Bypass Diodes

[Jim Avery]

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.

 

[Bill Kaszeta / Photovoltaic Resources]

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).

Hi Jim;

Is there any advantage to using Germanium or
Schottky diodes rather than Silicon for bypass diodes?

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.

In a gridtie system what reverse Voltage rating do the
bypass diodes need to be?

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.

Thanks.

Duane

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Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA
[email protected]

 

[CEC]

Hi,
In general, a Schottky diode has less voltage drop across it, so,
running the same current, over silicon vs. Schottky, will
give less heat inside the Schottky one (I think, about 25 % less
power dissipated).

Schottky diodes are in general rated with lower reverse voltage.
Schottky diodes are in general more expensive than the silicone ones.

Germanium diodes are sometimes used as RF rectifiers, with current at
0.01 A only. Those diodes melt-down very easily.

Thanks,
CEC

 

[Graham Parkinson]

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.

 

[William P.N. Smith]

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

Then why would the panel not work before he removed it, and work fine
after it's been removed? How would one even notice a Series
Protection Diode that was shorted?

 

[Bill Kaszeta / Photovoltaic Resources]

Siemens built bypass diodes into the ST40 junction box.
The problem with this is that a reversed connection to a battery
can really blow up the diode. I had several customers with
this problem.

For 12-volt systems you do not need the bypass diode
and can remove it.

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.

Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA
[email protected]

 

[Graham Parkinson]

Thanks Group, for the assurance that a diode is only "a complication" on a
single panel system

Don't miss the solar powered "significant findings" being released by NASA
in the press conference 2pm EST Tuesday. (Salty Brine found in trenches,
"BB"'s are _____ forms of _____)

Graham

 

[Anthoney]

I have been reading about installing bypass diodes within a photovoltaic panel ( DIY) to prect the cells from shadowing that may occur.
cosidering, 36 cells at 1.8w, 3.6 amps, 0.5v would a diode per 18 cells be sufficent? thank you