Solar panel output sensor for data logging?

[mrderik]

Greetings All -

     I am setting up a (Hobo brand) data logger to make a study of the solar potential on my property.  I've done the math on expected output based on a year long study of average W/m2 for our town.  But what I also want to determine is how much (actual) potential do I have for a fixed array on my roof.  So I would like to utilitze a ready made, weatherized solar panel (like a sunsei SE-400 which is rated at 15vdc & 400mA ie 6w) with the output (preferrably under ideal load) connected to one of the sensor inputs on my datalogger.  The logger has two adapters available for input ranges - Voltage between 0-5v or Amperage between 4-20mA.  I have the Voltage adapter, but I would have to buy the Amperage one.  I understand basic electrical theory and can breadboard a circuit to fab a convertor / adapter but need some assistance.  For reference, I'll be simultaneously logging the ambient W/m2 with a pyranometer. I am quite good at math and very good with spreadsheets so I could come up with formulas to determine full output in watts for variable volts based on available W/m2. Or am I making it more difficult than I need to and should just get a second Pyranometer, mount it on a flat board, angled and oriented the same as my roof and just do the math?  Any help would be greatly appreciated.  Thanks!

[GaryGary]

Hi,

I'm NOT very good at electronics, so others may have a much better solution.

But, couldn't you just hook up two resistors in series across the PV panel output as a voltage divider, and then hook the center point of the voltage divider to your 0 to 5V logger input?  The resistors could be something like:

R1 + R2 could be (15V)(0.4amp) = 37.5 ohm, and (R1(R1 + R2))(15V)  = 5V, so R1 = 12.4 ohm, and R2 = 37.5-12.4 = 25.1 ohm??

Seems like you might be able to calibrate it by using the pyranometer to measure solar input on the panel, and manually measure voltage and current across the R1+R2 to get the panel power output -- then correlate the voltage across R1 to the measured power output??

--

A question for you, since you seem to be into measuring solar radiation.

I have one of the Onset Computer (hobo) pendant data logger that measure sunlight in lumens/sqft.  I use this as a substitute for a pyranometer because I'm to cheap to buy the real thing.  It only costs $50, and it seems to work well, but I would like to be able to do some kind of rough conversion from lum/sqft to watts/m^2 -- any ideas on how to do this?

Gary