Posted by Merlin-7 KI4ILB on January 16, 2006, 6:23 pm
I thought I would start a new thread on this as there seems to be some
confusion with them...
<snip>
Well Steve, is this even necessary? If the cell is a reversed biased
diode, it should have a reverse leakage current of microamps. Its a
diode, after all, right?
Bobg
And a question of mine..
I will try and keep the numbers even for simplicity.
1: If a diode has a voltage drop of .7 volts
2: You have 500 watts of panels in a 12 volt system.
3: voltage at Pmax 18volts
How much of a loss (in watts) will you get by the voltage drop of .7 volts?
Is it possible to convert this loss into a percentage drop in performance?
That said let me tell you what I am doing with them.
I have other reasons for using blocking diodes, mostly used for testing
different brands of panels.
I have 4 different brands of panels at this time all wired to a central
junction box.In this box each solar panel is wired thru a diode. I take an
amp meter and can quickly check the shorted amps of each panel without
having to disconnect the leads etc.
However if I am losing more than an amp or two, I can take the diodes out
and replace them with fuses. It may take a bit longer than I like to get my
readings but it would help in getting the open voltages.
Posted by Duane C. Johnson on January 16, 2006, 7:42 pm
Hi Merlin;
> I thought I would start a new thread on this as
> there seems to be some confusion with them...
> Well Steve, is this even necessary? If the cell is a
> reversed biased diode, it should have a reverse leakage
> current of microamps. Its a diode, after all, right?
> Bobg
PV cells are not reverse biased they are forward biased.
Duane
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Posted by Eric Sears on January 17, 2006, 5:10 am
On Mon, 16 Jan 2006 18:42:36 -0600, "Duane C. Johnson"
>Hi Merlin;
> > I thought I would start a new thread on this as
> > there seems to be some confusion with them...
> > Well Steve, is this even necessary? If the cell is a
> > reversed biased diode, it should have a reverse leakage
> > current of microamps. Its a diode, after all, right?
> > Bobg
>PV cells are not reverse biased they are forward biased.
>Duane
It looks like we might need a bit more discussion on this topic to
help some of us (including me) to clarify some statements that have
been made.
Firstly, I have always considered that a pv cell in light, and above
battery voltage, can be considered as forward biased - though its
actually being biased by the light, rather than by an applied voltage
as in the case of an ordinary silicon diode.
As I understand it the biasing is caused by the energy of the photons
exceeding the bandgap voltage.
But secondly, if we are talking about the operation of a blocking
diode, then we are talking about the case when the pv cell is in
darkness (or at least, insufficient light to generate battery
voltage).
In this case, without a blocking diode, the cell is reverse biased by
the battery.
However, that said, a pv cell is not simply another diode - it is a
very large area diode, with considerable leakage in some cases.
One panel I measured the other day, connected to a 12 volt battery
conducted more than 100mA at night. I consider that rather excessive,
though these were cells capable of producing 6 amps or more in full
sun. 1 or 2 amp-hr loss at night is only a small percentage of what
the panel would produce even on a cloudy day.
I use 40 amp schottky diodes with these panels (I got about 50 at a
very good price some years ago), and these cut the leakage to
microamps, though they still drop about 0.35 volts at 5 amps forward
current. But that's much better than a regular silicon diode which
often exceeds 0.8 volts, and can lose as much as 10% of the generated
power when they get hot, or if they are only marginally large enough
for the current. (I have measured voltage drops of up to 1.2 volts on
some diodes).
A fet can be used with much lower losses, though I'm not sure about
using them with parallel panels.
Today, with two panels in parallel, (producing about 8 amps at the
time) I got an extra 0.75 amp when I shorted the schottky blocking
diode, though I think my long leads to the battery may also be having
some effect in reducing the voltage needed to drive current through
the battery.
Just my tuppence worth
Eric
Posted by phatty mo on January 17, 2006, 11:34 am
Eric Sears wrote:
> On Mon, 16 Jan 2006 18:42:36 -0600, "Duane C. Johnson"
>
>
>>Hi Merlin;
>>
>>
>>
>>>I thought I would start a new thread on this as
>>>there seems to be some confusion with them...
>>
>>>Well Steve, is this even necessary? If the cell is a
>>>reversed biased diode, it should have a reverse leakage
>>>current of microamps. Its a diode, after all, right?
>>>Bobg
>>
>>PV cells are not reverse biased they are forward biased.
>>
>>Duane
>>
>
> It looks like we might need a bit more discussion on this topic to
> help some of us (including me) to clarify some statements that have
> been made.
>
> Firstly, I have always considered that a pv cell in light, and above
> battery voltage, can be considered as forward biased - though its
> actually being biased by the light, rather than by an applied voltage
> as in the case of an ordinary silicon diode.
> As I understand it the biasing is caused by the energy of the photons
> exceeding the bandgap voltage.
>
> But secondly, if we are talking about the operation of a blocking
> diode, then we are talking about the case when the pv cell is in
> darkness (or at least, insufficient light to generate battery
> voltage).
> In this case, without a blocking diode, the cell is reverse biased by
> the battery.
> However, that said, a pv cell is not simply another diode - it is a
> very large area diode, with considerable leakage in some cases.
> One panel I measured the other day, connected to a 12 volt battery
> conducted more than 100mA at night. I consider that rather excessive,
> though these were cells capable of producing 6 amps or more in full
> sun. 1 or 2 amp-hr loss at night is only a small percentage of what
> the panel would produce even on a cloudy day.
> I use 40 amp schottky diodes with these panels (I got about 50 at a
> very good price some years ago), and these cut the leakage to
> microamps, though they still drop about 0.35 volts at 5 amps forward
> current. But that's much better than a regular silicon diode which
> often exceeds 0.8 volts, and can lose as much as 10% of the generated
> power when they get hot, or if they are only marginally large enough
> for the current. (I have measured voltage drops of up to 1.2 volts on
> some diodes).
> A fet can be used with much lower losses, though I'm not sure about
> using them with parallel panels.
>
> Today, with two panels in parallel, (producing about 8 amps at the
> time) I got an extra 0.75 amp when I shorted the schottky blocking
> diode, though I think my long leads to the battery may also be having
> some effect in reducing the voltage needed to drive current through
> the battery.
>
> Just my tuppence worth
>
> Eric
>
I noticed something strange,atleast to me..maybe this is normal for PV
cells/panels.
I usually have a diode in series with my panel,but one day while I was
experimenting in the bright sun,I bypassed the diode,and left it wired
to the battery bank like that...and forgot about it..
Now,The last time I did this when I was experimenting,the panel drew a
few microamps at night..This time it was drawing some actual
current..something like 100ma IIRC!! It actually drained the battery
bank a little bit overnight.
Why would it suddenly be drawing way more current at night than it has
before? (or more than I've ever noticed before?)
Does temperature make a big diffeence?
Can this damage the panels?
Posted by Windsun on January 17, 2006, 11:55 am
Are any of you using a real charge controller between the panels and the
batteries?
If so a diode is pointless. If not, good luck with your batteries....
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> > I thought I would start a new thread on this as
> > there seems to be some confusion with them...
> > Well Steve, is this even necessary? If the cell is a
> > reversed biased diode, it should have a reverse leakage
> > current of microamps. Its a diode, after all, right?
> > Bobg
>PV cells are not reverse biased they are forward biased.
>Duane