. I wonder if this accounts for the
greater efficiency of the 12V lamp,

yes, mains ones generally run at lower filament temp

and means that the mains lamp, being
relatively under-run, will last longer.

quite the contrary. Mains have very thin filaments, so they need to
underrun them to get them to last adequately.


I reduced the voltage on the 12V lamp until it had the same apparent
brightness as the mains lamp. At 8.7V both lamps had the same brightness and
colour temperature, as near as I could tell.

Can anyone cast any light (geddit?) on this? Are 12V lamps really so much
more efficient? If so, why?

yes, thicker filament enables higher operating temp for same life


The guys over on uk d-iyy will be able to pick this one up. *This is a
frequent topic.

more info on halogens, downlighting, dimming, and bulb life he
http://www.wiki.diyfaq.org.uk/index....egory:Lighting

Ignore any references to angle grinders.

Andy


NT