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Forum Index : Solar : sunlight intensity

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isochronic
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Joined: 21/01/2012
Location: Australia
Posts: 689
Posted: 11:27am 28 Sep 2017
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I am wondering about measuring sunlight intensity with a small solar cell.
The lightmeters for photography often just had a solar cell driving a current meter, with the output current directly proportional to light intensity.
So if I substitute a smallish resistor for the meter, the voltage across the resistor should be proportional as well ( as long as the cell voltage is well away than its maximum ), right ?
I have tried it (nominal 6 v cell, 1k resistor, approx 3v across resistor in direct sun) and it seems to work ok but I am wondering if it is accurate /linear etc. The small cells are widely available, I think they are amorphous on glass types.
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 4406
Posted: 12:41pm 28 Sep 2017
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I am pretty sure you need to measure short circuit current directly.
More like having millivolts rather than 3v across your measurement resistor.

Try buying a digital amp meter. These will display in any units with typically 50mV or 75mV input.

For example, a 20 amp meter would display 20.0 with 50mV input.
A 300 amp meter 300 with 50mV input.
A 10 amp meter 10.0 with 50mV input.

If your cell puts out 50mA, then use a 1 ohm resistor to get 50mV to drive the display to your chosen units to be displayed.
Could be Lux, Lumens, Candelas, watts per square metre or whatever units you want.
Cheers,  Tony.
 
yahoo2

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Joined: 05/04/2011
Location: Australia
Posts: 1166
Posted: 04:04pm 28 Sep 2017
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it all depends on what you are trying to measure.

there are several standards,
one calculates heat perpendicular to the earths surface measured in langleys using a sensor on a horizontal plane.
one calculates energy density perpendicular to the sun measured in sol.
there are others that pick specific spectrums only.

there is a device called a pyranometer that has a lens that captures total solar radiation. What I regard as the standard is a pyranometer thermopile sensor made by kipp and zonen, they give specs for
response time
spectrum spread
directional response
temp response
calibration stability
resolution

and an overall number for uncertainty

My davis sensor does not match the K&Z specs in any category yet they claim to have half the uncertainty. yeah.... right!

I set mine up next to the local airport weather station for a few days and compared the logged numbers with their published data. They need to be close, you cant compare two sensors even one kilometer apart because the local conditions variation is massive compared to seasonal or latitude variation.

Believing a sensor is a bit like believing the speedo in a car or a multi-meter. I hear guys with certain brands of car complain about speeding trucks passing them, it never dawns on them that its their vehicle that is traveling slow.

I think that is about all you can do, compare it to a known meter and take both sensors readings with a grain of salt under extreme conditions.


I'm confused, no wait... maybe I'm not...
 
isochronic
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Joined: 21/01/2012
Location: Australia
Posts: 689
Posted: 08:04pm 28 Sep 2017
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The millivolts range sounds like the go. Distantly I now remember some solar cell info, the imperfections include a parasitic resistance that bleeds off current, no doubt severely non-linear wrt voltage.
The digital meters work well but it is intended to interface to other systems. The aim is to have a measurement of intensity, sol or watts/square meter, which would then help determine the best current to draw on the spot.
Maybe plan B : use the cell as a photodiode, there are a lot of circuits on the web, and maybe whack an op=-amp onto it to amplify/filter/sort it out.
 
Phil23
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Joined: 27/03/2016
Location: Australia
Posts: 1664
Posted: 12:36am 03 Oct 2017
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I'm measuring "Solar Energy" with a simple panel from a garden light.

I think from memory it's loaded with about a 10ohm resistor & into an analogue input.

It's roughly calibrated to my pyranometer to read W/m².

Not totally precise, but good enough for my purpose.

Phil.
 
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