Once the position of the wire tails is checked using a piece of strip board the heater element can be stuck to the back of the prepared mirror glass (clear silicone sealant again, applied to the drywall tape and smoothed out to about 2mm total thickness to the aluminium foil) and clamped in position using spring clips -
Excess sealant between the layers can be squeezed out with the fingers to produce a tolerably smooth surface on the back. The assembly now needs at least a day to cure before finishing.
When the sealant has cured sufficiently - firm enough around the edges to be cut and sanded - the strip board can be cemented into place using 705 clear silicone RTV (very mobile, good grab and quick cure as a thin film, stable to 200C when cured).
The wire tails cut back and soldered to the strip board once the polymer coating is stripped off (burn'n'scrape) followed by the heater element profiled to the mirror glass using scissors and a foam-backed sanding block.
A piece of foil backed insulation cut from a damaged reflective windscreen sunshade is useful for temperature control: without it hysteresis (difference between cut-out and cut-in temperatures) increases.
Apertures are required for the strip board and a thermal switch for each element, the positions of the latter being the 'top' of the assembly dictated by wiring considerations.
The insulation will require trimming at a later stage but no adhesive is needed to hold it in position so it can be put to one side once prepared.
Silicone sleeves for the thermal switches bonded to the mica backing sheet using 705 silicone; the sleeve is clamped to the assembly with the switch inside so that the switch can slide out as required once the silicone has cured -
Axial 10kΩ thermistor prepared and soldered to the strip board then secured to the mica backing sheet with a blob of 705 silicone -
Bead type thermistor will work just as well as an axial type but the axial type is physically more robust. Hysteresis of the two types is different (bead type response tends to be faster) but the effect on steady state temperature of the heated mirrors in free air conditions is marginal.
Finishing details prior to installing temperature control board.
Vinyl tape added to shiny side of insulation -
The tape is more to prevent tearing of the insulation rather than provide electrical insulation; the shiny side doesn't conduct electricity.
Thermal switch wires trimmed and soldered to strip board so that the wires serve to keep the switches roughly central in their sleeves -
Wires added for the heater elements (red, white), thermal switches wired in series (brown) and thermistor (grey) -
Thinwall PVC sheath stranded cable used, 0.5mm² for the heater elements and 0.35mm² for the rest.
Insulation refitted and marked for trimming to clear the rim of the mirror shell -
3mm diameter Vidaflex heat resistant tubing to protect wires close to the heater element and KF2510 female connectors added, 2-way (heater elements) and 5-way (the rest) with one way in the connector left spare, this being dictated by arrangement of the temperature control board components.
Trial fit of heated glass assembly in a mirror shell -
So far so good: even without the thermistor and the rest of the temperature control circuit that goes with it each heated mirror when connected in series with the other will perform like any other heated mirror, this being the first of many tests for this arrangement, in this case to determine temperature differential between two elements (buggerall given that difference in resistance can be kept down to 0.01Ω and inconsequential compared to other variables in the system) and a working upper temperature limit for selection of appropriate thermal switches (45C for now with scope for higher temperature if nothing wilts in service).
The downside of permanent series operation is the slowness in initial warm-up (8-10 minute when ambient temperature is below 10C) and poor performance in sustained wet (and even not so wet) conditions. Hence the quest for something with a timed warm-up at full power for rapid warm-up before switching to low power mode with provision for over-ride at full power mode as required.