Diodes ?

[TwoBlocked]

Sorry to ask what is probably a simple question. Kinda new to PV systems. What I deal with is pretty simple. Like today, a single 200W panel keeping a couple 100 Ah batteries charged up. So I wire up a panel today and it has half a dozen terminals with two of them tied to the panel with another "middle terminal" between them. There are two diodes. One between the Neg and this "middle terminal" and the other between this "middle terminal" and the Pos. It was mentioned that this is so you can wire it up for 12V instead of 24V, but I dismissed the idea. Obviously the diodes would protect the panel from reverse polarity (and also indicate the polarity of the terminals - they were not marked...). But why two diodes, and why have a "middle terminal." My only guess was for a "functional ground" system? Can't wait to be educated by you fine folks.

 

[GoldDigger]

These are called bypass diodes. They are placed so that their forward, conducting, direction is the same as the direction of current in the working panel.

Since all of the cells of the panel are wired in series, the forward current of the illuminated panel would normally be no greater than the current of the weakest or least illuminated cell. If there is shade on a small area of the panel, the voltage of the working cells will try to force current through the shaded cell(s). This will both limit the power output of the panel and also potentially cause thermal damage to the shaded cell(s).
Each bypass diode will allow the current in the panel string to bypass (funny thing about that) the section containing the shaded or damaged cells, with the panel producing power but at a lower voltage. A panel can have anywhere from 2 to 4 bypass diodes (a design decision).

Bypass diodes are more important when the PV array consists of series strings of several (or many) panels. They are not as critical when each panel is connected to its own optimizer or micro-inverter, but even in those cases they allow partial energy production from a partially shaded panel.

They do not specifically protect the panel from an applied reverse voltage. In the olden days, when cell leakage currents were higher than they are today a system, particularly one with several panels or strings in parallel, would include a blocking diode in each parallel element to prevent reverse current from flowing through a panel or string which was not illuminated.

 

[TwoBlocked]

Thanks, I understand now how a diode would be reverse biased with the panel working and forward biased when it is not working, thereby allowing other panels in series to have a current path to the load. What I am still wondering is why there would be TWO diodes in series between the Pos and Neg with the junction of the two diodes being an additional terminal. I took a picture on my mobile phone but was not able to send it. I might try again later.

 

[jaggedben]

It essentially divides the module into two submodules. Then if the cells on one submodule are shaded (dirty, damaged) the other submodule still works better.

Most commercial modules with 6 columns of cells have them connected in three 'U' shapes, with three diodes. As seen here:

Google Image Result for https://www.mdpi.com/energies/energies-13-02472/article_deploy/html/images/energies-13-02472-g003.png

I'd guess if you only have two diodes your panel has only 4 columns of cells? (Doesn't have to be arranged that way, but would resemble what I've seen.)

 

[LarryFine]

Two diodes in series effectively behave as one.

 

[RumRunner]

Sorry to ask what is probably a simple question. Kinda new to PV systems. What I deal with is pretty simple. Like today, a single 200W panel keeping a couple 100 Ah batteries charged up. So I wire up a panel today and it has half a dozen terminals with two of them tied to the panel with another "middle terminal" between them. There are two diodes. One between the Neg and this "middle terminal" and the other between this "middle terminal" and the Pos. It was mentioned that this is so you can wire it up for 12V instead of 24V, but I dismissed the idea. Obviously the diodes would protect the panel from reverse polarity (and also indicate the polarity of the terminals - they were not marked...). But why two diodes, and why have a "middle terminal." My only guess was for a "functional ground" system? Can't wait to be educated by you fine folks.

Someone is “barking up the wrong tree”.

Your allusion regarding the purpose of those diodes are “theoretically correct”.

I say theoretically correct because (as you mentioned) they are protection from polarity reversal.

You did not mention the voltage rating of the PV .

I’m assuming this is for a residential use 12 volts to accommodate the generic house car.

Industrial and Commercial applications are usually -- around 24, 36 and 48 volts.
The middle terminal that connects the positive and negative terminals of the diodes provides an option to the installer to ensure congruity with the MPPT that is available on the market.

There are two types of diodes on the PV, they are:

1. The blocking diodes.

2. The bypass Diodes (these are is the ones being beaten to death.)

Generally the bypass diodes are inherently built-in to the PV array. They are not designed to be altered or removed by installers or they are non-serviceable by anyone outside the factory.

As mentioned by other posters, it shunts the under performing PV cell or if the sun rays being blocked. This is to ensure continued supply wihout losing power even though at a lower voltage.

On the other hand-- Blocking diodes prevent current going in reverse to cause battery discharge at night when there is no PV production .
These are what you have
They are usually mounted in the combiner box or at a location separate from the PV ARRAY, that are accessible to installers and technicians.

I’m aware that this comment will raise “hackles” and maybe cause an avalanche of unkind words among purported experts –but, I say “BRING IT ON”

 

[TwoBlocked]

Am thinking that the termination box with 6 terminals and two diodes in series (behaving as one - Larry) is a generic device that is used for different applications by the manufacturer. It is the manufacturer that would make use of the middle terminal in another application, not the installer. The diodes could only act as bypass diodes, not as blocking diodes, because they are in parallel with the solar panel, not in series. But if for some reason reverse polarity were applied, either a fuse would blow or the diodes would blow.

For general info, this is for an industrial application as a stand alone, 24V power supply going to a small PLC and HMI. There is one 200W, 38V (max measured) panel, two 12V 110 Ah batteries in series and one Sunsaver controller. Since this must provide enough power to keep the batteries charged in the dead of winter, the panel was set to a 60 deg vertical angle.

 

[ggunn]

But if for some reason reverse polarity were applied, either a fuse would blow or the diodes would blow.

Not if that reverse polarity is from batteries. Current from batteries is determined by load, and the high resistance of a reverse biased diode keeps the load to virtually zero.

 

[TwoBlocked]

Not if that reverse polarity is from batteries. Current from batteries is determined by load, and the high resistance of a reverse biased diode keeps the load to virtually zero.

The volage from the solar panel applies a reverse bias to the diodes and so they do not conduct, unless it is a bypass situation where the panel is not providing a reverse bias voltage. But if the voltage polarity to the solar panel is revered, say from batteries, the diodes become forward biased and then they do conduct without a load - a nearly dead short. Fuses would blow or diodes would blow.

 

[ggunn]

The volage from the solar panel applies a reverse bias to the diodes and so they do not conduct, unless it is a bypass situation where the panel is not providing a reverse bias voltage. But if the voltage polarity to the solar panel is revered, say from batteries, the diodes become forward biased and then they do conduct without a load - a nearly dead short. Fuses would blow or diodes would blow.

I don't understand how that could happen; can you draw the circuit?

 

[winnie]

If you connect the 'production +' of the PV array to the - of the battery, (and - to +) then I'd expect > Isc to flow through the PV cells and the bypass diodes to be forward biased.

I'd also expect a race to see which part releases smoke first.

Jon

 

[ggunn]

If you connect the 'production +' of the PV array to the - of the battery, (and - to +) then I'd expect > Isc to flow through the PV cells and the bypass diodes to be forward biased.

I'd also expect a race to see which part releases smoke first.

Jon

Why would anyone do that?

 

[TwoBlocked]

I don't understand how that could happen; can you draw the circuit?

Consider a circuit with just one of the solar panels and it's bypass diode. with the DC polarity as shown, the diode is reversed biased and does not conduct current. But if for some reason a battery provided an opposite polarity, the diode would become forward biased, resulting in a near dead short.

How could this happen? The conductors given with the panels I installed were colored black and white. There are two conventions I've seen for which is positive... In the installation I was involved with, the panel and the batteries each went separately to a controller that I expect would not allow the wrong polarity from the batteries to blow the panel. But what if there was not a controller? Maybe something someone hacked up? When installed during the day, the panel voltage would be higher than the batteries, but at night? But that is what blocking diodes would be for.

Hey, this is a bit new to me. I'm just saying how I see it and am interested in other views, too.

 

[ggunn]

Consider a circuit with just one of the solar panels and it's bypass diode. with the DC polarity as shown, the diode is reversed biased and does not conduct current. But if for some reason a battery provided an opposite polarity, the diode would become forward biased, resulting in a near dead short.

How could this happen? The conductors given with the panels I installed were colored black and white. There are two conventions I've seen for which is positive... In the installation I was involved with, the panel and the batteries each went separately to a controller that I expect would not allow the wrong polarity from the batteries to blow the panel. But what if there was not a controller? Maybe something someone hacked up? When installed during the day, the panel voltage would be higher than the batteries, but at night? But that is what blocking diodes would be for.

Hey, this is a bit new to me. I'm just saying how I see it and am interested in other views, too.

View attachment 2565285

Oh, I see. I thought that you were saying that it could happen in a properly designed and connected system.

 

[jaggedben]

Why would anyone do that?

By mistake. It happens. There's even a code section about marking polarity to help avoid it.

 

[ggunn]

By mistake. It happens. There's even a code section about marking polarity to help avoid it.

Well, OK, but there are all kinds of mistakes one could make in wiring a system that could or would result in equipment damage and/or injury.

 

[TwoBlocked]

By mistake. It happens. There's even a code section about marking polarity to help avoid it.

Yeah, but it can get ugly. NEC requires all grounded current carrying conductors to be white (or gray, or whit stripped or ...) and these colors not used elsewhere 200.6. But when you start with a battery with red (+) and black (-), ground the negative, take thenm to a terminal strip, and then have a cable with a black and a white wire, what then? It's a very common problem in industrial instrumentation. Whatever convention is already there, I just stick with it. And to make matters worse consider a 4-20 mA loop where there are maybe 4 devices in series and the twisted pair is black and white.