Calculating string amperages [RE-wrenches]

[boB Gudgel]

Hi...

You can think of PV module as a bunch of silicon diodes in series, like 
36 or more of them. The positive terminal is the anode and negative 
terminal the cathode.

The Voc happens to be appoximately equal to the forward drop of these 
series string of diodes and will start to conduct when presented with a 
voltage at around that point.  They will conduct when the sun is shining 
on them or not. This voltage drop comes down as their temperature rises, 
just like the Voc and Mpp voltage.  This diode characteristic is also 
why they are sometimes used as a voltage clamp when paralleled with a 
hydro or wind turbine.

Of course they don't have to work great as a diode and will have some 
leakage, so that's why they can act like a resistor at night.  Most 
charge controllers have a relay or back to back MOSFETs (like the PWM 
types CCs) to reduce this night time discharge.

I'm not sure, but I 'think' that PV modules may be better in the leakage 
department these days because of better manufacturing processes.

boB




R. Walters wrote:
> 
> 
> I'm passing Bill's  post to the list for everyone. Thanks BIll.
> My understanding is that one of the basic functions of a charge  
> controller is to prevent back feed of current to the modules. A  
> normal functioning solar module becomes a resistor without sunlight  
> on it.
> Would one of the Manus care to respond?
> Also, this comes back to the original start of this thread: where and  
> what are the parallels MC connectors for, if we can't use them?
> 
> Ray


boB Gudgel K7IQ
MidNite/Magnum  Solar/Energy
Washington AC


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