Blocking Diode Question please:

[timselectric]

Your not alone. We all do the same thing.
The great thing about this community is that we don't each have to test every scenario. Odds are someone already has.

 

[BackTOschool]

How do you buy someone a beer on here ?

 

[BackTOschool]

Sure they do, the Amazon one you found. Ignore the 'high' Vf. See if they actually make a difference.

As soon as you go higher voltage, your Schottky diodes will automatically have a higher Vf anyway. For example, this 200V (you have to keep in mind that your Voc is going to go up in cold weather) Schottky diode (20A, 200V):

https://eu.mouser.com/ProductDetail/Vishay-General-Semiconductor/V20PW22-M3-I?qs=7MVldsJ5Uaw6TU%2Fs0iCcEw%3D%3D

Vf = 740 mV

If you come to the conclusion that blocking diodes are really needed, you can invest in making a proper PCB etc. Before that, try one of the MC-4 versions and see if they really make a difference.

Thank you for your reply, really appreciated.

 

[BackTOschool]

They'd be fine, they'll just produce a bit more heat.

0.75v @ 10.2A = 7.65W heat/wasted energy.
0.22v @ 10.2A = 2.24W heat/wasted energy.

Worst case it's a 0.4% difference in the total maximum output of your array. Not worth worrying about.

Finally, can I please query:

"0.75v @ 10.2A = 7.65W heat/wasted energy.
0.22v @ 10.2A = 2.24W heat/wasted energy."

Mine are 117V and 10.2A they are not the same as above ? ( I'm probably misunderstanding something here so apologies)

thank you.

 

[octal_ip]

Finally, can I please query:

"0.75v @ 10.2A = 7.65W heat/wasted energy.
0.22v @ 10.2A = 2.24W heat/wasted energy."

Mine are 117V and 10.2A they are not the same as above ? ( I'm probably misunderstanding something here so apologies)

thank you.

You only need to care about the voltage drop across the diode, which is where the inefficiency and heat comes from. The total voltage of the system doesn't really have any effect on this.

 

[Checkthisout]

My bad....

 

[BackTOschool]

You only need to care about the voltage drop across the diode, which is where the inefficiency and heat comes from. The total voltage of the system doesn't really have any effect on this.

thanks again. I'm done!.... shows once again, you just need to ask the right people!

 

[curiouscarbon]

this thread helped me learn more about the function of diodes in solar panel configuration ?? thanks all for sharing.

---

on a related note, i assembled a 6S string of 5W (yes, 5.0) solar panels that hang vertically on a wall outside. neglected to include Bypass diodes in parallel with each panel. when all panels are in sunlight, it works pretty well. but as soon as one is shaded, power production goes down the drain. time to solder a fix ?

cheers again for this educational thread??️

 

[42OhmsPA]

They have bypass diodes, not blocking diodes.

I tend to use a Schottky diode from Mouser or some other component store specifically tailored to my needs. Otherwise, for small arrays, just go with the off-the-shelf MC4 ones. You also want to run some tests to see if they make a difference at all.

Thanks for the education. I know I've read that at least 37 times, it will stick now.
Edited my first post for clarity of other readers.

 

[Checkthisout]

If I had a system professionally installed, grid connected, permitted etc.

Where would they place or have placed the blocking diodes in the system?

The one system on a coworkers house has panels at all different angles all over his busy roofline but uses micro inverters at each panel.

 

[Cal]

That's helpful to know, thanks.
seems a consensus they have minimal impact at best. So I'm now leaning towards not include them.
Can I ask your opinion ( on the comments ) made by another purchaser on Amazon re MC4 30A 1000V blocking diodes.
"..... do not contain a Schottky blocking diode rather a "Schottky rectifier" and the reviewer went on to say "The mic rectifier has a much higher forward voltage drop, 0.750V"...."My 20A Schottky diode that I bought separately from amazon voltage drop of only 0.220V"

If I did choose to use them ( I have already purchased ) Do the comments above mean they are not suitable because they will use more power?
is it negligible impact overall, or does he have a valid point and they should not be used?
thank you.

It appears the diode is built into the MC4 connector. Amazon provides no diode spec. This diode probably has similar specs.
20A 1000V diode
At 10A, diode drop is 0.9V. Power dissipation is 9W. Thermal resistance is 4 C/W in free air. But his diode is contained in the plastic MC4 connector. Therefore thermal resistance could be a lot higher. In free air the diode temperature increases by 36 deg.C. If ambient is 30 C then diode temp is 66 C with free air cooling. Being in a case will make it hotter. Max diode temp is 175C

Probably not a good idea to use this configuration.

 

[octal_ip]

It appears the diode is built into the MC4 connector. Amazon provides no diode spec. This diode probably has similar specs.
20A 1000V diode
At 10A, diode drop is 0.9V. Power dissipation is 9W. Thermal resistance is 4 C/W in free air. But his diode is contained in the plastic MC4 connector. Therefore thermal resistance could be a lot higher. In free air the diode temperature increases by 36 deg.C. If ambient is 30 C then diode temp is 66 C with free air cooling. Being in a case will make it hotter. Max diode temp is 175C

Probably not a good idea to use this configuration.

Agreed. I originally experimented with some 60V TO-220 Schottky diodes for blocking (before I found they weren't necessary). With a small heatsink attached in open air, within a minute at 10A (0.54v voltage drop) they were burning hot. I can't see how these tiny encased diodes would last long.

 

[Checkthisout]

Help me out here.

Can't we test if we need these diodes by simply installing our system, then disconnecting the shaded side array and seeing what happens to the output at the charge controller?

Maybe make a little dc breaker and add a couple mc4 connectors and plug that in before sunup to use for testing purposes as a switch. Or just shut off the breakers to each string at the combiner box ?

Why would this be something you need to figure out during initial design instead of after install?

How often do you see blocking diodes in professional setups and what do UL listed blocking diodes look like?

 

[svetz]

They have bypass diodes, not blocking diodes.

Just as panels typically have the bypass diodes pre-installed, typically the SCC typically has the blocking diodes built in, it should say on its datasheet or manual.

 

[Checkthisout]

I propose the MPPT controller will pull the array voltage down to whatever the peak output voltage of the shaded side is and theirfore blocking diodes will never provide any noticeable benefit.

 

[octal_ip]

Help me out here.

Can't we test if we need these diodes by simply installing our system, then disconnecting the shaded side array and seeing what happens to the output at the charge controller?

Maybe make a little dc breaker and add a couple mc4 connectors and plug that in before sunup to use for testing purposes as a switch. Or just shut off the breakers to each string at the combiner box ?

Why would this be something you need to figure out during initial design instead of after install?

How often do you see blocking diodes in professional setups and what do UL listed blocking diodes look like?

Unless you know in advance that your parallel strings are going to be out of balance due to panel mismatch (in which case you probably shouldn't have them in parallel), I'd just connect them all up and use a clamp meter (or an inline ammeter with appropriate care) to measure current direction at various times of the day.

 

[Cal]

I propose the MPPT controller will pull the array voltage down to whatever the peak output voltage of the shaded side is and theirfore blocking diodes will never provide any noticeable benefit.

I don't believe that's the case. The MPPT controller will look for the maximum power point. The shaded panel will have a lower power point that the un-shaded panel. There will be several different power points. Just one is maximum. A good controller will find the correct power point and disregard the others.

Blocking diodes are not necessary.

 

[Checkthisout]

Unless you know in advance that your parallel strings are going to be out of balance due to panel mismatch (in which case you probably shouldn't have them in parallel), I'd just connect them all up and use a clamp meter (or an inline ammeter with appropriate care) to measure current direction at various times of the day.

I don't think it matters with an MPPT controller.

I think an MPPT controller will pull the voltage down as low as it needs to go to allow peak output.

If the shaded string in my array has 60 volts open circuit and the sunny side has 100 volts, the charge controller will pull the array voltage down to whatever voltage allows maximum wattage out of the array.

 

[octal_ip]

I don't believe that's the case. The MPPT controller will look for the maximum power point. The shaded panel will have a lower power point that the un-shaded panel. There will be several different power points. Just one is maximum. A good controller will find the correct power point and disregard the others.

Correct. It'd find the maximum power point close to where both strings are contributing as much as they can. It wouldn't be as optimised as an individual MPPT per string, but it wouldn't be far off, either. The MPPT voltage of a string of panels doesn't vary with lighting conditions as much as one might expect.

 

[Checkthisout]

So in short, the reason blocking diodes are hard to find in an easy to use form is because they aren't necessary with an MPPT controller and will only eat up precious juice and add a point of failure.

Also, a cursory check of the open circuit voltage of any given panel will show you that it takes very little light to get there.