PV Panel Diodes

[henjulfox]

Folks,

I've picked up some used 64 watt PV panels. The plan is to wire them in banks of 5, in series, to feed an Outback MPPT charge controller. They will receive some partial shading late in the day.

Under this situation I believe diodes are recomended. They have diodes installed but since they are of unknown age I figured I would replace them before bolting down the panels.

I've been to my usual store (ebay) looking for Schottky diodes in the range of 10 amp, 100 volts and came up empty. There were other non-Schottky diodes available at this capacity.

Anyone know if Schottky's exist in this range? Would another type be acceptable?

Thanks,

-Henry

[Nando]

If you are going to have 5 panels in series, ( You have not indicated the panel voltage), you can use a dual Schottky, one in series and the second part in parallel.

Also you only need to have 30 Volts break down and around 10 or 15 amps -- it will require heat sinking.

You are going to have some panels in the shade and for that reason alone, you need a diode in anti-parallel to the panel to insure that the panel is bypassed if is not producing any energy.

I have many, but at the moment, I do not remember the nomenclature, two diodes in a plastic power TO-3 size. 30 Volts , 15 amps, made by Motorola.

Where are you located ?.

http://www.onsemi.com/pub/Collateral/MBR41H100CT.PDF - 100 volts, 40 A (2 - 20A)

http://www.onsemi.com/pub/Collateral/MBR2060CT-D.PDF - 60 volts, 20 A (2 - 10)

www.mouser.com have them

www.digikey.com other place to check you can buy them via credit cards.

Nando

[henjulfox]

Nando,

Thank-you, Mouser.com was what I was looking for - a place that carries everything electronic.

I was puzzled at the diode ratings you suggested, 30 volts at 10 amps. A string of 5 in series would be 18 volts X 5 = 90 volts at 3.5 amp. I've studied the diagram on the back of the panel, done some back of an envelope sketching, scratched my head, and I think I've got it.

A "blocking" diode would be in series with the entire string. I would need only 1 per string and would need to be sized to carry the load of the entire string, in my case 90 volts @ 3.5 amp. It's purpose is to pull the entire string out of the circuit if the power is trying to flow backwards, such as night discharge.

A "bypass" diode (I'm still a little fuzzy on this one) is in parallel with each individual panel. 1 per panel sized to carry the load of 1 panel only, in my case 18 volts at 3.5 amp. It provides a circuit around an individual panel that is trying to absorb instead of provide current.

These particular panels are constructed as 2 X 6 volt circuits, therefore 2 diodes per panel, carying 9 volts at 3.5 amp.

From there, always over size a diode by at least 2 X the expected current, therefore your suggested 30 volt, 10 Amp suggestion.

Am I close?

-Henry

[Nando]

Henry:

You need to think a bit about how to connect them.

I can not follow your analysis of 18 X 5 = 90 Volts, - what is 18 volts ? The panel voltage --

Think panel = battery of the same voltage, output diode then all 5 in series = total voltage = #panels X 5 = 90 volts - ( 5 X 0.6 volt / diode )= 90 - 3 volts = 97 volts out.

Each diode ( anti-parallel) sees a panel voltage in reverse if using 30 volts at 10 amps then those all series represent 30 volts X 5 = 150 volts

The diodes voltage in series does not add, you are thinking in reversal voltage and for that case you need just one, the top connection if such is needed - and is not needed.

Mouser.com have some diodes that are 40 amps ( 2 -20 amps)

MBR40250 = 100 VOLTS 40 AMPS ( 2 - 20 )

MBR6045PT-D = 60 AMPS ( 2 30 A) 45 VOLTS

Nando