On 13/02/2012 08:24, harry wrote:
On Feb 12, 11:40 pm, Andy wrote:
On Feb 12, 6:31 pm, wrote:
The panel I am thinking of purchasing off eBay has a protection diode
built-in.
I bought one of Maplins - it cost me a car battery. The supposed
diode was more of a didn't.
The PV cells are themselves diodes.
Yes. But leaky ones - left to its own devices a PV array connected
across a battery without a charge controller or a protection diode will
discharge the battery at a rate that may be comparable to the charge it
can put in during the daytime at this dark dismal time of year.
A crude basic PV and diode combo won't do much by way of charging the
nominally 12v lead acid battery until its terminal voltage reaches 14v
or so. This condition may not be met at all in diffuse winter light.
A Schottky diode in series is a wise precaution as is a low voltage
automotive fuse to avoid serious embarrassment in the event of a short.
They are penny items at Maplin
http://www.maplin.co.uk/1a-schottky-...6354&u=mapauto
Thin connection wire becomes red hot rather quickly with the current
that a 12v car battery can supply.
The OP would be better off with a mains powered battery charger.
The other point worth making in a DIY group is that since silicon is
expensive and mirrors are cheap if you want to get the most out of a PV
array and can afford the space a pair of mirrors will help. Simplest
configuration is \_/ - half a hexagon with the PV array at the bottom.
No good on a roof, but useful in smaller devices. It does tend to cook
the PV and some yellowing of the plastic may occur if you overdo it.
More sophisticated non focussing flux concentrators use truncated
parabolas with the focus of each at the opposite edge of the array. In
this configuration it is possible to prove that any light that enters
the front aperture will eventually hit the PV cells.
It gives a gain of around 3-10x in diffuse light depending on the
geometry. The technique originates from HEP bulk scintillator designs.
An article with some design details online at:
http://home.iitk.ac.in/~aradhya/cpc.pdf
Not checked in detail all the way through but it looks about right.
--
Regards,
Martin Brown