Car door mirror heating pads

[Dave Plowman (News)]

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?

[Colin Wilson]

Many wing mirrors are used across a range of other vehicles - sometimes for other manufacturers... perhaps its a fairly standard bit of kit ?

(IIRC some Porsches use bog standard VW wing mirrors)

[Andrew Gabriel]

Oh, what a shame. After some stoned youths came past and kicked my wing mirror off, the heater pad was the only part still intact. I didn't try taking all the broken glass off it though -- it was chucked out. I did go along to the court hearings though...

[Dave Plowman (News)]

It's possible, but I'd guess someone would have found this out already if so. The option is obviously to get a vaguely similar glass and remove the element. Checking on some prices for cars which have both, the heated ones seem to be about 10 quid more.

[The Natural Philosopher]

I am very surprised that they are NLA. Scrapyard? You could experiment with e.g. bits of nichrome wire epoxied to that back.

[Dave Plowman (News)]

Likely to be fooked too.

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.

[Andrew Gabriel]

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.

[Dave Plowman (News)]

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.

[The Natural Philosopher]

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.

[Matt]

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.

[manatbandq]

I thgink you'll find it's more than that when you include the photo-plotting and other tooling costs. If you can find one that is =A325 then I'd dearly like to know.

I would use a piece of thermally conductive pad such as silpad but it ain't cheap (Farnell sell it).=20

MBQ

[Matt]

It's been a while since I knocked some pcbs up and my "local" supplier plus an alternative have since gone to the wall. pcbpool.com who I've used a few times in the recent past are 39 euros on 8 day delivery for single sided eurocards including vat and tooling - 39 euros = 26 quid Postage is a couple of quid extra though.

Doublesided one off price is 49 Euros = 33 quid

I remember buying some about 10 years ago rather than using messy heat sink compound on a processor heatsink. A piece slightly larger than the size of an A4 sheet of paper was about 15 quid which I though a bit expensive then - i just checked and it's now 43 quid for a 300mm x

300mm sheet! The problem with using this in the heated mirror application is it needs to be clamped which might not be too easy to achieve. I reckon an epoxy adhesive would work much better.