Date: Fri, 17 Jan 2003 10:19:36 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Rear, Side, Front,
Defroster FISH Wires... Can These Be Purchased?
In-Reply-To: <009a01c2be0a$a6021560$0100a8c0@MEDION1800>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 04:27 AM 1/17/2003, Robert Steven Fish wrote:
>Anyone seen anything like this for sale out there?? Can I build this type
>of thing from scratch with some uninsulated wire and a resistor or two??
Short answer - yes. Medium answer: you would need Nichrome or equivalent
resistance wire or conductive/resistive paint. Wire is probably a lot
cheaper -- paint is very likely easier to deal with though prolly insanely
expensive -- they get $10 or some such for a bottle with maybe half a cc of
copper paint for repairing breaks in defroster grids.
Long answer: If using Nichrome, you would need to completely bond the wire
to the glass so as to prevent any hot spots. If using paint of course that
takes care of itself. In either case you would need terminals bonded to
the glass. And you'd need to do some engineering -- starting with how much
heat you would want to apply, or could apply without endangering the
window. Honda recalled many thousands of Accord rear windows in the '80s
because they were cracking from too-powerful rear defrosters.
Once you know how much heat you want, and where, you need to design a
pattern of resistance that will provide that amount of heat in the areas
you want. That done, you must figure the lengths involved in the (probably
parallel-wired if paint and series if wire) circuit, and calculate (and
obtain) the right thickness of wire to give the proper resistance in the
designed length -- or calculate and lay out the width of the paint lines to
give the same effect. Paint will give you much more flexibility to vary
the resistance per inch at particular spots -- note how the width of the
paint stripe varies in different parts of a rear-window installation. If
you're using Nichrome, you can only join pieces mechanically with
fasteners, or by spot-welding -- hence, you will want to try to use a
single piece of wire from end-to-end, whereas with paint you can run
multiple stripes in parallel if that's desirable. In either case you have
to calculate resistance at the operating temperature, as it certainly
varies markedly in Nichrome and probably in paint as well [well, it does if
you're running the Nichrome red hot, which you won't be. Temperature
coefficients should be readily available on Google. For paint info suggest
starting with Google/Thomas Register search for conductive paint or
resistive paint, try to get tech info and application/termination notes
from mfrs. Likewise the glass mfrs for info on how abrupt a
temperature-gradient their products can handle, and what systems are
available for bonding terminals to glass -- may be possible to use ones
from junked/broken rear windows and attach with mirror-mounting adhesive, I
don't know.
You can fudge the values somewhat by using an external resistor to reduce
the power available to the heating grid, but of course then you will have a
hot resistor to deal with -- mounting, shielding etc.
Dealing with wire, it may be a problem finding Nichrome that's either thin
enough for your purposes, or not already formed in a tight helical coil for
a heating element. Fractional-kilowatt elements for heaters, toasters,
dryers etc running on house voltage are easy to get. Straight wire of
given dimension may not be.
Sounds like lots of fun if you're up for it, a nightmare if not.
:)
david
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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