Hi,
Chuck is quite "honest" that his "pyrometer" is a "fun" project, with his conclusion that "In spite of the limitations, the simple device shown here is useful as a learning tool.". The current from his PV panel is ridiculously large, necessitating the use of copper wire as the "load resistor"; a really bad idea because of the large temperature coefficient of copper*.
To make a "scientific" measurement you don't normally use a "load" resistance at all, but measure directly in the current domain; typically using an Op-Amp configured with a "Virtual Earth" input that gives zero voltage drop (<~1mV) across the panel/diode. Alternatively, configure the amplifier as a Current-to-Frequency converter (aka an oscillator), which can also perform the ADC function with high accuracy and dynamic range. By remaining in the current-domain, it's possible to use a single photodiode and a $1 microcontroller to achieve a dynamic range from peak sunlight down to moonlight, or almost 1,000,000 : 1 (20 bit resolution), but that's not the purpose of this thread.
As said above, the main aim of this project / thread is to make practical and reliable long-term outdoor measurements. For these, there are much more important practical design issues, such as the Enclosure / Window, with its impact on the Cosine response of the sensor, and the reliability (and low resistance) of the electrical connections, etc..
*EDIT: For reference: Davis quote the Temperature Coefficient of their 6450 Solar Radiation Sensor as +0.12% per degree C, and the TempCo. of copper is +0.4% per degree C (a load resistor is also likely to get hotter than a solar panel, of course). The TempCo. of a low-resistance Carbon Film resistor is around 0.02% (200 ppm) and Metal Film around 0.001% (10 ppm) per degree C.
Cheers, Alan.
Chuck is quite "honest" that his "pyrometer" is a "fun" project, with his conclusion that "In spite of the limitations, the simple device shown here is useful as a learning tool.". The current from his PV panel is ridiculously large, necessitating the use of copper wire as the "load resistor"; a really bad idea because of the large temperature coefficient of copper*.
To make a "scientific" measurement you don't normally use a "load" resistance at all, but measure directly in the current domain; typically using an Op-Amp configured with a "Virtual Earth" input that gives zero voltage drop (<~1mV) across the panel/diode. Alternatively, configure the amplifier as a Current-to-Frequency converter (aka an oscillator), which can also perform the ADC function with high accuracy and dynamic range. By remaining in the current-domain, it's possible to use a single photodiode and a $1 microcontroller to achieve a dynamic range from peak sunlight down to moonlight, or almost 1,000,000 : 1 (20 bit resolution), but that's not the purpose of this thread.
As said above, the main aim of this project / thread is to make practical and reliable long-term outdoor measurements. For these, there are much more important practical design issues, such as the Enclosure / Window, with its impact on the Cosine response of the sensor, and the reliability (and low resistance) of the electrical connections, etc..
*EDIT: For reference: Davis quote the Temperature Coefficient of their 6450 Solar Radiation Sensor as +0.12% per degree C, and the TempCo. of copper is +0.4% per degree C (a load resistor is also likely to get hotter than a solar panel, of course). The TempCo. of a low-resistance Carbon Film resistor is around 0.02% (200 ppm) and Metal Film around 0.001% (10 ppm) per degree C.
Cheers, Alan.