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When you need to shed some light on the subject . . .
Solar-Powered Skylight
with fluoro “backup”
by Ross Tester
Why pay a lot of money for a skylight to be installed when you can
take advantage of modern technology – solar panels and ultrabright
LEDs – to achieve effectively the same thing – for $$$ less!
S
kylights are a great idea for dark
or dim rooms. I should know, I
installed two on my roof about
30 years ago to light up an internal
bathroom and kitchen.
But they’re not cheap – expect to pay
at least at least a couple of hundred
dollars each – and then about the same
for installation.
You have to cut a suitable hole in the
roof itself, then cut an appropriate size
hole in the ceiling, install a light well
or tube, fit the whole lot . . . it’s not a
real simple job and of course, there will
always be a ceiling joist or roof support
just where you want to fit it!
Here’s an alternative: an electronic
or “solar” skylight.
You simply mount a solar panel on
the outside of the roof in an appropriate
spot and use its output to drive some
of the ultrabright 20W LEDs which
we featured in our LED Floodlamp
(August 2012).
These can be located wherever you
need extra light – unlike a conventional
skylight, there’s no need for a physical
connection between the two (of course,
there’s an electrical connection but that
can be many metres, if need be).
Installation should be a lot simpler
36 Silicon Chip
(well within the capabilities of home
handymen/women) and you can position the LEDs exactly where you
want them, not where roof trusses and
ceiling joists and battens dictate they
must go.
This was the scenario presented
to us by Oatley Electronics, the same
people who provide the kit for the 10
and 20W LED floodlamps. For a little
over a hundred dollars, they supply
a large (1200 x 600mm) “First Solar”
FS-272 solar panel and four 20W LEDs.
The panel is rated at 72W; 90V opencircuit – so it’s got a “bit-o-bite!”.
No batteries, no controller
The LEDs are connected in series/
parallel and wired straight to the solar
panel. There is no battery to charge,
Four of these 20W LED arrays are supplied in the Oatley Electronics kit, along
with a 90V/72W Solar Panel (see photo above right). The LED here is shown not
too far off life size.
siliconchip.com.au
therefore there is no controller needed.
Neither is there a current-limiting
circuit required for the LEDs as the
system is basically self-regulating. But
more on this anon.
While we could see the merit in
this simple system, we thought that it
could be expanded somewhat. After
all, no-one wants four very bright LEDs
mounted on the ceiling. At minimum,
they would need some sort of diffuser
and some means of heatsinking.
Second problem we thought of was
almost a “duh” moment. The LEDs
would only be on during daylight
hours (ie, when the sun is allowing
the panel to produce power). Duh!
Reminds us of the Irish flashlight
company that went broke producing
solar-powered torches . . .
What do you do at night – you’d still
need a more-or-less traditional light, or
perhaps you would then need a power
supply to drive the LEDs?
The lightbulb moment
Then we thought “why not combine
both of these ideas and mount the LEDs
inside the light fitting?” We considered
various types of light fittings which
came with diffusers and, while most
would be quite acceptable, we finally
settled on a twin 36W fluoro “troffer”.
Why did we choose this fitting?
They’re not the prettiest ever made but
they do have a couple of big advantages
– first, they’re cheap. Second hand,
you can almost always pick them up
for next to nothing ($10 regularly on
ebay, for example) but if you’re a typical handyman/hobbyist, the chances
are you’ve got one stored away somewhere. We certainly did!
Even new, they are often sub-$30
or so.
The other advantage of a fluoro light
fitting is that there is plenty of room
to work with. We tried a couple of different variations on the mounting-theLED theme but finally settled on one
scheme which worked well for us – you
might find that another method works
better for you.
Incidentally, we did go out and buy
a brand-new fitting just to make sure
we could use it. We could!
The fluoro fitting we bought was
a “Clipsal” brand double 36W unit
(model TB236NEL) which our local
electrical wholesalers had on “special”
for less than $30 – complete with triphosphor tubes. By the way, these
fittings are referred to in the trade as
“troffers”. Just make sure when you
get one that it comes with a diffuser.
Another advantage of using a new
unit is that these days they come
completely wired with cord and 3-pin
230V plug (electricians simply install
them and plug them into roof cavitymounted mains sockets).
If yours is a relatively modern home,
the chances are your lighting is installed in the same way, so you won’t
be breaking any laws by plugging in
a new fitting. You might like to fit a
smaller (twin 18W) fitting if you think
that the large one will look out of place.
And yet another reason for using a
new fitting is that it will probably also
have an electronic ballast fitted instead
of the old-style iron-cored ballast –
that will save you a few dollars over
the years as iron-cored ballasts waste
quite a bit of energy in the form of heat.
(Hey, don’t knock it: have you looked
at YOUR power bill lately?)
So now that we had both the LED
mounting method AND the diffuser
problem solved, we went about fitting
the troffer with the LEDs.
Heatsinking
Our first thought was that the steel
case of the troffer might be adequate
as a heatsink for the LEDs – but this
thought quickly diminished as we
smelled some LEDs getting a little
upset (despite prodigious globs of
heatsink compound).
So we elected to mount the LEDs on
small pieces of thick aluminium and
firmly bond them to the cases – again,
with plenty of heatsink
The Clipsal
TB236NEL T-Bar Troffer
we purchased for this project. It’s shown here
fitted with the four LED arrays, just visible through the
Perspex diffuser. A diffuser is essential for use in domestic situations
– the light from the four LED arrays is simply too bright without one.
siliconchip.com.au
1.2m
The “First Solar”
FS-272 Solar Panel drives the four
LED arrays direct – no controller is
required. In bright sunlight, maximum
output is 72W and the four LEDs are
each rated at 20W (80W total).
compound on both the back of the LEDs
and between the aluminium and steel.
The aluminium we used was actually some offcuts of flat bar which
we happened to have on hand from a
previous job. It’s about 4mm thick and
about 30mm wide; each piece about
100mm long.
Ideally, we would have liked it to be
about 50mm or 60mm wide in order to
mount the LEDs square-on but elected
to use what we had rather than buy
more (aluminium is expensive these
days!).
To secure the LEDs to these heatsinks
we turned them through 45° and used
only two of the four mounting holes.
This secures them more than adequately, especially with a large dollop
of heatsink compound under the LED.
Each bar was drilled with four 3mm
holes – two for attaching the bar to the
troffer and the other two, countersunk
from the underside, were for attaching
the LEDs with M3 12mm countersunkhead screws, nuts and shakeproof
washers. Using countersunk-head
January 2013 37
The 20W LED arrays
are connected in
series/parallel. We
didn’t worry about a
power switch because
“normal” skylights
don’t have any means
of turning the light
on and off. While
ever there is light, the
LEDs will light up.
While ever there is
bright sunshine, the
LEDs will light up
brilliantly!
+
72W
SOLAR
PANEL
screws ensures that the maximum
metal-to-metal contact is made when
the bar is screwed onto the troffer.
Incidentally, in use the troffer case
immediately under the LEDs gets
barely warm, so it is achieving the aim
of getting rid of the heat.
The first thing we did when we
mounted the LEDs on their heatsinks
was to clearly mark the heatsink with
a marker pen + and -. As we explained
in the LED floodlight, the markings on
the LEDs themselves are quite difficult
to see (impossible in low lighting) so
you need to make sure you can’t make
a wiring error.
LED placement within
the fitting
We had a bit of a quandary here but
soon proved – by trial and error – that
it didn’t really matter too much where
we placed the LEDs within the fitting.
In the end, we made up two fluoros
with two different LED arrangements.
In one, the older fitting which we had
on hand, we placed the four LEDs
equidistant down the centre – ie, arranged between the two fluoro tubes.
This worked pretty well and had the
added advantage (at least in the fitting
we “doctored”) that we didn’t have to
remove the cover to give access to the
bitey bits.
The new troffer we bought specifically for this project was a bit more of
a challenge. It had a centre U-shaped
guard to hide the mains wiring and
placing the LEDs on top of this put
them too close to the perspex diffuser
(in fact, virtually touching it).
We figured the diffuser would either
discolour quickly or worse, melt under
the heat.
And removing the guard we didn’t
think would be politically correct!
In the end, we tried the LEDs arranged down the sides of the troffer.
38 Silicon Chip
+
–
–
+
+
4x
20W
LED
ARRAYS
Other fittings?
–
+
–
around and short to any other wiring.
Most fluoro fittings have plenty of
cutouts and tabs which can be used
as handy anchor points for cable ties.
–
This appeared to give a nice light coverage so this is what we decided on. Sure,
all four LEDs are quite visible when
alight, even through the diffuser – but
does this really matter? We think not.
Again, the pictures tell a thousand
words.
Because the LEDs get rather warm,
they need to be mounted so that they
are well away from ballasts, starters
and so on. It also looks best if they are
evenly spaced along the fitting. We
tried staggering them as well as across
from each other but the light output
didn’t seem to be affected either way.
As we mentioned earlier, the LEDs
are connected in series/parallel. In
other words, two LEDs are connected
in parallel, then those two are wired in
series with the other pair (also wired
in parallel).
The wire used must have a rating
of at least 2A (3A or more is better)
and its insulation needs to be rated at
250V or more.
If you use an old fitting without the
wiring guards of modern-day fluoros,
all low-voltage wiring needs to be kept
well separated from the mains wiring.
Use plenty of cable ties to ensure that
in the unlikely event of a cable coming loose, it would not be able to move
Of course, the LED positions we
decided on are not the only options,
nor are 36W dual fluoro lamp fittings.
We’ve already mentioned the possibility of using dual 18W fluoros but possibly bearing some further investigation
would be the use of some “oyster”
light fittings, the ones fitted with a
metal base. That would be needed to
dissipate some of the heat given off
by the LEDs.
One major difficulty here would be
that the amount of light from the LEDs
might be considered far too high – it
would be much more than the light
from the (usual) 32W round fluoro tube
they usually come with (or in some
cases, a pair of CFLs).
We haven’t tried these so can’t give
any guidance, except for the reminder
to keep the LEDs well separated from
other wiring and also to keep the
maximum distance between LEDs and
diffuser. Once again, wiring needs to be
250V rated and firmly anchored with
cable ties, etc.
Whatever you end up using, make
sure you use copious amounts of heatsink compound and, again, mount the
LEDs on individual heatsinks (thick
aluminum sheet?) which are themselves made secure to as much metal
as possible.
Mounting and connecting
the solar panel
No mounting hardware is supplied
with the solar panel but brackets etc,
are quite widely available so you can
suit yourself how you mount it. Like
all solar panels in the southern hemi-
Here’s how we mounted the LED array on the aluminium bar offcuts. Ideally, the
bar should be a little wider to allow “square on” mounting but this arrangement
works quite well. Note the large polarity markings on the aluminium – these are
to make sure that Murphy bloke doesn’t put in an appearance.
siliconchip.com.au
Here’s how we placed the four LED arrays (with heatsinks) in the quite old fluoro fitting we had on hand – obviously before
wiring and without the tubes or diffuser. The LEDs actually fit the between the two tubes – there’s not a huge amount of heat
given off in the forward direction and the tubes seem to cope quite well with it (they actually get fairly warm themselves).
sphere, it needs to be mounted on a
north-facing roof (or backyard support) with the angle above horizontal
dependent on your latitude.
There’s plenty of information on the
net about best solar panel positioning.
Unfortunately, the connectors on
the “First Solar” panel are not the
usual 4mm standard you’ll find on
the vast majority of solar panels. We
couldn’t find suitable male and female
connectors so the best option appears
to be cutting the connectors off and
soldering your wires directly to them.
Ensure that you identify which are
the positive and negative terminals –
and mark them. It won’t work if you
get it back to front!
And just be warned, the solar panel
produces a significant voltage even in
subdued light (50V or so), so you don’t
want to get across that (especially with
your soldering iron!).
Speaking of wires, it makes sense
to use a cable which has minimum
voltage drop between the panel and
the LEDs. While the total current will
only be an amp or so, small diameter
cable (ergo, higher resistance) will lose
more power than a larger diameter
cable (more copper, lower resistance).
For the same reason, keep your
cable run to the minimum possible.
Mains-rated, polarised heavy-duty
Figure-8 cable should be suitable but
again, watch polarity. If it isn’t the
traditional red/black (the best choice),
make sure you know which cable is +
and which is – (while there is no real
convention, traditionally we’ve made
the stripe negative).
Where any cable needs to pass
through a roof, ceiling, case etc, ensure
that it is adequately protected against
chafing and damage – things do move!
In use
One of the things we wanted to
compare was the light output between
the four LEDs and the two fluoros. On
a fairly sunny day (sunlight through
whispy cloud but throwing a distinct
shadow) we measured the output of
the LEDs, with diffuser fitted, as 170
lux at a distance of 2.2m.
The fluoros, same conditions, came
in at 270 lux. Obviously, the fluoros
are half as bright again as the LEDs.
But does this matter?
Arguably not – a traditional skylight
is intended to fill in “light holes” and
apart from the size of the skylight itself,
you don’t have much control over how
much sunlight is transmitted down to
the room.
It’s the same with the LED version
– because it has no batteries to store
power, you’re basically at the mercy
of the weather. You should get lots
of light on a bright sunny day; under
heavy overcast it will be less – possibly a lot less.
But we have to qualify this by saying
that even under a heavy overcast day,
with the solar panel producing only
about 50V open-circuit, we still got
some light from the LEDs.
Of course, if lack of natural light
is a problem due to the weather, you
can simply flick the light switch and
let the fluoros take over.
We found no problems running both
the fluoros and LEDs at the same time
– in fact, we didn’t even worry about
putting a switch in for the LEDs.
One final thing – from what we’ve
read, the solar panel supplied (with
Cadmium Telluride cells) is much
more forgiving when it comes to the
shading problems you’ve probably
heard about with older panels.
In fact, walking in front of the panel
(and casting a significant shadow over
it) only dropped the output by a volt
or so.
We’ve heard of many panels which
lose dramatically more than this when
even very lightly shaded.
SC
Where from, how much:
The K-328 kit from Oatley Electronics
includes the First Solar 1200 x 600mm
solar panel and four 20W LED arrays. It
retails for $109.00 inc GST but note that
due to the significant weight of the solar
panel (12kg) there is a freight charge
depending on distance.
Contact Oatley Electronics on (02) 9586
3564; email sales<at>oatleyelectronics.
com; write to them at PO Box 89 Oatley
NSW 2223, or visit their website: www.
oatleyelectronics.com
Here’s a view of the fluoro fitting with LEDs but without the diffuser in place. We found that exact placement of the
LEDs was not necessary – they performed well wherever they were placed. We’ve spaced them out to allow better heat
dissipation.
siliconchip.com.au
January 2013 39
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