Hello,

I have a project to build a photodiode circuit with microcontroller to measure solar irradiance. The function of OpAmp is to limit the analog voltage to A/D converter, as shown in attached picture below. Vout will be connected to A/D converter (Vout = Ip * R1).

My problem is, how to measure irradiance? (power per diode area). For my understanding in measuring power, I will use a voltmeter to measure voltage across photodiode, and multiply it with the photocurrent (Vp * Ip). Is that correct?

Kindly appreciate any help and suggestion. Thanks in advance!

 

(Some text removed for clarity)

My problem is, how to measure irradiance? (power per diode area). For my understanding in measuring power, I will use a voltmeter to measure voltage across photodiode, and multiply it with the photocurrent (Vp * Ip). Is that correct?

Nope. The output of the opamp is the photo current times the value of the feedback resistor. So if you have a 10k feedback resistor and 10 uA of photo current, then the (Ideal) output of the opamp is 100,000 uV or 0.1 volt.

What you want for irradiance is watts per square meter. Your photodiode will give you microamps per watt per square meter, so you only need to calibrated your instrument by adjusting the feedback resistor for your particular photodiode, so the output voltage is in terms of volts per watt per square meter.

Note: The coefficient for microamps per watts per square meter is only good for one particular spectral distribution and sunlight changes a little in spectral resolution during the day and through the seasons, so if you can accept the resulting inaccuracy, you can use a simple photodiode instead of a very expensive spectroradiometer.

You can find the typical solar irradiance for your location for calibration purposes at sites like the URL below. Be prepared to do some math with the data from some sites.
http://home.iprimus.com.au/nielsens/solrad.html

NASA's SPARKS project at the Ames Research Center used to have a 1/4 sun simulator. If you can find such a simulator, calibration would be easy. Remember, that number only works for one spectral distribution.