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Blocking Diode Question please:

octal_ip said:
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.
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thanks again. I'm done!.... shows once again, you just need to ask the right people!
 
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??️
 
upnorthandpersonal said:
They have bypass diodes, not blocking diodes.

articles-solar3.gif


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.
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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.
 
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.
 
BackTOschool said:
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.
Click to expand...

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.
 
Cal said:
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.
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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.
 
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?
 
upnorthandpersonal said:
They have bypass diodes, not blocking diodes.
Click to expand...
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.
 
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.
 
Checkthisout said:
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?
Click to expand...
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.
 
Checkthisout said:
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.
Click to expand...
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.
 
octal_ip said:
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.
Click to expand...

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.
 
Cal said:
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.
Click to expand...
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.
 
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.
 
svetz said:
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.
Click to expand...

This is true, but those serve a different purpose - to prevent power from flowing from your battery back to the panels. If you add multiple parallel strings to the same charge controller, the charge controller blocking diodes will prevent power from the battery from flowing back to the panels, but not from power flowing between the parallel strings of panels.
 
upnorthandpersonal said:
This is true, but those serve a different purpose - to prevent power from flowing from your battery back to the panels. If you add multiple parallel strings to the same charge controller, the charge controller blocking diodes will prevent power from the battery from flowing back to the panels, but not from power flowing between the parallel strings of panels.
Click to expand...
The MPPT will drop the voltage down to whatever results in highest wattage output from the array which will be at or below the voltage of the shaded string.
 
Checkthisout said:
The MPPT will drop the voltage down to whatever results in highest wattage output from the array which will be at or below the voltage of the shaded string.
Click to expand...

I'm not disputing that. Only that the blocking diodes in the MPPT are not the same as blocking diodes in parallel solar strings.
 
I tested my array with and without blocking diodes. for two weeks each way. Same amount of sunlight (which actually took me a couple of months to accomplish). There was no noticeable difference. I believe that any benefit from the diodes was being eaten up by the heat loss at the diodes. So I decided that it was better to remove the extra failure points. Since I wasn't gaining anything in return.
 
It's also worth noting that my array was very unbalanced. Two strings of ten and one string of nine, facing east. Six strings of ten and one string of nine, facing west. I would call it extreme testing. lol
 
timselectric said:
It's also worth noting that my array was very unbalanced. Two strings of ten and one string of nine, facing east. Six strings of ten and one string of nine, facing west. I would call it extreme testing. lol
Click to expand...
Extreme indeed, I'm amazed you didn't see a difference in that scenario.
A great example of the value of real-world testing.
 

This video shows it's better not using blocking diodes.

There's two panels: 24V, 100 W and 12V, 50 W panel. The 50W panel could be considered being a 100W panel with shaded conditions, producing just 50W.

Individually the panels produce:
24V, 100W panel outputs 5.85 A into a 12V battery, with Vmp = 34V
12V, 50W panel outputs 3.08 A into a 12V battery, with Vmp = 18V

When the two panels are in parallel they output 6.29 A, with Vmp = 20V

Conclusion:
If a blocking diode is used then output current is 5.85 A. Without a blocking diode output is 6.29 A. There's better performance without blocking diodes. This is a worst case test. If all panels are the 24V variety then the shaded panel Vmp will be significantly greater than 18V. Output current will then be significantly greater than 6.29 A.
 
to deal with this issue, using multiple MPPT instead of paralleling arrays facing different directions to one MPPT is the approach i plan to implement.

being able to see more granular data about PV performance is also a motivation for me

either way, parallel connecting them to one MPPT without supplemental blocking diodes also seems just fine..

thanks again everyone
 
Checkthisout said:
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?
Click to expand...
I see them used in the Combiner box, see the schematic.

4-1 Solar Panel Combiner Box – 4 circuits to 1 (1 x 63A max output) – Watts247 Wholesale

watts247.com watts247.com
 
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