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Hello,

I have now build the Solar radiation sensor and fine tuned its calibration.

At first I had a gain of about 26 (assuming the same values taken in the example published in a different post on solar radiation measurement) and at mid-day the values read were capped at 1400W/m2 in Cumulus. So I've adjusted R1 and R2 to get the exact values giving me a gain of 6.875. Then I readjusted by measuring the PV cell output at a constant irradiance and reading the board output to get the same gain.

Now the values are pretty similar to the national irradiance map published here in Singapore and refreshed every minute! It's a color mapped, so not extremely precise, but when the zone of my weatherduino is of the same color as the neighbourhood, we can deduce the value of the irradiance. I still need more fine tuning though.

My question is on Cumulus: how to adjust the original green gaussian center? Currently it's entered at 3am.. I need to center it around 12pm, right? I tried to find some configuration on the trendT.html or the js files but no success.

Thanks!

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27-08-2017, 00:03
(This post was last modified: 27-08-2017, 00:04 by

jgveill.

*Edit Reason: typo*
)

Happy to have found this thread today as I was looking to have an inexpensive way to measure sun irradiance. I looked for cheap pyranometer but the cheapest one is way too expensive for a weather station built just for fun.

Looking at this project, the most difficult part is certainly the low voltage output and the need to have an opamp. Somewhere in the thread, someone (alan I think) came with the idea to use a panel with more cells. I think the real benefits were not seen. The graph shown on the thread's first page is for one cell giving 0.5 to 0.6 volts. Uncle_bob calculations are based on that : one cell solar panel. If you use more cells you multiply the voltage by the number of cells, current being the same, which allows to get rid of the opamp.

To give an idea of the values we could get with 10 cells solar panel, target voltage is then 1,6 volts. So for 1600 W /m2 we now have 1600 mv or 1 mv /w / m2 (10 times lower). With arduino internal ADC, this gives 4.88 W / m2 as the smaller step (again 10 time better than original design), which is quite good and really easy to do. Using 2 resistances for shunt to make a voltage divider (total becomes 17,33 ohms) we could use the 1,1 v arduino ref voltage and get 1,07 W /m2 as the smaller step and again no opamp needed ... Too good in fact for other imprecision factors like no real good reference irradiance.

Some time ago, to make tests for building a sun irradiance meter, I ordered a 10 cells panel which is a 5 volts solar panel. In fact this is a 2 times 10 cells to have higher power. I can expect higher short circuit current. As expected, open voltage is 5 to 6 volts. I will make calculations and test this but here's what I expect with this panel :

Short curcuit current : 0,27 amp for 810 W /m2 (bad conditions for a test today so I took a nice irradiance number to facilitate calculations)

Shunt resistance : 3 ohms

Maximum irradiance : 1500 w / m2 (1000 would probably be better for Quebec)

Max voltage measured: 1500 mv

1500 W / m2 -> 1500 mv or 1 mv / W / m2

arduino gives 4,88 mv / step -> 4,88 w / m2

Also, with arduino, there's an easy way to get better precision : 16 bits ADC which has 4 inputs to do other things (like wind direction which is analog output for me).

J Guy

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27-08-2017, 02:36
(This post was last modified: 27-08-2017, 02:40 by

jgveill.)

I agree with you we are measuring voltage drop across a shunt but ohm's law says that when in series voltages sources (like batteries) add. Do we agree that cells are "like" batteries ? When in series cells give higher voltage, when in parrallel, cells give higher current. So here, we have 10 series cells giving an open circuit of 5 volts. To keep each cell in the current source mode (as explained in the link), we HAVE to keep each cell to 0,1v and this is done with the shunt. But as total voltage is 10 times higher (parrallel batteries) we have to have 1 volt voltage drop in the shunt. This way each cell will have a voltage drop of 0,1 v exactly like shown in the link.

In fact, another way to see it is to imagine having 10 reference circuits (like the one in the link) and put them in series. Total current will not change, but the total voltage from end to end of the 10 circuits will be increased by a 10 factor (like we add batteries voltage when batteries are in series). Some people working with BWP34 add some in series to higner the generated voltage.

if you think cells are batteries, and we should easily agree on that, it's clear that total voltage from 10 series cells increases by ten. For having each cell in the current source mode, which is the basis of the theory, we have to have each cell at 0,1 volt. With 10 series cells, total solar panel voltage MUST be 10 X 0,1 volt which gives a total voltage of 1 volt for 10 cells and easier treatment.

J guy