Exponential, linear or logarithmic scaling...

[Externet]

Hi.
Which of the three do I want, and how to tailor a passive circuit for it ?

A couple of solar cells in series with an analog VU meter -galvanometer- canibalized from defunct audio equipment.

(-)------------cell1----cell2(+)--------------10Ktrimmer---------------(+)VUmeter------------(-)

At maximum insolation, the variable resistor set to full scale. When cloudy or one cell covered, still points to about 75% of scale. Covering one cell, also about the same reading, does not point to centre of scale.
What circuitry would display properly as incident solar power ?

 

[crutschow]

The solar power equals voltage times current so you need a power meter if you want to display the power it outputs.
Voltage by itself from a solar cell is not a good indication of power.

 

[barry]

The solar power equals voltage times current so you need a power meter if you want to display the power it outputs.
Voltage by itself from a solar cell is not a good indication of power.

This is not as simple as you would like.

Measuring the current from the solar cell tells you nothing about the incident solar power, it only tells you how much power the load draws.

Further, unless you know the efficiency of your solar cell theres no way to determine the incident solar power. You also need to know the I-V curve for the solar cell. Read this.

 

[KlausST]

Hi,

I'm with Crutschow,
P = V x I
should always be true in a DC system.

This is the electrical power provided by the solar cells.

Klaus

 

[BradtheRad]

Wondering the same thing about my own solar panels, I put one in the sun and loaded it with a variety of resistors. Measured voltage on one meter and Amperes on another. Wrote down results.

I typed the data into a homebrew computer program which plotted graphs. As you can see, high A goes with low V and vice-versa. For all measurements the program calculated Watts = V x A.
Maximum Watts could not be discovered until the program had plotted all data points. Notice maximum Watts appears at a midway position of the V & A plots.

V, A, W are on linear scales. Load resistance is log scale.

 

[KlausST]

Hi,

You say "maximum watts" ...

With P = V x I
one calculates actual electrical power.

One can not estimate "maximum" power, neither with V, nor with I, nor with both....unless you know the V-I curve of the PV cell.

Maybe the OP can clarify what he is after...

On a loaded PV cell (driving an inverter or a battery charger)...it's even more difficult.
For example an MPPT charger will continously vary V and I to find MPPT ... until the battery is (almost) full.
Then it leaves MPPT mode and draws less power than theorectically possible. So I drops, V rises...

Klaus

 

[FvM]

Not open circuit voltage but short circuit current of a solar cell or photo diode is proportional to light intensity. In practice, also a load resistor that keeps the voltage sufficiently below open circuit voltage at maximal irradiation will do the trick.

 

[KlausST]

Hi,

I agree, with a short circuited PV cell one can determine I_short and with the use of the V-I curve one can determine theoretical possible power.
But then the cell is not able to produce useful power. So the cell power is wasted. And when there are several cells in series, then all the cells (power) are wasted.

Klaus

 

[FvM]

Post #1 doesn't say anything about using the solar cell as power source at the same time. In case of arbitrarily loaded cells, intensity could be calculated as a nonlinear function of cell current and voltage.

I noticed however, that the post is talking about series connected cells. Short circuit current doesn't work as average intensity criterium for uneven irradiated series connected cells, rather for minimum cell irradiation.

 

[barry]

Hi,

I'm with Crutschow,
P = V x I
should always be true in a DC system.

This is the electrical power provided by the solar cells.

Klaus

The question was how to measure incident solar power, NOT the power out of the cell. You could put a cell with 10% efficiency out there with a cell with 40% efficiency. Put the same load resistor on both. The incident solar power is the same, the measured power is certainly not.

 

[crutschow]

Certainly if the TS wants the actual solar radiated power hitting the cells and not the cell output power, then that's a whole different problem.

 

[Externet]

Thanks, gentlemen.
The purpose is just making an instrument from junk (10cm2 garden light solar cell and VU meter) to display an almost decent value of insolation.
Like saying "It is not showing 100% now; there should be some cloud or the angle of incidence is not perpendicular"
Not for measuring power generated by the cells nor the cells are doing any work other than pushing current trough the meter. To find how clear the sky is at the moment, Voltage from solar cells is not a good indicator of solar incidence; current is better for that.... I think.

(-)------------cell1----cell2(+)--------------10Ktrimmer---------------(+)VUmeter------------(-)