Bypass Diode in Small System

whackamole · June 2011

I've got my Sunsaver MPPT connected to 2 of my 6 BP SX20U 20w solar panels connected in series for 24v, charging a 12v battery. The other 4 panels are in a box in the garage at the moment - don't ask ... time just flies when you are a hobbyist - and I'm going to hook them up as 2 x 3-panel 36v strings.

When adding 3rd panel for a 36v string I'm reading I really should be using a bypass diode for each panel to handle one panel getting shaded, and the other two trying to pass their current through the shaded panel.

I also understand most newer panels have bypass diodes built in for strings WITHIN the panel. My panels' spec sheet says I can add bypass or blocking diodes in the junction box - it is big - and there is no bypass diode in their right now (ie nothing in the junction box, and no bumps anywhere in the panel or backing that might indicate a diode.) So three questions

a) The panels have two 6v/18 cell strings in them. Should I install one bypass diode for each 18 cell string? It seems I"m finding as I read I could be damaging a cells in a panel if part of it gets shaded (even if not in a panel string.)

b) Should I install another bypass diode around each WHOLE panel, when connected in a 3-panel 36v string?

c) The host store shows a bypass diode with 3 legs. I expected two. (positive and negative) Can someone point me in the right direction on this? My junction box is a 6-terminal one, and it is just not clear to me how I'd connect a 3 legged bypass diode, for (a) or (b) above.

BB. · June 2011

Re: Bypass Diode in Small System

It appears that they have the wrong TO-220 case... The data sheet they have for the part shows only two legs (plus the metal tab if it is that version).

http://lib.store.yahoo.net/lib/wind-sun/MUR-DataSheet.pdf

From NAWS faq--They say that to bypass every 18 cells... The idea is to keep the reverse voltage across shaded cells from getting over ~12 volts (too high, and you can damage the solar cells--which are just giant diodes themselves).

If you put two diodes (across every 18 cells), you will be fine. You will not need another on each panel.

-Bill

whackamole · June 2011

Re: Bypass Diode in Small System

Thanks for the information.

Because each diode will handle 1/2 the panel's cells, and each panel has a ~1.2 amp 16.9 volt rating, can each diode be rated to handle 1/2 of this?

I also forgot to confirm one other thing. While the charge controller renders blocking diodes unnecessary for the purpose of preventing battery current leakage into the panels at night, do I need a blocking diode on each string just to prevent some sort of problem where one string puts current into the other? This part was a little confusing to me when I was reading about diodes.

BB. · June 2011

Re: Bypass Diode in Small System

The panels (internal strings) are in series... So the 1.2 amps will flow through both diodes equally. The diodes from NAWS are are rated at 15 amps (with proper heat sinking). They should work fine at that low of current.

When you install panels in series--the Imp/Isc should match fairly closely (within 10% or better). (i.e., a 1.2 amp panel in series with a 1.1 amp panel). You cannot/should not put a 1.2 amp panel in series with a 5 or 10 amp panel (it will not work correctly and can damage the smaller panels).

The blocking diode is just to limit leakage current at night... Not really needed on 12 volt systems, and should be installed on 48 volt systems (if no charge controller is installed--most people install charge controllers this days).

There should be no need to install a blocking diode per string otherwise (that I am aware of).

-Bill

mike95490 · June 2011

Re: Bypass Diode in Small System

BB. wrote: »

The panels (internal strings) are in series... So the 1.2 amps will flow through both diodes equally. The diodes from NAWS are are rated at 15 amps (with proper heat sinking). They should work fine at that low of current......

Parallel diodes don't always share current equally.

whackamole · June 2011

Re: Bypass Diode in Small System

OK this should be pretty straightforward. I'll assume each diode needs to handle the entire panel's rating plus some fudge factor.

I realize these are small panels, but theoretically, when would I need to add a heat sink for diode?

mike95490 · June 2011

Re: Bypass Diode in Small System

whackamole wrote: »

OK this should be pretty straightforward. I'll assume each diode needs to handle the entire panel's rating plus some fudge factor.

I realize these are small panels, but theoretically, when would I need to add a heat sink for diode?

You will have to read the data sheet for the diode. Small SMT diodes need the mounting substrate to carry the heat away, Larger diodes may have enough surface area and lead mass, to be ok.

Bypass Diode in Small System

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