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Solar cell powered battery charger
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Mark Fergerson
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PostPosted: Thu Aug 28, 2003 9:24 pm    Post subject: Re: Solar cell powered battery charger

Spehro Pefhany wrote:
Quote:
On Thu, 28 Aug 2003 08:07:38 -0700, the renowned Lizard Blizzard
NOSPAM@rsccd.org> wrote:


Mark Fergerson wrote:

[snip]

Next question is how many red LEDs I'll need to equal the Iout of a
small PV cell.

Lessee... A LED chip is about a square mm.


You're being too kind by more than 10:1 there. We use special LED dice
that are larger (and more expensive, and brighter) in some of our
displays so I know the die sizes.

When I said "a sh*tload" I meant it; I have lots of
assorted display board from dead VCRs and like that, many of
which contain SMT LEDs which I planned to use to approach
the equivalent surface area of a PV. Yeah, I know, lots of
persnickety hand-soldering.

OTOH it doesn't look all that good efficiency-wise. Guess
I'll bite the bullet and go with a Si PV.

Mark L. Fergerson
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Mark Fergerson
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PostPosted: Thu Aug 28, 2003 9:26 pm    Post subject: Re: Solar cell powered battery charger

Lizard Blizzard wrote:
Quote:
William P.N. Smith wrote:

Mark Fergerson <mfergerson1@cox.net> wrote:

<bitching about>

Quote:
the limited lifetine of standard Si PV cells

Decades aren't enough?


Yeah, I agree. I see no point in messing with .5V single cells. I
bought some SPL-60 solar cells for $3.50 from All Electronics, have the
solderable contacts etc. They're good for 3V at 40 mA. For more info,
see
http://www.allelectronics.com/cgi-bin/category.cgi?category=565&type=store

So, how long are they good for?

Mark L. Fergerson
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Andre
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PostPosted: Thu Aug 28, 2003 11:34 pm    Post subject: Re: Solar cell powered battery charger

Lizard Blizzard <NOSPAM@rsccd.org> wrote in message news:<bil459$3tat4$2@hades.csu.net>...
Quote:
Mark Fergerson wrote:

Duane C. Johnson wrote:

snip

I have made an inverter that works on single
PV cells. See:
http://www.redrok.com/images/pvcellinv01.gif


This is a newer version of a ckt you previously posted using LEDs as
PV cells to get the +5V, yes? I saw the older one some time ago and lost
your name and the URL or I'd have asked you sooner; can you please
provide a description of the ckt's operation?

I'm interested because I have a sh*tload of LEDs lying around unused
and would like to fiddle with the ckt because I think it's more amenable
to miniaturization than other (vaguely) similar ckts I've seen, and the
limited lifetine of standard Si PV cells (I want to try a bunch of red
LEDs for the low V supply part instead of a Si PV). BTW, any data on the
lifetime and efficiency of LEDs used in this manner?

In my experience the LEDs can generate only sub-microamp currents.
After all, it takes square inches of solar cell to generate milliamps,
and you have a LED with a chip that's no bigger than a millimeter on a
side.

Interesting - I have heard that the superbright 10mm LEDs can go up to
a milliamp.

Seems that the best results can be had from infra-red LEDs (lower
barrier voltage, so higher output under load).

-A

Quote:

Mark L. Fergerson
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Duane C. Johnson
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PostPosted: Fri Aug 29, 2003 5:30 am    Post subject: Re: Solar cell powered battery charger

Hi Lizard;

Lizard Blizzard <NOSPAM@rsccd.org> wrote:
Quote:
William P.N. Smith wrote:
Mark Fergerson <mfergerson1@cox.net> wrote:
Duane C. Johnson <redrok@redrok.com> wrote:

I have made an inverter that works on single
PV cells. See:
http://www.redrok.com/images/pvcellinv01.gif

can you please provide a description of
the ckt's operation?

Gee, that's easy, and it's not even my circuit:

U1{A,B,C} make up an oscillator, coupled to U1{D,E,F},
which provide a driver to the two FETs.

Q1 drives a simple step-up through D1 to provide 5V to
drive U1, the two LEDs provide a shunt to keep the voltage
from rising too high.

Q2, T1, D3 provide a step-up with a transformer boost to
charge the battery. D2 provides overcharge protection
and bootstraps U1.

The two LEDs can apparently also provide some bootstrap
via their PV action.

and the limited lifetine
of standard Si PV cells

Decades aren't enough?

Yeah, I agree. I see no point in messing with .5V single
cells. I bought some SPL-60 solar cells for $3.50 from
All Electronics, have the solderable contacts etc.
They're good for 3V at 40 mA. For more info, see
http://www.allelectronics.com/cgi-bin/category.cgi?category=565&type=store

Actually there is a lot of merit to the solar battery
that you found for small applications.
(I say battery because it is composed of a number
of cells strung in series.) I like simplicity.
For very low powered applications this would work well.

However, for more power hungry applications this
small solar battery doesn't make it.:
1. The absolute power is low at 0.12W.
2. The efficiency is pretty low at about 3%.
Or the specific power is about 3W/ft^2.
OK, this just means that I need 5 of these to equal the
efficiency of a quality .5V PV cell at 15% efficiency.
3. The specific cost is about $29/W. And heck it's not
even mounted yet.

I found some 4" round PV cells about a year ago
or about $10, 1.3W, .5V, 2.6A, 15%, 15W/ft^2, @ $8/W.
There are probably better buys out there.

Much better buy if the low voltage can be inverted
to something useful.

The reason for pursuing the inverter technology was to
reduce the size and cost of small PV systems.
It doesn't take a lot to improve on $20/W to $50/W
with a little electronics.

Next step is to concentrate on single cells, maybe
5X to 10X, with a common grounded rear contact.
The idea is to have a power MOSFET and transformer
winding associated with each cell.

Duane

--
Home of the $35 Solar Tracker Receiver
http://www.redrok.com/electron.htm#led3X [*]
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Duane C. Johnson
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PostPosted: Fri Aug 29, 2003 5:55 am    Post subject: Re: Solar cell powered battery charger

Hi Andre;

Andre <testing_h@yahoo.com> wrote:
Quote:
Lizard Blizzard <NOSPAM@rsccd.org> wrote:

In my experience the LEDs can generate only sub-microamp
currents. After all, it takes square inches of solar
cell to generate milliamps, and you have a LED with a
chip that's no bigger than a millimeter on a side.

Interesting - I have heard that the superbright 10mm LEDs
can go up to a milliamp.

The green 10mm Lumex LEDs I use can generate up to
200uA or so if I carefully aim at the sun.

Quote:
Seems that the best results can be had from infra-red LEDs
(lower barrier voltage, so higher output under load).

Not exactly. Yes the lower band gap voltage can have
higher currents because there are more suitably energetic
photons to get lots of electrons. That's the good part.

That's also the bad part. Since the voltage is low the
trade off is lower power.

I think the best single band gap color is in the green
range. This is because the sun is brightest in the green
and the voltage is fairly high.

This is all good for light sensors as the voltage is
about 1.7V per cell.

Cost is another thing. Gallium Indium Phosphide is
very expensive. To expensive for PV panels.
Nice for generating medium cost light as the amount
of material is small.

Quote:
-A

Duane

--
Home of the $35 Solar Tracker Receiver
http://www.redrok.com/electron.htm#led3X [*]
Powered by \ \ \ //|
Thermonuclear Solar Energy from the Sun / |
Energy (the SUN) \ \ \ / / |
Red Rock Energy \ \ / / |
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1825 Florence St Heliostat,Control,& Mounts |
White Bear Lake, Minnesota === \ / \ |
USA 55110-3364 === \ |
(651)426-4766 use Courier New Font \ |
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Duane C. Johnson
Guest





PostPosted: Fri Aug 29, 2003 6:49 am    Post subject: Re: Solar cell powered battery charger

Hi René;

René <rjz@xs4all.nl> wrote:
Quote:
"Duane C. Johnson" <redrok@redrok.com> wrote:
Anyway, I have made an inverter that works on single
PV cells. See:
http://www.redrok.com/images/pvcellinv01.gif

Just curious: (for my education)

How does C5 (150 pF) carry enough signal to the driver
gates while R3 pulls the voltage towards Vcc and the
much larger C6 (100nF) loads C5 into a virtual
AC short (C1)?

I don't know. Its been about a year since I worked on
it and reused the plug board. Possibly I made a mistake
on the value. Trust me, it did work though.
Maybe C6 was 100pF.

C6 helped to hold off driving the BS107 when voltage was
low. At very low voltage the oscillator out does not
swing from rail to rail as it does at higher voltages.
C6 and R3 bias the input high.

OK here is the tricky part. There is an unseen protection
diode on CMOS inputs to VCC. Depending on how much voltage
swing the output from the oscillator has the average
voltage gets lower. The protection diode clamps the
oscillator swing on the high side bringing the low
swing low enough to switch the gate.

The result is very low current draw from VCC until the
things can get started.

There isn't any regulation in this circuit.
The 2.2M resisters holds the fet driver input high
when starting. This allows the 2 LEDs to get VCC
up to the point the oscillator will start. At this point
the small fet can just start to invert the .5V PV power
to raise VCC.

Once VCC gets higher the oscillator drives harder
bringing the power MOSFET into the picture.

BTW, don't substitute other CMOS gates for the CD4069.
The CD4069 is an un-buffered gate.

Quote:
What stops the oscillation if there is no light?

- René

Duane

--
Home of the $35 Solar Tracker Receiver
http://www.redrok.com/electron.htm#led3X [*]
Powered by \ \ \ //|
Thermonuclear Solar Energy from the Sun / |
Energy (the SUN) \ \ \ / / |
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1825 Florence St Heliostat,Control,& Mounts |
White Bear Lake, Minnesota === \ / \ |
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Watson A.Name - 'Watt Sun
Guest





PostPosted: Fri Aug 29, 2003 12:08 pm    Post subject: Re: Solar cell powered battery charger

In article <3F4E3B34.8010603@cox.net>, mfergerson1@cox.net
mentioned...
Quote:
Lizard Blizzard wrote:
William P.N. Smith wrote:

Mark Fergerson <mfergerson1@cox.net> wrote:

bitching about

the limited lifetine of standard Si PV cells

Decades aren't enough?


Yeah, I agree. I see no point in messing with .5V single cells. I
bought some SPL-60 solar cells for $3.50 from All Electronics, have the
solderable contacts etc. They're good for 3V at 40 mA. For more info,
see
http://www.allelectronics.com/cgi-bin/category.cgi?category=565&type=store

So, how long are they good for?

We had a discussion on these recently here. These are coated with
epoxy, and someone said that they fail if you coat the PV cells with
epoxy because of the diff in thermal expansion, I believe. But they
said they came out of some equipment so I would think that they would
not have a problem for years if the mfgr wanted to stay in business.


Quote:
Mark L. Fergerson


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William P.N. Smith
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PostPosted: Fri Aug 29, 2003 2:10 pm    Post subject: Re: Solar cell powered battery charger

"Duane C. Johnson" <redrok@redrok.com> wrote:
Quote:
Duane C. Johnson wrote:
I have made an inverter that works on single
PV cells. See:
http://www.redrok.com/images/pvcellinv01.gif

I chose a BS107 small MOSFET for it's very low gate
voltage. There are better MOSFETs but that's the one I had.

How would one select a MOSFET with a gate voltage under 5 volts for
full saturation and a peak current rating of about an amp?

I know how FETs work, but haven't used them, and don't really know how
to select them...

Thanks!

--
William Smith wpns@compusmiths.com N1JBJ@amsat.org
ComputerSmiths Consulting, Inc. www.compusmiths.com
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Duane C. Johnson
Guest





PostPosted: Fri Aug 29, 2003 3:22 pm    Post subject: Re: Solar cell powered battery charger

Hi William:

"William P.N. Smith" <wpns@compusmiths.com> wrote:
Quote:
"Duane C. Johnson" <redrok@redrok.com> wrote:
Duane C. Johnson wrote:
I have made an inverter that works on single
PV cells. See:
http://www.redrok.com/images/pvcellinv01.gif

I chose a BS107 small MOSFET for it's very low gate
voltage. There are better MOSFETs but that's the one I had.

How would one select a MOSFET with a gate voltage
under 5 volts for full saturation and a peak current
rating of about an amp?

In my circuit, this was not needed. That's why the use
if the TO-92 packaged BS107. It is used to bootstrap
the voltage up to a higher operating range needed for
the "logic" gate power MOSFET.

Quote:
I know how FETs work, but haven't used them, and don't
really know how to select them...

Good engineering practice would suggest that the gate on
a MOSFET be driven just as the manufacturers says.

However we can often go outside the suggestions in
special applications. In this case the IRLZ44 is a
50A transistor. I also know the gate threshold voltage
is about 2 volts. At 2.5 volts the transistor is
pretty well turned on.

Of course, there is nothing so instructive as some parts
and a plug board with which to experiment. Some of the
best AhHa moments happen when you just try something
ant then understand the outcome.

Quote:
Thanks!

Duane

--
Home of the $35 Solar Tracker Receiver
http://www.redrok.com/electron.htm#led3X [*]
Powered by \ \ \ //|
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White Bear Lake, Minnesota === \ / \ |
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Duane C. Johnson
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PostPosted: Fri Aug 29, 2003 4:53 pm    Post subject: Re: Solar cell powered battery charger

Hi Mark;

Mark Fergerson <mfergerson1@cox.net> wrote:
Quote:
"Duane C. Johnson" <redrok@redrok.com> wrote:

I chose a BS107 small MOSFET for it's very low gate
voltage. There are better MOSFETs but that's the one I had.

OK.

The LEDs don't do regulation as there is 10K in series
with them.

I use them as a bootstrap power source to get the
oscillator supply voltage over about .9V. The oscillator
I have here is about the lowest VCC I can get to run
without using germanium transistors.

I've been parting out old handheld CB radios; plenty of
Ge transistors. But I think the IC takes up less space,
those cans are huge. ;>)

Anyway once the voltage gets to .9V or so the oscillator
starts and Q1 converts power from the PV cell to something
over about 2.5V where the large power MOSFET takes over.

So the green LEDs' Vout droops a bit; but VCC stays high
enough that the gates in U1 can oscillate; that's what I
thought.

Q2, T1, D3 provide a step-up with a transformer boost
to charge the battery. D2 provides overcharge protection
and bootstraps U1.

Yep, crude but effective.

IIRC in the older version there was no transformer, just
a boost coil like L1; but in the newer one you seem to have
it connected as sort of a hybrid balun/autotransformer
thingie. BTW it's a standard small audio type, yes?

My cad program doesn't have very many choices
for transformer symbols.

This transformer is a large ferrite core with a few
turns on the primary. The secondary is determined
experimentally.

The inductor was a small ferrite core.

Quote:
Anyway my confusion was how the thing started. I forgot
there'd be _some_ voltage on the NiCd. Silly me.

True, I just wanted to guarantee that the thing would
even with a totally dead NiCd.

Of course there is another solution.
A small button lithium cell would power the oscillator
for 10 years or more.

Quote:
The two LEDs can apparently also provide some
bootstrap via their PV action.

Yes, a few uA of charge current.
The green LEDs I use have the capability of generating
1.7V. However the the oscillator consumes this current
so I put 2 LEDs in series to get some improvement in
starting time.

I have plenty, I could just stack them up...

and the limited lifetime of standard Si PV cells

Decades aren't enough?

Nope, I'm thinking about an application similar to Watt
Sun's "forever flashers", but I want to charge a supercap
instead of a NiCd, which is why I mentioned stacked green LEDs.

The "BEAM" guys have things called Photo Poppers that do this.

Quote:
Next question is how many red LEDs I'll need to equal the
Iout of a small PV cell.

Mark L. Fergerson

Duane

--
Home of the $35 Solar Tracker Receiver
http://www.redrok.com/electron.htm#led3X [*]
Powered by \ \ \ //|
Thermonuclear Solar Energy from the Sun / |
Energy (the SUN) \ \ \ / / |
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1825 Florence St Heliostat,Control,& Mounts |
White Bear Lake, Minnesota === \ / \ |
USA 55110-3364 === \ |
(651)426-4766 use Courier New Font \ |
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Mark Fergerson
Guest





PostPosted: Fri Aug 29, 2003 7:33 pm    Post subject: Re: Solar cell powered battery charger

Duane C. Johnson wrote:
Quote:
Hi Mark;

Hi back, and thanks for all the explications;

Quote:
Mark Fergerson <mfergerson1@cox.net> wrote:

IIRC in the older version there was no transformer, just
a boost coil like L1; but in the newer one you seem to have
it connected as sort of a hybrid balun/autotransformer
thingie. BTW it's a standard small audio type, yes?

My cad program doesn't have very many choices
for transformer symbols.

Don't feel bad; if I bother to do any schematizing on my
puter I use a symbol font in MSPaint.

I'm so old I usually use pencil and graph paper.

Quote:
This transformer is a large ferrite core with a few
turns on the primary. The secondary is determined
experimentally.

Um, "large"? I was kinda hoping to keep this small. Time
to get out the bits 'n' pieces and see how small "large" can
be made.

Quote:
The inductor was a small ferrite core.

Got lots of those.

Quote:
Anyway my confusion was how the thing started. I forgot
there'd be _some_ voltage on the NiCd. Silly me.


True, I just wanted to guarantee that the thing would
even with a totally dead NiCd.

There seems to be some disagreement on that, but that was
my concern. Yes, I know a totally dead NiCd often won't
recharge, but what the hell.

Quote:
Of course there is another solution.
A small button lithium cell would power the oscillator
for 10 years or more.

That takes all the fun out of it though.

Quote:
... I want to charge a supercap
instead of a NiCd, which is why I mentioned stacked green LEDs.


The "BEAM" guys have things called Photo Poppers that do this.

Thanks for that pointer; I've been looking at what the
BEAMers have been doing but somehow missed Photo Poppers.

Mark L. Fergerson
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