Dave Plowman (News) wrote:
In article ,
Andrew Gabriel wrote:
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.

In a similar situation where I wanted to heat a panel, I
epoxied a chain of resistors on the back. Each one is quite
low power. You will have to be careful of water ingress and
electrolosis thought.

I'm now considering using fibreglass sheet as used on PCBs with nichrome
wire in parallel runs secured by holes at either end of the run, and the
whole lot covered and secured by fibreglass resin. Any obvious problems
with this approach? I have spare glasses so can experiment.

None, as long as you don't let the temperature get too hot.

An old acquaintance of mine was a VERY junior engineer during the
commissioning of a large fiberglass radar dish that had a petal of
nichrome inside it to de-ice it in winter..on being ordered to 'show how
the de-icer works' he duly switched it on, and, receiving no further
instructions, left it on. Only the smell of burning and the brown flower
petal shaped pattern emerging from the £100k dish eventually alerted them..

...well fortunately its polar pattern was unaffected, so they toned down
the heater voltage, repainted it and filled the cracks, and of course he
took the blame for the whole design cockup. We both left the company at
similar times. My crime had been to criticize the 'wire girl' who was
supposed to wire up prototype board for us, but who usually spent her
time knitting, and sunbathing on the roof where we tested the dishes. I
learnt after my resignation that she was the section heads mistress.

This has very little to do with car mirrors I am afraid, but it might
lighten up a dull Monday.