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Diodes or not diodes
By ZooT, Section Controls Posted on Thu Sep 27, 2007 at 07:05:02 PM MST
collecting electricity from multiple sources

I'm planning on hooking up both a set of 3 HF 15 watt panels, and a 200 watt genny this weekend.
I've got a xantrex c-35 that I'm going to be using in diversion controller mode.
I'd suppose that using a single source might negate the need to use blocking diodes to keep the battery from feeding the panels or genny but when using more than one source, should I use blocking diodes to keep the genny from trying to feed the panels and the panels from trying to feed the genny?

Diodes or not diodes | 10 comments (10 topical)

Re: Diodes or not diodes (3.00 / 0) (#1)
by kurt on Thu Sep 27, 2007 at 02:16:30 PM MST

well i would put a blocking diod in the positive wire going to the battery of the solar panels and as far as the wind mill goes every type of wind turbine i know of eather requires a blocking diod or some bridge rectifiers in the line someware in eather case that should do fine.

IRC inst.
just a personal rant carry on.

Re: Diodes or not diodes (3.00 / 0) (#2)
by boB on Thu Sep 27, 2007 at 04:07:49 PM MST

A PV module will not start conducting from an external DC source
until the voltage goes above the modules open circuit voltage.
Then is starts to look like a forward biased diode (actually, a
bunch of diodes in series)...

So, a 12V module will take up to, and probably more than
around 21 Volts before it starts conducting. This voltage
of course depends on the temperature of the PV module and
how many cells it has in series. The Voc is a fairly
good estimate though for a typical mono/poly crystaline
module.

boB

Re: Diodes or not diodes (3.00 / 0) (#3)
by Slingshot on Thu Sep 27, 2007 at 04:18:04 PM MST

What is Voc at night?

[ Parent ]

Re: Diodes or not diodes (3.00 / 0) (#5)
by TomW on Thu Sep 27, 2007 at 05:23:06 PM MST

Slingshot;

Uh, Volts Open Circuit at night should be zero or possibly a few millivolts from bright moon / stars. Anyway, well below daylight volts for certain.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

[ Parent ]

Re: Diodes or not diodes (3.00 / 0) (#6)
by Ungrounded Lightning Rod on Thu Sep 27, 2007 at 06:23:08 PM MST

But the Voc that's significant is the one when there's actually light. That's a good measure of the voltage drop as a string of diodes - which doesn't change when the light goes away.

[ Parent ]

Re: Diodes or not diodes (3.00 / 0) (#7)
by boB on Thu Sep 27, 2007 at 07:26:27 PM MST

>> What is Voc at night?

OK... OK...

What "would be" the open Voc... This can be measured with a current
limited power supply, even in the day.
Place the module face down, like on a rug (OR, just wait till dark)
and apply voltage to it until the current starts to rise.
That will be approximately Voc. Temperature dependent, just like
Voc with light. I think it's just slightly higher than real
light induced Voc.

boB

[ Parent ]

Re: Diodes or not diodes (3.00 / 0) (#9)
by Slingshot on Mon Oct 15, 2007 at 09:06:09 AM MST

In that case, "dark" panels should not draw current from the batteries, even without a blocking diode. Is that true in practice?

Do the so-called "self-regulating" panels, which are merely a lower number of cells to reduce overcharge, require anything to prevent dark discharge?

[ Parent ]

Re: Diodes or not diodes (3.00 / 0) (#10)
by ghurd on Tue Oct 16, 2007 at 02:14:57 PM MST

They draw a little, but not a whole lot. A blocking diode won't hurt anything because of the power curves on standard PVs. Every controller I can think of has nighttime discharge protection built in.

There were a lot of different kinds of "self-regulating" PVs. Probably forget the diode and get a decent controller. Decent solar controllers have a much lower forward voltage drop than a diode, and with self-regulating PVs, you need all the voltage you can get.
Self-regulating PVs tend to be small, so a ~$32 SG-4 would do a decent job of it.

G-
Ghurd.info
[ Parent ]

Re: Diodes or not diodes (3.00 / 0) (#8)
by Opera House on Fri Sep 28, 2007 at 05:56:54 AM MST

I have two 75 watt panels in parallel and the leakage current at night is 65ma if I remember the test right. Panels are basically stacked diodes, but very imperfect ones. I don't see what VOC has to do with Chinese apricots being a glut on the market this year.

[ Parent ]

Re: Diodes or not diodes (3.00 / 0) (#4)
by wooferhound on Thu Sep 27, 2007 at 04:37:07 PM MST

I am building the almost the exact system that you are. I said that I was going to put this system in last weekend, well, so far I have put 4 full days into this project with at least 2 more days work before it is completed. Next week I'll be posting pictures and descriptions of my system.

http://www.fieldlines.com/story/2007/9/3/22483/29925

I personally put a single diode on the output of every single panel of my system. When finished there will be 5 total panels. The wind Generator will require a Diode Bridge Rectifier on it's output.
W o o f -={(

Diodes or not diodes | 10 comments (10 topical)

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