Alt-BEAM Archive

Message #10969



To: beam@sgiblab.sgi.com
From: Wilf Rigter rigter@cafe.net
Date: Sun, 27 Feb 2000 13:19:39 -0800
Subject: [alt-beam] Re: Solar Cell tests


Very clear and simple analysis!

regards

wilf

CyberBug wrote:

> Hi there,
>
> I saw your tests and I was curious about your results, so I did some
> calculations and this are the results (or explanations of your results):
>
> 1. When charging a capacitor the solarcell works like a baterie and resistor
> in series, so to make the calculations we need to now the solarcell's
> resistance, and that would be:
>
> A: 3.5v @ 6 mA
>
> Ra = 3.5 v / 6 mA
> Ra = 583 ohms aprox.
>
> B: 2.7v @ 16 mA
>
> Rb = 2.7 v / 16 mA
> Rb = 169 ohms aprox.
>
> 2. Now we test for parallel configuration. As you can see in the schematic,
> you have a net formed by V1, V2, V3 and V4.
> According to Kirkchof's law the algebraic sum of all the voltages in the net
> must equal 0. So:
>
> 0 = V1 + V2 + V3 + V4
>
> We know V3 and V4 (the solarcell's voltage), but since they are not equal
> they have to compensate through the resistors, so we now have to calculate
> the real output voltage. First we have to indicate de polarities of each of
> the voltages, in a CCW fashion you get the polarities noted (based on the
> known voltages), then we can solve for the know voltages:
>
> V2 + V3 = V1 + V4
> V2 + V3 = 3.5v + (-2.7v)
> V2 + V3 = 0.8v
>
> Now we know that resistors Ra + Rb hold a voltage of 0.8v. The real output
> voltage is taken between the Ra and Rb therefore resulting a voltage divider
> between the two. Keeping this in mind we know calculate the voltage presente
> at Rb:
>
> Ra = 583 ohms
> Rb = 169 ohms
> Ra + Rb = 752 ohms
>
> V3 = 0.8v x (Rb / (Ra + Rb))
> V3 = 0.8v x (169 / 752)
> V3 = 0.8v x 0.211
> V3 = 0.179v
>
> Now for V2:
>
> V2 + V3 = 0.8v
> V2 = 0.8v - V3
> V2 = 0.620v
>
> Now V3 and V4 are in series and this is the real output voltage so:
>
> Vo = V3 + V4
> Vo = 0.179v + 2.7v
> Vo = 2.879v
>
> The real output voltage represents the voltage that you would have if it was
> only one solarcell. The output current I is the sum of both solarcell's
> current:
>
> I = 6 mA + 16 mA
> I = 22 mA
>
> The resulting solarcell is 2.8v @ 22 mA (hence faster charging, more on this
> later)
>
> 3. Now we test series configuration. Again, as you can see in the schematic,
> this is much simpler. We now have two voltages V1 and V2, they are in series
> and the resulting output voltage Vo is:
>
> Vo = V1 + V2
> Vo = 3.5v + 2.7v
> Vo = 6.2v
>
> But now, we have the same problem we had with the voltages in parallel but
> with the current. We have two different currents, but not to worry, we
> already know the resistance of each solarcell, so:
>
> Rt = Ra + Rb
> Rt = 583 + 169
> Rt = 752
>
> Applying Ohm's Law:
>
> I = Vo/Rt
> I = 6.2v / 752
> I = 8.2 mA
>
> The resulting solarcell is 6.2v @ 8.2 mA (slower than A + B in parallerl but
> faster than A alone).
>
> So now we have:
>
> A: 3.5v @ 6 mA (11.5 sec)
> A + B series: 6.2 @ 8.2 mA (6 sec)
> B: 2.7v @ 16 mA (3.5 sec)
> A + B parallel: 2.8v @ 22 mA (2.5 sec)
>
> What can you guess from above?
> Right, the larger current charges faster, even though voltage is smaller.
>
> But then again, all of the above results are theoretical, in practice they
> vary. You were suposed to have 2.8v total, but in practice you had 2.4v,
> although it did charged faster because of the current enhancement.
>
> Now for the charging times, if you could see voltage vs. time in a graph you
> would see that a charging curve, as you get closer to the maximum voltage
> the voltage variations are smaller and much more longer in time. You start
> charging very fast but it keeps getting slower until you reach the top
> voltage. If the charging time was linear you would see the same time going
> from 1.1v to 1.2v than going from 1.4v to 1.5v.
>
> Hope this was useful, if you or anyone has any comments please let me know.
> See ya.
>
> ----- Original Message -----
> From: FCO Enr.
> To:
> Sent: Thursday, February 24, 2000 3:35 PM
> Subject: Solar Cell tests
>
> > greetings,
> >
> > I did some tests with some various solar cell configurations
> > and here is some of the data I gathered as well as some questions
> > for those of you who can explain this phenomena.
> >
> > First off, I tried various combinations to find the most efficient
> > low light (my ambient light level) setup. The winner is surprising
> considering
> > even my Solarbotics catalogue seems to point in the wrong direction.
> >
> > A - SC2422 (3.5v @ 6ma)
> > B - SC2433 ( 2.7v @ 16ma)
> > ( I also experimented with the SCPDs but I didn't write down and time
> > the results so I won't include them).
> >
> > The goal was to see how fast the circuit could gain .01v in my ambient
> > light. I did my best to avoid any movement of cells.
> >
> > A 11.5 seconds
> > A + B in series 6 seconds
> > A + B in parallel 2.5 seconds
> > B 3.5 seconds
> >
> > Now this is pretty odd considering it seems like wiring these
> > cells in parallel improved their low light capability (not by much
> > considering the space it takes up).
> >
> > WEIRD: When the voltage in the circuit reached 2.2 something volts
> > I noticed it was takin MUCH longer.. from 2.23v - 2.24 it took
> > 1 minute 7 seconds!!! I looked at my meter again and it hasn't
> > budged from the 2.24v mark in over 2 mins!!
> >
> > I also noticed the times I stated above were between 1.4 and
> > ` 1.58v. It seems like the charging between 1.1v and 1.2 is almost two
> > times faster! Why does it vary depending on circuit voltage and why
> > did the voltage sort of plateau at 2.24v?
> >
> > Now the light level hasn't changed AT all and I tried again when it got
> > to 2+ volts the cycling was a LOT slower to get the same 0.01v
> > WHY? So this also means if I get the lowest rated trigger it would
> > improve low light capability DRASTICALLY!
> >
> > I would appreciate any feedback on this.
> >
> > Andy
> > fco@total.net
> >
> > hrdware's digital sanctuary - mindless oddities
> > http://www.angelfire.com/pq/hrdware
> >
> > The Factory - TF Map Making Tutorial
> > http://www.planetfortress.com/factory
> >

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