piclist 1999\03\08\140435a >
Thread: Solar Panels? - Why PWM Works
www.piclist.com/techref/io/pwm/index.htm?key=pwm
picon face BY : Wagner Lipnharski email (remove spam text)



Eric Borcherding wrote:
> There is a need to understand that PV cells are not linear
> they a bowed - the V*I max in the bow can be tracked.

Where can we find a solar cell data sheet, with informations
like that?

Remember that the best power transfer happens when the
external (load) impedance is equal to the internal (cell)
impedance, so at that point, the voltage-drop at the
internal (cell) impedance is the lowest possible with
the maximum possible output current. Lots of folks doesn't
understand that, this is culture, so here we goes:

10Volts-----INT.IMPEDANCE(10 Ohms)----x---LOAD(15 Ohms)
Circuit current V/R: 10 / 25 = 0.4 A
Power transfered to load RI^2: 15 x 0.4 x 0.4 = 2.4 W

10Volts-----INT.IMPEDANCE(10 Ohms)----x---LOAD(5 Ohms)
Circuit current V/R: 10 / 15 = 0.66 A
Power transfered to load RI^2: 5 x 0.66 x 0.66 = 2.178 W

10Volts-----INT.IMPEDANCE(10 Ohms)----x---LOAD(10 Ohms)
Circuit current V/R: 10 / 20 = 0.5 A
Power transfered to load RI^2: 10 x 0.5 x 0.5 = 2.5 W

As you can see, when get the max transfered power when
both impedances are equal. This is applied to sound,
video, RF, power, pressure, including mechanics.
Look what happens when the equalization changes a bit:

10Volts-----INT.IMPEDANCE(10 Ohms)----x---LOAD(11 Ohms)
Circuit current V/R: 10 / 21 = 0.476 A
Power transfered to load RI^2: 11 x 0.476 x 0.476 = 2.49 W

10Volts-----INT.IMPEDANCE(10 Ohms)----x---LOAD(9 Ohms)
Circuit current V/R: 10 / 19 = 0.526 A
Power transfered to load RI^2: 9 x 0.526 x 0.526 = 2.49 W

The general formula is:

PowerLoad =   Voltage x LoadImpedance
           --------------------------
           (SourceImped + LoadImped)^2
or
PowerLoad =    Voltage
           -------------------------
           (SI^2 / LI) + 2xSI + LI

I wonder if the bowed curved you said is not related to
optimum power transfer when both impedances are equal,
that a pwm current can do, caused by the current slack
raising time.

Wagner

<36E40397.EBEBAA24@earthlink.net> 7bit

See also: www.piclist.com/techref/io/pwm/index.htm?key=pwm
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