sizing schottky for solar

[MVIDSMITH]

I have home built panels of 88, 110, 160, and 220 watts.

I want to put a blocking diode in them before I hook them up. Each one goes to a different charger and or pump.

Is there a formula of how to size a schottky diode so I put the right one in?

Any help out there?

[ghurd]

Usually 2 or 3 times the short circuit current.

Usually 20V for a 12V system.

PV to direct drive pump needs no diode.

'Charger'? Meaning solar controller?  It should have the blocking provision built in.

If the PVs have 36 or more cells, the gain from a Schottky compared to a regular diode is not much.  Sort of expensive to get Schottkys for panels that big.

Did you build the panels?  Would be an interesting story.

G-

[tanner0441]

Hi

Just a point a 20V diode on a 12V panel could see 22 - 23V open circuit, which is OK until you reconnect it.

Brian

[ghurd]

That is Vf, not Vr.

For Vf, the highest measurable voltage is about 0.5V depending on the part number.

The highest Vr will be the battery voltage.  20Vr is fine for a 12V system.

G-

[boB]

Why do you want to add a blocking diode ?   Are you connecting the module to something that generates voltage that could forward bias the module ?

boB

[MVIDSMITH]

The panels I built are simply soldered together using 5 " cells.

different makers of cells have different power ratings.(I only use one type of cell per panel) There are 36 in series to get me the voltage to charge a 12 v battery. I also want to put 2 of them together in a series parallel hybrid.  since these are more or less just a bunch of cells strung together wouldn't it be wise to put in a some sort of blocking diode?  what if one of my two panels was shaded ? Or even if I string 3 or 4 together.

Isn't there a potential to lose power through the panels at night.

I know how to solder these things together  (very delicate like weak potato chips)and I have a little knowledge about electronics but I'm trying to gain a better understanding of why each of the components is needed and how to determine the best match for everything.

Thanks for any help

[ghurd]

"want to put 2 of them together in a series parallel hybrid"

I do not know what that means, but I have a feeling it is not good.

Need to clarify that.

Compensating with bypass diodes for shaded 6V sections of panels can help, in systems of 36 or 48V.

It can help with a MPPT controller.

Usually it will not help.

"(very delicate like weak potato chips)" is about the toughest I ever heard the cells described.

G-

[OuttaSight]

If you're just connecting panels in series to the controller then you shouldn't need a blocking diode at all (as the controller will have one or a MOSFET or relay to disconnect the panels at night).

If you're connecting several panels in parallel then you should use a blocking diode on each panel (or series string... I'm running 24V batteries so have parallel connected series pairs with one blocking diode per series string) so that you don't get shading losses from two unshaded panels back feeding a shaded one.

If you're making some weird hybrid (like I had for a while) with two 40W panels in parallel and then in series with a 80W panel then put blocking diodes on each of the two 40W panels (but not the 80W one).

This stops a shaded 40W panel being back-fed from the other one and provides series back-feed blocking for the whole group at the same time (as my hybrid array was also in parallel with other panels).  You may also have to beef up the internal bypass diodes on the 40W panels as a shaded one could cause the current from the 80W panel to course through just one panel and overheat the cells in it if the bypass diodes in that panel can't let the extra current bypass them (a 40W shaded panel in a pair looks electrically the same as a 80W panel with all the cells half covered up).

Schottky diodes aren't worthwhile at 24V and above. Actually it's really hard to get ones that can withstand the parallel back-feed Voc of the other series strings (can be 50V for just two amorphous 12V panels in series) so I just use P600D diodes that are good for 200Vr and you only lose 0.8Vf.