(27-08-2017, 00:03)jgveill Wrote: Somewhere in the thread, someone (alan I think) came with the idea to use a panel with more cells.
Hi,
Welcome to the forum. No it wasn't me, it was Werk who has proposed using the multiple diodes of a single PV panel right from the start. I was originally working with a single BPW34 (which can be made to give excellent results even interfacing directly with a microcontroller) but I'm now a "convert" that the PV panel is a good solution for this particular application.
If you look at the cell (on the right hand side of the picture) that Werk supplies with his kit of parts, it has at least 3 and probably 6 separate sections (diodes) in series. Although the Op-Amp does have some voltage gain, I don't believe that is its main purpose. For example, a method to calibrate (change the voltage gain) of the interface is needed and a "1 Ohm Potentiometer" is NOT the way to do it !
IMHO, the advantages of the PV panel are much more "practical" than theoretical. One important requirement of a Solar sensor is that it must be absolutely horizontal, which is much easier to do with a small PV panel than a packaged photo-diode. Also, the sensitivity range (or conversion efficiency) of a photo-diode may be quite large (+/- many tens of percent) whilst a PV panel will be "expected" to be optimised for a specified/maximum output from the solar radiation.
Bear in mind that the purpose of the PV panel is to convert solar energy ("heat") into electrical energy, but from then on, "heat is your enemy", because nearly all, electrical components "dislike" getting hot (becoming inaccurate and/or unreliable). So large currents (i.e. large panels) are to be avoided, particularly noting that if the PV panel is "Short-Circuited" then the electrical energy, which would normally have been fed out to do useful work, remains in the panel and heats it up.
PV panels (and silicon photo-diodes in general) have a very significant temperature coefficient. Davis (whose equipment is often considered as "reference quality") quote the temperature coefficient of their (silicon photo-diode) pyrometer, but I'm quite surprised that they don't appear to make any provision to actually measure that temperature.
Cheers, Alan.