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PICList Thread
'[EE]: Solar Pic voltage'
2002\02\07@144147 by Aaron Lahman

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I've tried searching on the net for characteristics of photovoltaic cells,
and am now well informed as to their chemistry, efficiency and so forth, but
can't yet explain my observation.  I'm attempting to power a PIC
microcontroller and LCD as a clock using a memory back up capacitor (1.0F)
and two 5-cell solar panels.  They're rated at 3.0v / 50mA in full sun, but
washington weather rarely sees that.  However, the pic and LCD only requires
2mA, and the pic will turn off the LCD at 4.5v and below, so then the
current draw drops to well below 1mA... the cap will go from 4.5v to 2.0v
over about 20 hours.

Heres the problem: when i run the solar cells in series in low lighting
conditions (only 2v per cell with no load) and attach a small load (a charge
pump circuit, the charge pump only consumes a few uA once fully charged,
minus cap leakage), i'll only measure 1.5 across the two elements.  the top
one will get 2v drop across it, and the bottom one gets -.5v across!?  If i
replace with only one solar unit (either one), i get the same 1.5v drop, and
the charge pump reaches the same max voltage (i think it was about 2v max, 3
stages - i'm aiming for 6v).  Diagram:

1.5v    +----------+          1.5v  +-------+
        |          |                |       |
       ---         |               ---      >
        - ,\       |                - ,\    >   R-load
        |          |                |       >
        |          > R-load         |       |
-0.5v    +          >           0.0v +-------+
        |          >
        |          |
       --- ,\      |
        -          |
        |          |
0.0v     +----------+

Does anyone know of any resources out there that can explain the effect i'm
seeing?  Is the solution obvious and i'm just missing it?   And third, how
do i eliminate this effect?

Thanks in advance.
-Aaron

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2002\02\07@145922 by David VanHorn

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Are they equally illuminated?
Try putting a load on the two panels separately, and checking them in
place. If one is putting out more current than the other, then you would
see as you describe.

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2002\02\07@185827 by Aaron Lahman

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I don't have equipment to measure the current in low lighting (it's slightly
less than 1mA, by inference) but I still get the effect if i switch the
order of the two solar elements... it's always the one on top - providing
the positive voltage - that has 2v across, and the one on the bottom
having -0.5v.  They are set in a position where they receive approx. equal
lighting at the same angle (side by side on a desk under a lamp a few feet
up).

> {Original Message removed}

2002\02\07@192943 by Thomas C. Sefranek

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I know you have already checked this but...

Do you have the bottom one in backwards???

PV panels are current generators, add a small series resistor
and re-run your test.

Aaron Lahman wrote:

>I don't have equipment to measure the current in low lighting (it's slightly
>less than 1mA, by inference) but I still get the effect if i switch the
>order of the two solar elements... it's always the one on top - providing
>the positive voltage - that has 2v across, and the one on the bottom
>having -0.5v.  They are set in a position where they receive approx. equal
>lighting at the same angle (side by side on a desk under a lamp a few feet
>up).
>
>>{Original Message removed}

2002\02\07@220716 by Roman Black

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Aaron Lahman wrote:
>
> I don't have equipment to measure the current in low lighting (it's slightly
> less than 1mA, by inference) but I still get the effect if i switch the
> order of the two solar elements... it's always the one on top - providing
> the positive voltage - that has 2v across, and the one on the bottom
> having -0.5v.  They are set in a position where they receive approx. equal
> lighting at the same angle (side by side on a desk under a lamp a few feet
> up).


Do you have a series (pass) diode? :o)
-Roman

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2002\02\08@130444 by Joris van den Heuvel

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This may be a dead end, but here goes nothing ;)

Have you considered putting a rather large-ish capacitor accros the solar
panel's terminals to increase their peak current? To me it seems as if the
step-up converter you're using demands peak current and the high output
resistance of the solar cells prevent this.


Regards,
Joris.

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2002\02\08@143440 by Aaron Lahman

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I should have provided a more exhaustive list of things i'd tried, but on
the other hand, i'd forgotten half the things i'd tried.  Maybe i need to
keep notebooks on all my little hobby projects :)

I tried putting a diode in series, both between and after (+ side) the solar
panels.  No effect.  Tried a capacitor (about 50uF and also tried 0.1uF)
from - to + of both solar cells together and each individually.  No
effect... well, i got a more stable voltage reading, but nothing more.

After careful consideration, i've decided that an old wall-wart would be
just as fun of a power source.  Less mobile, but i can still use the memory
backup cap to keep time when unplugged.  It can keep time for a few days at
least off of that thing.  Using internal RC, sleeping, and a 32.768kHz
crystal on timer1 only draws a few uAs on the average

To everyone who threw out some thoughts, thanks for your time.  I'm not
giving up... just postponing.  I'll try it again another day when i think of
another solar project.

-Aaron

> {Original Message removed}

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