Hi Mike
On (16 Nov 97) MIKE ROSS wrote to Alec Cameron...
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re this bit of rough math-
 AC> what is "lost" is 0.7 of the 30 volts available on open cct. .7 div
 MR> by
 AC> 30 =   2.33%. Less than a wee cloud passing by.
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 AC> If you find the concept of 0.7 being an emf unconvincing, then put a
 AC> voltmeter across the diode while increasing/ decreasing the panel
 AC> output eg by shading it. You will see that the 0.7v does not change.
 MR> Gotta disagree a bit on the .7 volt diode value. If you look at the spec
 MR> sheets of common rectifier diodes you'll see numbers closer to 1 volt at
 MR> a stated current. This is a much more appreciable loss.
I pulled that number out of air. Of course there are as you say different
diodes. And the characteristics vary especially with new developments. Shut
your eyes, open them again and hell! ten new semiconductors have been
designed.
 MR> In theory the drop should be about .7 volts but the difference is due to
 MR> the bulk semiconductor material itself has some resistivity between the
 MR> junction area and the connection. At rated current this is an additional
 MR> loss.
This "resistivity" is not finite but is dynamic and temperature responsive.
Mustn't be though of as a published value as with metals. Two important
features:
                Inverse temperature coeff- the conductivity increases greatly
                with forward [load] current, so the heat loss is nicely
                limited
                Reverse voltage dependent- the leakage current increases
                greatly if the blocked voltage is increased.
 MR> What he should be using is a Schottky type diode. This has a forward
 MR> junction voltage of about .25 volts. With bulk resistive losses this
 MR> comes up to about .5 volts in practice or about half the loss of an
 MR> common rectifier diode. The difference isn't much but if you're trying
 MR> wring all the efficiency possible then it's another option.
My point is that with panels driving batteries, maximum efficiency is NOT the
proper goal. Maximum energy into the battery is what we want, and we accept
the penalty that the panel will be heated by its own resistive losses [Ohm's
Law] as well as by the sun light.  Cheers...ALEC
... ........Genealogists do it yesterday
--- PPoint 1.92
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