Smokin' out the bypass diode

System
System Posts: 2,511 admin
in Advanced Solar Electric Technical Forum #1
(As mentioned in a previous thread) I have some Evergreen off-spec panels. I am assured by members of this community that these have bypass diodes. The Evergreen literature also states this indirectly.

It boils down to this. When I completely cover 1 of 15 panels in a string, I would expect to get a 1/15 drop in power, assuming the diode(s) on the "killed" panel is doing its job. There may be some other factors at play, so let's just say a 1/10 drop in power. But I'm getting a 50% drop in power. This tells me that the "killed" panel is holding things up WAY too much.

Not to disrespect or doubt anyone, especially Guppy and B.B., but is there any sure-fire way to verify the function of these diodes and/or make sure they actually exist? Without taking apart the access panel? Or doing a homemade "load test" and smoking them out, quite literally? (I hear things don't work as well once the smoke's all leaked out of 'em.) I have an uneasy feeling that mine aren't working, or aren't even there.

Multimeter testing........

My multimeter has a selection for diode testing. However, I don't know what reading I'm looking for. Various websites http://www.electronicrepairguide.com/testing-diode.html
warn that testing a Schottky diode is notoriously fraught with innacuracies, and that it is almost impossible without first isolating the diode. This is currently not an option, as this would require opening the panel.

So... I get a known good, or new Schottky diode. (Just happened to have a pack of 100 arrive in the mail yesterday - how convenient - LOL). The readings on all of them are consistently 0.225. 0.225 WHAT, though? Well, I do two in parallel, and the number goes down. Three in parallel, and the number lower still...now it's 0.180. So, I'm thinking a lower number is what I'm looking for. Next... I go out to one of my Evergreen 180w panels, at NIGHT (moonless night). Cover it with a blanket anyway. Measure the leads... I get 1.6, give or take, "units of whatever"...both ways. Apparently this panel doesn't have the same characteristics as my raw diodes. But, without knowing ALL the variables, I am still left with only innuendo and suspicion.

"Bench" test? ...I'm NOT going to do this unless someone "talks me into it"...

Take the suspect panel out, lay it face-down, and cover it up really good. Run a current through it, say 50% of rated (That would be roughly 3A). This would simulate what's happening in the string when shade covers one panel.
Then measure voltage across the leads of the panel.... It SHOULD be the drop voltage of the Schottky (0.35v) give or take a tenth of a volt. Right??

Any other ideas?

Thanks.
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Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Smokin' out the bypass diode

    Basically, a Silicon Solar Cell (as I understand it) goes into high resistance when "exposed to dark"... So any voltage drop/current flow would be the result of bypass diodes.

    A diode is a highly non-linear device. Where a resistor has a fixed R and is measured by the equation R=V/I... The diodes voltage is an exponential equation and the "0.2 volts" for a Schottky Diode is just an approximation--as the current goes down, the voltage will fall at bit, as the current is raised, the voltage will rise a bit (diode IV curves are also affected by temperature too).

    So, in your case, if you measure anything between 0.1 and 1.0 volts (or so), while forward biasing your dark solar panel, then you can be sure there is a diode in the circuit... And you will have to trust that the manufacturer installed an appropriate diode in the appropriate location for optimum performance.

    I went looking around for a Power vs Voltage curve--but I could not find one really quickly--but, basically, from looking at the IV curves, a 10% change in operating voltage (from Vmp to 1.1xVmp) appears will drop the current to near zero amps.

    So, just placing two strings in parallel, but one with 10 panels, and one with 9 panels, the MPPT controller could have two "stable" operating points... One that found the MPPT operating point of the 10 panel string (with zero contribution from the 9 panel string), or if it continued to draw more current (and drop the voltage further), would find the max power point of, basically, two 9 panel strings (and "waste" the contribution of the 10th panel in the 10 panel string)...

    And, the ability of the MPPT converter to find either of those two points would depend on its particular MPPT algorithm. Some MPPT controllers just look for Voc (open circuit) and set Vmp as a percentage of Voc and that is it (tends to be off-grid battery charger type of controllers). So, it would only see the 10 panel voltage and makes its decision--and basically ignore any available contributions of the 9 panel array.

    Or, if the controller continued to increase the current to 2x of the 10 panel array's measured output current, it would see another voltage support step where I*V=P would be higher than the first 10 panel only measurement...

    However, that would depend on the controller's software design.

    Would it see a 15/14 panel string--I do not know... But since Solar Guppy (and maybe boB too) actually writes MPPT algorithms (and does competitive analysis too) for various MPPT based controllers--I would have to go with whatever he says.

    But, in the end, if you wanted to see an operating array and prove the diodes work--cover one panel in each string and see if you Pmp drops by a bit more than 1/15th (allowing for diode losses) vs two strings of 15 uncovered.

    You should be able to see this too by monitoring your array's voltage and current numbers too (assuming your controller has those readings available).

    Lastly, Kyocera has a couple of nice PDF catalogs that are a nice A-Z primer of Solar power (home and off grid, maps, design guidelines, etc.). They also have (I assume) list pricing of a wide array of components (from panels, to inverters, charge controllers, racks, breakers, boxes, electrical/mechanical hardware, etc.)... I will post just the link to their support page--since I am not sure if this is direct competitor to Wind-Sun or Wind-Sun is actually one of Kyocera's retailers...

    Kyocera Solar Energy Catalog (industrial and whole catalogs)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Solar Guppy
    Solar Guppy Solar Expert Posts: 1,989 ✭✭✭
    #3
    Re: Smokin' out the bypass diode

    Yes, find something else to do with your time :roll:

    Your panels are fine, they have the diodes and there is nothing wrong

    You will only get a 1/15th power drop if

    a) its a single string
    b) the load ( mppt inverter ) retrack's to the new vmp point

    Since you have multi-strings, pulling out ( buy covering ) a 22 volt vmp panel will make that string be operating at least 30 volts higher than vmp and this is NOT a linear scale

    http://www.locoengineering.com/Solar%20Info.htm

    look at the last graph on this page, notice as the operational voltage moves closer to the open circuit voltage the power produced goes to zero and also notice its only a change in the voltage of 5 volts ( for a 30 volt panel )reduced the power by 1/2.
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