On Mon, 25 Sep 2006 01:23:20 +0100, "Dave Plowman (News)"
wrote:

In article ,
The Natural Philosopher wrote:
Bit O/T, but the heaters for the mirrors on both mine have failed and
are NLA - although plain glasses are still available. Anyone think of
a source for the elements only?


I am very surprised that they are NLA. Scrapyard?

Likely to be fooked too.

You could experiment with e.g. bits of nichrome wire epoxied to that
back.

Yes. The original is a sort of fibre pad with wide tracks on it - rather
like a PCB using something other than copper, and is just glued on. Seems
modern mirrors have the element built in like a heated rear window.

It can be done with copper direct onto a pcb - preferably on thin
(0.8mm) laminate.

Assume you want to raise the temperature to around 30 deg above
ambient while consuming around 2.5A Using standard tables @ 2.5A 70u
FR4 copper requires a minimum track width of around 0.7mm to avoid
exceeding thermal limits. Typical mirror heater current consumption of
2.5A @ a nominal car battery voltage means a heater element resistance
of around 5.5 ohms is required.

0.7mm copper track on 70u copper requires 16000mm of track to provide
circa 5.5 ohms. Assume a minimum 0.2mm gap between tracks. Therefore
0.9mm is needed "per track run"

Assume the mirror is 150mm wide

The mirror needs to accommodate:

16000mm/150mm = 106 tracks

106 tracks @0.9mm = 95mm minimum height - probably less than the space
that is available.

Conveniently the arrangement will fit on a standard Eurocard PCB which
could then be routed to size. Termination would probably be best with
soldered and bolted connection with bolt through lucar 0.25" tabs You
could probably knock up a layout in an hour or so with a freeware pcb
cad package. The one off cost will be about 25 quid from one of the
many online pcb manufacturers dropping to about 2 or 3 quid for 100
off quantities.

Bond the pcb copper side directly to the back of the mirror using an
appropriate adhesive. Normal slow setting epoxy would probably
suffice.


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