Date: Fri, 6 Jun 2003 10:19:44 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: So... what is the answer for these poor lights?
In-Reply-To: <f0510030fbb06ca97d05e@[203.167.170.90]>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 07:07 PM 6/6/2003, Andrew Grebneff wrote:
>NonQH lights give a weak yellow beam,
[compared to halogen lights]
> due to having low-resistance
>filaments, poor reflectors and drude lens pattern and finish.
Can we settle on "because the halogen lights give more visible light
output for the same power consumed?" Each light will have the same
filament resistance at working voltage because that is what determines the
power consumed. The non-halogen lights might even have a *higher*
resistance when cold, because they have less temperature rise to get to
their normal working point. Reflectors and such aren't part of the light
unless they're sealed-beam; and if they are the quality is still
independent of whether they're halogens.
> QHs
>burn much hotter, giving a brighter whiter light
Right. Brighter matters, whiter is mostly aesthetics...of course the
actual output is the same, but the halogen lamps shift more of it into the
visible range. Well over half is still infrared.
>A QH (quartz-halogen) bulb is filled with a halide gas mix, which
>reduces sublimation/oxidation
Actually (just learned this recently myself), it's the high-pressure gas in
the bulb capsule that reduces the sublimation (there's no oxidation, any
few molecules of free oxygen get snapped up the first time the bulb is lit;
some bulbs contain a highly reactive "getter" to scavenge oxygen, as was
done with vacuum valves/tubes). The trace amount of halogen -- usually
bromine -- is necessary because in order to safely attain high pressures
the bulb capsule must be small, and tungsten from the filament rapidly
blackens the small capsule. The halogen cycle takes up the deposited
tungsten and redeposits it on the filament as metallic tungsten -- however
unfortunately it doesn't go back to the hottest parts which is where it's
needed, but rather the coolest parts. It's no longer darkening the bulb,
but it isn't helping the filament life.
> of the filament and allows a
>higher-resistance filament to be used, which glows more intensely.
Same resistance (at working temp), but less...hmmm, less effective
radiating surface I reckon, keeps the temperature up. Undoubtedly means a
physically smaller filament, which would also have smaller conductive
losses through its ends. Trivial example -- two meters of 6mm^2 wire and
one meter of 3mm^2 will have the same resistance at equal temperatures and
dissipate the same power -- but to dissipate that power the smaller wire
will get hotter [which raises the resistance and lowers the power output,
so it will actually stabilise at some point where it's using less power but
is still hotter than the larger wire -- if you want same power output hot,
you have to start with slightly different conditions cold].
>With QHs you don't really need a relay, except that VW's light
>switches are low-quality and don't handle the power throughput well.
>A relay bypasses the switch, which then only activates the relay; the
>current to the light misses the switch entirely.
Right.
>You can't legally (or morally) use South African main headlights, as
>the low-beam cutoff is on the wrong side for LHD countries.
Right.
>You could
>get these cheaply in Germany, I bet (7" round lights, so-called
>semisealed with separate H4 bulbs), where most cars of the late 70s
>and many of the 80s would have had them as standard.
They're readily available here in many FLAPS -- Hella and Bosch brands are
common, maybe $30-50 each with included bulb. There are mail-order imports
(Asian) much cheaper but the slight info I have on their quality is not at
all encouraging.
david
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
