Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Mon Jul 18, 2016 2:37 am
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
A couple of GIF animations to illustrate operation of the system including mirror operating voltages (yellow) and total current draw (green).

Full power mode
  • Only the right hand switch contact moves
    Heaters in parallel operating at 12V
    Switch LED bright
    Total current consumption 4.00A indicated
    (4.003A including switch LED current)
Image

Timer mode
  • Only the left hand switch contact moves
    Timer circuit energised, relay PW LED lights
    Heaters in parallel operating at 12V
    Switch LED bright
    Total current consumption 4.01A indicated
    (4.008A comprising 0.003A switch LED current plus timer circuit current of 0.005A)

    At 90 second: relay coil energised and relay contacts change over
    Relay UP LED lights.
    Heaters in series operating at 6V
    Switch LED dim
    Total current consumption 1.08A indicated
    (1.078A comprising 0.003A switch LED current plus relay current of 0.075A)
    System remains in this state until switch turned off, when timer immediately resets
Image
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Mon Aug 01, 2016 11:14 pm
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
First failure: the timer relay in prototype 1 died during a wet 2-hour trip requiring low power operation for much of the journey: next trip out the timer wouldn't play. A new relay sorted the problem.

After a bit of head scratching after finding no faults in the wiring I think the most likely reason is the alternator peak voltage of about 14.5V being too much for the relay: the operating voltage should be within 10% of nominal 12V. Oops.

Fix would appear easy enough: remember to give the timer relay a rest from time to time. Yeah. Right. :roll:

A better fix is to add a voltage regulator for the timer relay using one of these -

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With the two capacitors required to operate the L7812 as a voltage regulator (one of a number of possible functions) a thin strip of Veroboard is the logical way to stick things together.

Extending the strip to include the switch LED ballast resistor then lends itself to adding adjustment in the form of a preset potentiometer with its own ballast to prevent track burn at maximum LED current.

Veroboard (2.54mm pitch, 0.1mm hole) sub-assembly layout using a Molex 5-pin 2.54mm header for a mating wiring plug -

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Board assembly and plug still are small enough to pass through the switch securing nut (19mm internal diameter).

Theoretical diagram -

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Physical diagram of complete system (click on image for larger view) -

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Components
A.thermal switch 45C....B.LED repeater lamp....C.screw connector 5-way
D.heater elements assembly....E.harness connector 4-way....F.harness internal junction
G.rotary switch....H.harness connector 2-way....J.timer relay base
K.Veroboard sub-assembly....L.line fuse 5A....M.connector
N.dash/instruments ground..........
Prototype 2 wired up and working once plugged into a pair of mirror heater assemblies, then soak tested for 8 hour at about 15V without load (but only because the 6-24V stabilised power supply can manage about 1A at most before chucking wobblies) -

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Relay survived and barely warmed up at all during the soak test: promising. :)

Having something to twiddle with a screwdriver to adjust the switch LED brightness will save an amount of messing about: a preset can be adjusted without disconnecting anything or having to get the soldering iron out to try a different ballast resistor.

Incidentally, the L7812 has some interesting features: it drops out when the supply voltage falls to its regulated output of 11.9V so guards against running a battery flat if somehow the system stays on, can withstand input up to 35V and has thermal overload protection. With up to 1A regulated output - about 12W - it's well able to cope with relay power consumption of 0.9W and b-all switch LED demand and whatever voltage the vehicle charging system chucks out.

Sat on hands now waiting for some more heater sets to arrive so that a spare pair of mirror heads can be done, i.e. with new heater elements rather than the test set which has gotten a bit crumpled and greasy already.

The spare heads will wired with thinwall PVC cable to replace prototype 1 heads wired with speaker cable, then prototype 2 can be installed and tested for real once I figure a reliable way of insulating the Veroboard assembly without obscuring the header or preset.

Meanwhile that bit of Veroboard has given me another idea ...
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Mon Dec 19, 2016 3:18 pm
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
There have been some significant changes since discovering that handlebar heaters come in two flavours: die-cut foil or carbon track -

Image

Quick test: temperature indicator on the carbon film element had gone through the range and to black again by the time the foil element had managed 35C, all in under a minute.

First thought: may be best to have thermal switches on each element instead of just one in each mirror head.

Like so -

Image

For now the wiring is kept separate for each element, for some testing -

Image
  • 1. Dual digital thermometer - ambient ('IN') and sensor ('OUT') temperatures displayed.
    2. Regulated voltage.
    3. Control board v5.3.
    4. Rotary switch.
    5. Element power indicator.
    6. Thermometer 'OUT' sensor.
This control board is a long way from the original piece small enough to pass through the switch securing nut but having a 12V regulated supply up to 1 Amp with appropriate heat sinking opens up a lot of possibilities: handy for running all sorts of gadgetry that might sweat a bit or even pop at higher voltage, the timer delay board in particular with a 5-12V operating range which came faulty anyway and needed some serious molesting just to obtain a decent time range out of it.

The control board itself is a test piece, changing almost daily as different bits arrive and I can add or tweak some aspect of its operation or layout, the ultimate aim being a compact circuit that needs little but a couple of small screwdrivers to install and adjust. For size I'm hoping a board a bit smaller than the H3Y-2 relay and base will carry everything needed plus a bit extra. Load sensing circuits for both mirrors might fit too, the origin being the crude double diode affair already on the board, lower right below the two 5 Watt bulbs used as dummy load on each output. Maybe.

Anyway, to the tests: basically, plot temperature against time for the different modes of operation, see what shakes out.

First problem encountered was the way the thermal switches operate: the assumption that 5 Amp rating equates to power handling capacity is wrong. The switches are vane type so the actual capacity is more like 0.1 Amp (100 mA) if the specified MTBF (mean time before failure) of 10000 operations is going to be achievable: 5 Amp is what the contacts will tolerate. Meanwhile the vane will warm up as switch current increases much above 200 mA ... which means that the way the switches are wired in they are responding to internal heating, not heater element temperature. Oops.

Fix is to add a relay to take the load of the heater element. A PCB mount 3 Amp relay with a freewheel diode across its solenoid coil terminals (to quench back-EMF spikes to protect voltage sensitive components on the control board) is small enough to tuck in a corner of the mirror enclosure and the bit of Veroboard that the relay is mounted on can be used for additional components as required.

The second problem was the capacity of the stabilised power supply used for testing. For no-load conditions when the control board only is being tested (200 to 600 mA consumption according to version) the supply output is steady enough (flatline 0.001 V drop at 1 Amp, to 0.1V at 2 Amp with about 5% ripple) but for full load testing something in needed that can whack out at least 5 Amp without sweating.

Fix is a 30 Amp stabilised supply for CCTV cameras and suchlike, 30 Amp being when a cooling fan comes as standard which hopefully should compensate for loading outside of its design parameters, i.e. at maximum of 14.5V. Cheap too. :)

With lots of power available the problems really started ...

The ready made heaters are a bit wild when fed enough power to run flat out: the resistance change with temperature is non-linear and erratic, no two elements having similar properties even when matched for resistance at ambient temperature of 20C. Track burn also has occurred but this may be due to handling rather than defects.

Even with a relay in each mirror head to handle switching loads the thermal switches cut-in temperature changes over time and the hysteresis is very variable, up to 15C for some switches.

These two problems result in nearly 20C difference between two mirror heads at full power and some strange switching at quarter power, both mirror head heater assemblies more often being 'off' than 'on'.

Um.

Time for a rethink maybe ...
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Mon Dec 19, 2016 7:02 pm
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
After rather a lot of testing and head scratching one conclusion is unavoidable: ready made heater elements are way too wild for what I want from them; no matter how good the temperature control at least one out of four elements can be expected to do strange things and completely ruin any attempt at achieving stable operation.

This likely isn't a problem when attached to a lump of metal as intended but when stuck to the back of a mirror glass one or more elements can become much too hot before the exterior surface of the glass reaches steady state temperature around 40C. After a couple of burn-throughs in element tracks and several incidences of bits separating despite using high temperature black silicone RTV I decided on a different tack.

The answer: home brew heater elements made using 32swg Constantan wire on mica composite sheet, ultimately as two elements per mirror head heater wired in parallel to give a nominal resistance of 5.44Ω for each heater assembly.

Template -

Image

Transferred to 0.7mm thick mica composite sheet (aka microwave guide cover) -

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Constantan wire threaded into place, glass side -

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Reverse -

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Wire used here is polymer coated simply because that's what came of finding a bargain on eBay. Useful though: makes threading a lot easier than bare wire.

Incidentally, nickel-chrome wire has been tried but (a) it's hard to use because it's a close cousin to piano wire in hardness and propensity to tie itself in knots and kink and (b) predictable change of resistance and length with temperature is large enough to cause problems with construction and temperature regulation. Constantan doesn't have such bad habits. :)

The tails are essential in order to fit the backing sheet, 0.5mm thick mica composite -

Image

Like so (dry run) -

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After some experimentation general purpose clear silicone sealant won as one of the few that actually stuck to the mica composite as well as being cheap.

Special tool for applying silicone and clamping sheets together while the silicone cures -

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The loose overlay of silicone baking sheet (taped to the underside) is to aid extraction of the cured assembly: not much (if anything) will stick to this type of sheet but it has to move enough to peel off from the cured assembly without having to bend the mica sheets. Thinner silicone baking sheets from Poundland seem to work better than thicker sheets from elsewhere.

First, however, the mirror glasses from Land Rover mirrors have to be prepared. When dismantling a mirror head the rubber seal peels off more easily when warm; likewise the glue holding the glass top and bottom cuts through more readily when warmed up.

Image

The plastic film on the back of the mirror glass must be removed and glue remains on the mirror shell should be cleaned off.

Image

After cleaning the back of the glass (acetone, finish with dry cloth) three strips of 50mm wide self-adhesive aluminium foil tape can be stuck on; accurate butt jointing isn't necessary because the main purpose of the foil is as a sacrifical layer to ensure that the glass and heater element can be separated if either needs replacing. If anything small gaps between strips of foil can be an advantage in dispelling air bubbles under the foil. The foil also spreads the heat a little but the effect is marginal.

Image

This is followed by a cross-laid layer of self-adhesive drywall joint tape -

Image

This tape to provide a cushion for the Constantan wire on the face of the heater element, at the same time creating an inter-layer of silicone sealant, the two together intended to fully secure and protect the wires.

Things now can be a bit messy ...

After a generous lathering of silicone sealant on the backing sheet has been smoothed out to an even layer about 1.5mm thick the the two sheets can be put together ready for clamping. The tails need to be hooked over the corners before clamping the assembly together -

Image

After a few hours of the assembly being clamped together this is what comes out once the baking sheet peels away from the assembly (difficult at first but soon becoming easier as the baking sheet lifts off from the Melamine surface) -

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Much of the silicone will not have cured yet which is ideal for the next stage but it's also an open invitation to stick to anything that comes too close, like a shirt sleeve ... :oops:
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Tue Dec 20, 2016 1:37 am
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
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 -

Image

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).

Image

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.

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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.

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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.

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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 -

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Axial 10kΩ thermistor prepared and soldered to the strip board then secured to the mica backing sheet with a blob of 705 silicone -

Image

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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 -

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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 -

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Wires added for the heater elements (red, white), thermal switches wired in series (brown) and thermistor (grey) -

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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 -

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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.

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Trial fit of heated glass assembly in a mirror shell -

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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.
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Tue Dec 20, 2016 4:04 am
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
With the heater assemblies completed the mirror head shells need preparing for some electronics.

Alteration of the upper ball seating retainer: the retainer can be pulled out using pliers after ensuring that the end of the ball seating socket cap screw is clear of the retainer -

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Strip board cut and shaped to lie on the retainer without overlapping the edges, conductor strips running across the width of the shaped board.

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A slack fit is good enough -

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Retainer drilled through for wiring, hole aligned with the upper ball seating when lower plate fitted -

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Strip board drilled through at symmetrical points each side, 3 rows in from the longest edge, 2 rows in from the short edges -

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Retainer drilled through using strip board as template, holes broached out for M3 stand-offs and screws -

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Strip board and retainer assembled -

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Trial fit in the shell -

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The assembly is held in place by the locating ribs of the plastic retainer engaging the the upper ball seating. When the mirror assembly is installed and the ball seating socket screw is tightened the end of the screw locates in a hole in the retainer, thus preventing movement of the temperature control board.

Since taking this photograph it has transpired that the stand-offs have to be the other way up on the board so that the screw heads face the upper ball seating because there's no room for studs and nuts when the lower plate is fitted. The stand-offs also have to be taller to make installation wiring easier but at this stage getting the electronics working is the main event.

To be continued ...
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: Mirror heaters, in development

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Post by VDUB384 » Club admin » Tue Dec 20, 2016 9:30 am
Joined: Wed Sep 12, 2012 9:01 am
Posts: 1400
Good luck with your heated mirror project I've never had them nor I have I really needed them.
Dave :thumb:
Whilst good maintainece is the best prevention"If its not broken don't fix it."
Image

Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Tue Dec 20, 2016 12:54 pm
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
Heated mirrors came about from getting used to them when I had a Volvo barge and the fact that Land Rover mirrors on my CF2 are just about useless in wet weather and can be dangerous, e.g. when changing lanes in Motorway traffic.

I don't remember having this problem with Wingard type mirrors on my previous CFs, maybe because they wobble enough when the van is moving to shake off water droplets, something I noticed particularly on my first CF with sliding cab doors and later with my last CF350 which had longer mirror arms to clear the Hawson body sides. There wasn't so much traffic then though, or so many numpty drivers oblivious to everything except their mobile phones ...

I don't remember having wet weather problems with Vitaloni mirrors either (standard on CF2 and later Facelift models) but they were crap even when new which I found to my cost when trying them as replacements for tired Wingard type in the 1980s. They develop a habit of falling down or even falling off because the ball socket and clamp isn't robust enough for normal adjustment including swivelling the heads in to avoid damage by passing traffic and certainly is far too weak to survive the occasional knock and bump that other mirrors can survive.

Hence Land Rover mirrors and arms as a better option - readily available and consistently cheaper than Wingard and Vitaloni mirrors - but there's something about fitting them to my CF2 that makes them so bad in icy, misty and moderately wet conditions that heaters have become essential especially in heavy traffic when it's impossible to stop to clean the mirrors.

Developing the system past simple switching like the first version reliant on an unreliable Facelift/CF2 rocker switch, i.e. throwing electronic components about until I'm happy with the result, is something I enjoy especially with so much that is available for pennies these days. It's also something that has kept me amused while I've been recovering from being ill. :)

Creating this blog may help others too: heater elements can be made for any size and shape of mirror using relatively cheap materials and ordinary tools, the main parameters being wire diameter and length per element to achieve about 1kW/metre at 14.5V (about 30 Watt for 8x5 Land Rover mirrors). Wiring and switching then is a matter of choice according to circumstances, from series connection with simple on/off switch to as complicated as one likes.
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

Re: Mirror heaters, in development

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Post by VDUB384 » Club admin » Wed Dec 21, 2016 8:29 am
Joined: Wed Sep 12, 2012 9:01 am
Posts: 1400
I've never had any problems with the mirrors on my cf mind you the mirrors on mine are the old type they used on lorries and like you say they get that much vibration water retention doesn't occur.
Dave
Whilst good maintainece is the best prevention"If its not broken don't fix it."
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Re: Mirror heaters, in development

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Post by Phil Bradshaw » Club admin » Mon Jan 09, 2017 5:58 pm
Joined: Sun Sep 02, 2012 7:15 pm
Posts: 3825
The temperature control board for inside each mirror head electronically is relatively simple: a common collector pair of BC548 transistors driving a 12V relay and controlled by a thermistor connected in parallel with an adjustment potentiometer. Additions to the basic board are 3 indicator LEDs and provision for connecting the 6-LED amber repeater on the outside of each head.

Circuit schematic -

Image

Screw connections for incoming wiring -

1: Common ground for control circuit and repeater lamp.
2: 12V regulated supply positive.
3: Repeater lamp supply from corresponding turn signal circuit.
4: Heater supply positive.
5: Heater supply negative.

Circuit as a schematic for the 2.54mm pitch strip board within the mirror head with KF2510 male connectors for the thermal switches and thermistor (5-way, 1 way unused), remote adjustment potentiometer (3-way), LED repeater lamp (2-way) and heater elements (2-way). Connector and screw terminal colours on the schematic indicate colours of corresponding wiring to the board.

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'See through' schematic showing board tracks and cut-throughs -

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This arrangement allows fine adjustment of operating temperature - about one turn of the potentiometer screw for 2°C change in steady state temperature on the mirror glass surface in still air conditions at 18C ambient - and has a natural hysteresis of ±1.5°C in like conditions in full power mode. Over-heat conditions are catered for by the two thermal switches connected in series in the regulated 12V input to the board, these switches being 45C nominal at present.

Indicator LED functions -

Yellow - 12V regulated input on.
Red - Heater on.
Blue - Heater off.

Normal operation: yellow + red during warm-up, changing to yellow + blue alternating with yellow + red when regulated temperature reached and maintained.

Failure of 12V supply and heater supply: no indicators lit.

Failure of 12V supply to board: blue only, no heating.

Failure of heater supply to board: yellow only, no heating.

Overheat condition, full power mode: blue only until both thermal switches closed and system returns to normal operation.

Overheat condition, low power mode, overheat here meaning reaching regulated temperature when left switched on unnecessarily, e.g. in dry conditions: yellow only or yellow + blue depending on which heater reaches its steady state regulated temperature first. The corresponding effect on the switch LED is alternating between dim (low power) and off or between dim and bright; either can be selected by swapping the mirror wiring connections at the control unit. My choice is alternating between dim and off rather than dim and bright because the latter appears to indicate the system cycling between low and full power modes.

To be continued ...
  • What is real is not the external form but the idea, the essence of things. Constantin Brâncuși

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