Solar diode savvy needed.

Discussion in 'General Electronics Chat' started by Dave 2000, May 3, 2015.

  1. Dave 2000

    Thread Starter New Member

    May 3, 2015
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    Hi all, first post and all that, as is the norm I need some help with a little project and hope you guys and girls can help? I have posted this question on solar forums and it resulted in a lot of bickering and information mixed with misinformation and no conclusion. I am hoping by getting the information from the 'horses mouth' it will be valid and proper.

    First the basics:

    4x4 used for camping, presently has one solar panel to keep battery topped up and run fridge, works fine over a couple of days but, after a longer term the battery voltage starts to fall, this is because other devices get used, so I will eventually have two solar panels connected for any camping for more than say a weekend.

    Presently fitted panel is 110 W/6.6A/16.7V and the next one I am about to purchase is 100W/5.62A/17.8V

    This was chosen as it is a flexible one and will be glued to the top of the 4x4.

    The issue is they are going to be paralleled two or three times per year so I guess I should put a diode on the positive lead of each of them, as one will often be shaded.

    I can only find 6 amp Schottky diodes from Maplin (50v) and the guy behind the counter was not electronics savvy.....'yeh I know', and as they had nothing else shown in the catalogue he felt they would be fine as the max current was going to be 6.6 and there was 'some' tolerance.

    I bought 6 thinking I could match two for each panel but after searching I found (on this site) that this practice is frowned upon?.. Will there be sufficient tolerance in the typical 6 amps I have to cover the possibility of 6.6 amps?

    TIA......oh and no fighting!

    regards

    Dave
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Dave 2000

    Thread Starter New Member

    May 3, 2015
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    Arrrrghhhh!!! I was only in the UK for a few days and had popped in to Maplins on the quick as hobby shops here in Spain are a bit thin on the ground. Now WHY didn't the guy recommend this? He had the catalogue and said the 6 amp was the best he had, may be I misunderstood him and he meant this was all he had in stock!

    Thank you so much for that Bertus, I just knew I had come to the right place!

    Now I will need to email them, and then when they have them in stock get a family member to pick them up and post them over as Maplin do not do post outside the UK.

    Thanks again

    regards

    Dave
     
  4. wayneh

    Expert

    Sep 9, 2010
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    I speak from a position of complete ignorance about what is available to you, but a suitable Schottky diode is not an exotic part. I would think anyone selling electronics would have something for you.
     
  5. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Are you sure that the panels do not have diodes already built-in? Most larger panels do.

    The way to find out is simple. Take a 12V power supply (your battery). Put your DMM in DC mA mode between the positive lead of the panel and the battery. When illuminated, current should flow from panel to battery.

    If you make the panel totally dark, either of two things will happen. The reverse current from battery to panel will be ~zero, meaning there is a diode inside that panel. Or, it there is substantial reverse leakage (10s to 100s of mA), then an external diode is needed.

    Do the experiment for both panels... add the diode only to the one that doesn't have one.

    If there are diodes in both panels already, they can be paralleled with no external diodes...
     
    Last edited: May 3, 2015
  6. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    It's unlikely you will need an isolation diode to parallel panels that close and in the sun at the same time. The Voc of the shaded panel will still be high with little or no reverse current flowing into that panel unless it's completely dark. Almost all commercial panels (not just a solar cell) with a similar rating already have bypass diodes in a module behind the panel to handle shading issues.
     
  7. Dave 2000

    Thread Starter New Member

    May 3, 2015
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    Hi and thanks for the replies.

    @wayneh, I totally agree, during the ordering earlier I was asked to complete the Maplin online survey, I did point out that having staff that do not understand ohm's law is a bit lame, :eek: if anyone reads these surveys it might bode well for the future?

    @nsaspook, I have tried the shading test in the past, and can confirm there are bypass diodes between the sub panels to allow for partial shading, but am unsure if they will stop leakage to the overall panel. I am guessing that they may well do but I am a belt and braces kind of guy. I have just ordered the diodes Bertus suggested and had them sent to one of my sisters addresses, she can forward them to me later.

    I was unsure how to test the panel for reverse protection so now thanks to MikeML ;) I do. I think I am right in saying that if I had additional diodes no harm will be done to the amperage available but my forward voltage will fall? I think the controller I have should still see more than 14.4 volts so will still have something to regulate.

    Thanks again all, I will come back with an update, best way the panel I have will have reverse current protection as will the new one, worse case scenario I have ten diodes in the spares box, I may just fit the 16 amp versions just for the he11 of it. :)

    regards

    Dave
     
  8. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    Nothing wrong with being safe and the extra diodes provide some protection from shorts at the panel but I just fused for protection as solar panels are current sources not resistors even at very low light conditions and as long the shaded Voc in low light condition is above the loaded voltage nothing will reverse flow into that panel from any number of parallel panels.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    IME: Maplin charge about 2 -3x what similar or better parts cost from suppliers such as Farnell - who have facilities in several countries.

    Every time I've dealt with Maplin by post - they *ALWAYS* sent the wrong parts. On one occasion I was buying resistors in quantity from the nearest shop, they didn't have any stock of one value, but said since I'd spent more than the minimum they'd have the warehouse send them to me. Naturally the wrong value arrived, I sent the strip of 10 resistors back with a letter explaining this and they sent back *ONE* resistor, still the *SAME* wrong value - it took a few weeks of phone calls and emails to actually get the parts I'd paid for.

    The same happened when I ordered a significant quantity of turned pin IC sockets - that took a couple of months to sort out!
     
  10. tom_s

    Member

    Jun 27, 2014
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    10a/30a bridge rectifiers are common and may be cheaper than going for a specific diode
     
  11. Dave 2000

    Thread Starter New Member

    May 3, 2015
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    I think as I have them (subject to ian_fields comments) and they are correct I will fit one in the + of each panel followed by the fuse.

    During the survey I did point out they were expensive, delivery wise this will be my first experience.

    Ah the er....'bridge rectifier' of course why didn't I think of that :confused:................tom you have over estimated my electronics knowledge mate. :D

    regards

    Dave
     
  12. camerart

    Active Member

    Feb 25, 2013
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    Hi,

    I have two solar panels on my van, with two control boxes and two batteries, with a switch over switch. One of the panels has a diode and one doesn't. As I understand it, the one without a diode is more affective, because the diode takes a little power out, it seems that the control box doesn't allow return voltage.

    Or you could put them both in series, but check that if you use one, the controller can take the higher input voltage.

    These controllers work as voltage pumps, so when the sun is low, will pulse the proper charge voltage.

    I think the flexible panels have the lowest output, where the polycrystaline ones have the highest output.

    Camerart.
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Something to consider as a solution to both problems (unequal panel capacities and joining their outputs) is to do what the home power people do. Put and inverter or buck/boost converter on each panel and then sum the outputs into a common storage device. Being able to get useful energy out of the panel in the shade even while the one in the sun is dominating things should outweigh the lower efficiency. Higher cost, more complexity, a different level of electronics, but with different rewards.

    ak
     
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Bridge rectifiers are pretty rare in Shottky-Barrier type, which pushes the price sky high.

    My experience of checking various types of rectifier with a DMM diode check function suggests that the higher speed diodes have lower Vf - SB rectifiers are of course in a class of their own.

    In actual fact; the only junction that comes close to the "textbook" 0.7V is the zener type.

    But the vast majority of bridge rectifiers will be the slowest variety with Vf at least 0.6V - examination of the datasheet usually indicates that the actual Vf at full rated current can be 0.8V or more!

    On the DMM check; SB rectifiers vary from about 0.1V to a little over 0.2V, depending on how chunky the rectifier - but of course the DMM check isn't real world current. Get the biggest rectifiers you can afford.
     
  15. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Assuming:

    1. that both panels are >~18V open circuit.
    2. they are unequal current capacity.
    3. they both have an internal reverse-blocking diode.

    This means that their outputs can be "ored" with no diodes, no converters, no nothing, paralleled right to the input of a single charge controller.

    You guys are over-complicating this thing.
     
    Last edited: May 4, 2015
    nsaspook likes this.
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