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Bypass Diodes

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Posted by Jim Avery on February 22, 2004, 7:42 pm
 
Is there any advantage to using Germanium or Schottky diodes rather than
Silicon for bypass diodes ?
In a gridtie system what reverse Voltage rating do the bypass diodes need to
be ?

Thanks.




Posted by Duane C. Johnson on February 22, 2004, 8:24 pm
 
Hi Jim;



Good luck finding Germanium diodes %^(
But seriously, there is no logical reason to use anything
but conventional silicon diodes for "Bypass Protection"
diodes. Assuming the current rating is at least 150% of
the panel's short circuit current.

Of course you still need Schottky "Series Protection" diodes.


Count the number of silicon cells being bypassed.
The the voltage rating should be at least .65V per cell.
Then double it, or at least 150% to be safe.


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Posted by Bill Kaszeta / Photovoltaic Re on February 23, 2004, 2:49 am
 There is a question that should have been asked.

Q: What is the required current rating for a bypass diode?

A: As listed below by Duane, but if the diode is in a junction
box on the rear of the module, it must be derated for the
highest temperature that can occur, or placed on a heatsink
outside of the box.  Depending on ambient temperature,
air flow around the modules, etc., the J-box can reach
90C.  This is why most PV modules need 90C rated wiring.

Most PV manufacturers have specific information on this
subject and the required data sheets should be packed
with new modules.  This is a UL listing requirement.

An under-rated diode can cause severe damage to a PV
module if bypassing is necessary (shading or cracked cells).



Bill Kaszeta
Photovoltaic Resources Int'l
Tempe  Arizona  USA
bill@kaszeta.org

Posted by Graham Parkinson on February 28, 2004, 3:29 am
 Our new (to us) Siemens ST40 panel came with a shorted diode in the junction
box.

I removed the diode and the cell works ok now (after I took it down to
remove the diode...)

Question is do we need the diode on a single panel, single battery setup.
The manual states that the diode is to ensure that multipanel setups don't
feed back into a shaded cell.

With only one panel I assume that the diode is not needed.   We do have a
Morningstar charge controller which I think has a diode to prevent the
battery from back feeding.

Thanks, Graham

Memo - Test panel _before_ installing atop large tree.



Posted by Duane C. Johnson on February 28, 2004, 11:38 am
 Hi Graham;



That would be a "Series Protection Diode".
Not a "Bypass Diode".


Probably not as the leakage current in the modern
PV panels is quite low.
Besides many chargers have their own Series Protection
Diode.
You will also get a slightly higher charging current
without this diode.


If you do replace the Series Diode choose a Schottky
diode rated at least 1.5 times the short circuit and
about double the open circuit voltage of the panel.
 

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---> Re: Bypass Diodes Duane C. Johnso...02-22-2004
| ---> Re: Bypass Diodes Bill Kaszeta / ...02-23-2004
| | ---> Re: Bypass Diodes Graham Parkinso...02-28-2004
| |   ---> Re: Bypass Diodes Duane C. Johnso...02-28-2004
| |   | | ---> Re: Bypass Diodes Duane C. Johnso...02-28-2004
| |   | `--> Re: Bypass Diodes William P.N. Sm...02-28-2004
| |   `--> Re: Bypass Diodes Bill Kaszeta / ...02-29-2004
|   `--> Re: Bypass Diodes Graham Parkinso...03-02-2004
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