solar panel blocking diodes

Ian_Edwards

Ian_Edwards

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I'm about to install 4, nominal 12 volt (17.5 volt open circuit) 50 watt semi flexible solar panels, in parallel.

The data sheet says I should install blocking diodes, which makes sense. I guess I need 4, one for each panel.

I can buy these ready made up in MC4 packages that just plug together, but they are quite expensive at around £12 each, and they are bulky and I have limited space and don't want to clutter up the deck with lots of connectors.

I'm considering a DIY solution, buying the 10 Amp Schottky Diodes and packaging the up in Epoxy.

The question is what's the best form of 10 Amp Schottky Diodes to buy, RS have a wide range, mostly in a 2-Pin TO-220AC package, and I've selected an maximum reverse current of 100 micro amps, and these sell at around the £1 each:

Vishay VS-MBR1045-N3 Schottky Diode, 45V 10A, 2-Pin TO-220AC
Taiwan Semi MBR10100 C0 Schottky Diode, 100V 10A, 2-Pin TO-220AC
Taiwan Semi MBR10150 C0 Schottky Diode, 150V 10A, 2-Pin TO-220AC
Vishay MBR1060-E3/45 Schottky Diode, 60V 10A, 2-Pin TO-220AC
Vishay VS-MBR1035-N3 Schottky Diode, 35V 10A, 2-Pin TO-220AC

Any ideas on which type would be best?
Or doesn't it make any difference?
Or any better suggestion?
And
Would I need to incorporate a heat sink? They'll have a forward current of around 5 amps Max.








 
Ian,
I've no particular experience installing solar panels, but I am an electronics engineer which may make me over-qualified.
There's little to choose between any of the diodes you list. I might choose either the first or the last because, on cursory inspection, they have a lower forward voltage drop thus wasting less of the precious energy.
You will have to attach them to a heatsink. A couple of square inches per diode is probably enough if exposed directly to air.
You might also consider a different diode packaging option, such as this: SDT40A100CT, which puts two diodes into a single package. Might make your wiring simpler, depending on how you locate your panels.
 
Thanks for the reply, I was struggling to see the difference between them, there are obviously some trade offs, a higher tolerance of reverse voltage for example, but it isn't clear to me how the differences affect the application as a blocking diode.

I plan to mount 4 diodes on an aluminium heat sink, I've got some 5mm stuff in the garage and then pot them in epoxy.
That way I can use the diode package as summing point.
I'll need some space around the the diodes to secure the cables. It'll have 4 @ 2.5mm cross section wires coming in from the panels and one 6 mm^2 wire going out to the MPPT controller. I'll make the Aluminium heat sink about 150 mm x 75 mm (6" x 3" in old money) and I guess that'll make a big enough heat sink.

Once again, thanks for the prompt reply.
 
LadyInBed said:
If you are using a regulator, doesn't that act as a blocking diode?
If you want a regulator, I have one (PU1024 12/24v 10A) that you can have for the postage cost.
Click to expand...

Not for 4 panels connected in parallel, where one could be in the shade and another in full sun. In that situation the panel in the shade could be seeing a reverse voltage of, say 15 volts. The MPPT controller will act as a blocking diode at night, when there's no output from the panels, and prevent discharge from the battery.
 
LadyInBed said:
If you are using a regulator, doesn't that act as a blocking diode?
If you want a regulator, I have one (PU1024 12/24v 10A) that you can have for the postage cost.
Click to expand...

Yes and no, I think.
A single panel connected to a regulator wouldn't need a blocking diode, because as you say the regulator will/should prevent the battery discharging through the panel when it is dark.
For panels connected in parallel where one panel may be lit and the other in shade, without blocking diodes some of the current produced by the lit panel will reverse flow through the unlit panel. Whether this happens in real-life I wouldn't want to swear to - I'm not sure if, on a boat, it's possible to have one panel so dark that this happens.
Blocking diodes are not to be confused with bypass diodes. Bypass diodes are needed for series-connected panels.
 
Richard10002 said:
I thought solar panels had built in diodes?
Click to expand...

My 4 100W panels certainly do. I bought them from a specialist solar panel Guy and told him that I needed everything to be enable to use all combinations of series and parallel so I could experiment with configurations. He sold me the 4 panels, all the cables, a selection of MC4 dual connectors and a regulator.

If anything else was needed he would definitely have wanted to sell it to me. ;)

Richard
 
PaulRainbow said:
Link to the panels you are buying please ?
Click to expand...

https://www.photonicuniverse.com/en...ing-for-motorhome-caravan-camper-rv-boat.html

Photonic Universe, seem to have a good selection of panels and where amongst the best prices I could find.

I don't see anything in the specification of the panel that I bought to suggest that they incorporate Schottky diodes to limit reverse current flow, when at least one panel of set set of panels connected in parallel, is in the shade.

Photonic Universe also sell blocking diodes in an MC4 compatible package with the following note:

"This MC4-compatible adaptor with a 10A blocking diode allows you to connect several solar panels in parallel (to common positive and common negative terminals) and run them through a single solar charge controller. It eliminates the risk of back flow of power through solar panels, particularly when different sized solar panels are used in the same array, or when one or more solar panels are prone to shading"

see the following link.

https://www.photonicuniverse.com/en...nels-to-the-same-solar-charge-controller.html

In reality, the panels will still work when connected in parallel, without blocking diodes, but the array won't be as efficient, and you'll never know how many amp hrs your missing.

I also think people get confused between bypass diodes within and array, which allow the array to continue functioning, albeit at a lower efficiency, when one cell is in the shade or is damaged. I'm fairly sure that all modern panels have bypass diodes. I'm not so sure that they all have blocking diodes.

If your still unsure, check with your supplier, that's what I did, and Photonic Universe are probably one of the largest supplier of solar panels, at least for the leisure market in the UK.

My issue is that the commercially available diode packages are relatively big, and want to minimise the clutter (and trip hazards) on the deck. The panels will be mounted on top of the coach roof, under the boom, which will inevitably cast a shadow on one or more panel. The deck at that point is essentially a cover over the recesses which carry the halyard and reefing pennants back to the cockpit, so I have ready access to the underside of the cover, However, there isn't much space so I'm looking for a neat solution.
 
Last edited:
Richard10002 said:
I thought solar panels had built in diodes?
Click to expand...

I think you have to check with the manufacturer's literature and specs.

I know my small Spectralite panel has a blocking diode but I dont think that is by any means universal. I suspect rather the opposite

Marlec's brochure and the instruction manual both state that all the panels in the range have blocking diodes

https://www.marlec.co.uk/wp-content...Lite-Solar-Brochure-011015.pdf?v=79cba1185463

I was surprised to see the picture of my own boat on the brochure!
 
VicS said:
I think you have to check with the manufacturer's literature and specs.

I know my small Spectralite panel has a blocking diode but I dont think that is by any means universal. I suspect rather the opposite
Click to expand...

I have 2 x Canadian Solar CS6P 245P which I bought from Bimble with some cable and a Tracer BN MPPT 40A controller, and just connected it all up, (in series), with fuses into and out of the controller, and it seems to work fine. I dont know much about solar and stuff, but I had heard of diodes and IIRC, when I opened up the electrics junction box on the panels, I recall one or more diode looking things across the terminals in the box - I think there are three terminal type things, +ve -ve, and another in the middle.

The Canadian Solar brochure on line says "J-Box 3 diodes"

I'd never heard of bypass and blocking diodes until now, and perhaps I dont need to worry either at all, or until I decide to change my panels from series to parallel ???
 
Ian_Edwards said:
https://www.photonicuniverse.com/en...ing-for-motorhome-caravan-camper-rv-boat.html

Photonic Universe, seem to have a good selection of panels and where amongst the best prices I could find.

I don't see anything in the specification of the panel that I bought to suggest that they incorporate Schottky diodes to limit reverse current flow, when at least one panel of set set of panels connected in parallel, is in the shade.

Photonic Universe also sell blocking diodes in an MC4 compatible package with the following note:

"This MC4-compatible adaptor with a 10A blocking diode allows you to connect several solar panels in parallel (to common positive and common negative terminals) and run them through a single solar charge controller. It eliminates the risk of back flow of power through solar panels, particularly when different sized solar panels are used in the same array, or when one or more solar panels are prone to shading"

see the following link.

https://www.photonicuniverse.com/en...nels-to-the-same-solar-charge-controller.html

In reality, the panels will still work when connected in parallel, without blocking diodes, but the array won't be as efficient, and you'll never know how many amp hrs your missing.

I also think people get confused between bypass diodes within and array, which allow the array to continue functioning, albeit at a lower efficiency, when one cell is in the shade or is damaged. I'm fairly sure that all modern panels have bypass diodes. I'm not so sure that they all have blocking diodes.

If your still unsure, check with your supplier, that's what I did, and Photonic Universe are probably one of the largest supplier of solar panels, at least for the leisure market in the UK.

My issue is that the commercially available diode packages are relatively big, and want to minimise the clutter (and trip hazards) on the deck. The panels will be mounted on top of the coach roof, under the boom, which will inevitably cast a shadow on one or more panel. The deck at that point is essentially a cover over the recesses which carry the halyard and reefing pennants back to the cockpit, so I have ready access to the underside of the cover, However, there isn't much space so I'm looking for a neat solution.
Click to expand...

Those are very good panels. I suspect though that you will waste more power by fitting diodes to them than you would by partly shaded panels. You will get some partial shading with coachroof mounted panels, but none of them will be totally shaded, so any losses should be small. You will however be getting a voltage loss from all of the panels that are in the Sun, all of the time they are in the Sun.
 
Richard10002 said:
I'd never heard of bypass and blocking diodes until now, and perhaps I dont need to worry either at all, or until I decide to change my panels from series to parallel ???
Click to expand...

Blocking and bypass diodes are not a reference to the actual diode itself, but rather to what it's doing. A blocking diode will stop reverse current, whilst a bypass diode will allow cells, or even a whole panel, to be bypassed if it is shaded.
 
Ian

You've not mentioned how you then plan on connecting the wires. I did similar to you although when I bought 4 diodes I did not realise I'd clicked on four packs of ten!

As to making the connection I seem to remember the legs fitted perfectly into these connector blocks and the thick cables crimped perfectly into the matching lugs.

http://www.furneauxriddall.com/acatalog/Block-Connectors.html
 
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