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need stepdown from 50v dc OCv to below 25vdc

 
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wolfdogg



Joined: 30 Sep 2007
Posts: 7

PostPosted: Sun Sep 30, 2007 8:05 pm    Post subject: need stepdown from 50v dc OCv to below 25vdc Reply with quote

I am upset, I ordered a solar panel and was sent a COMEPLETELY wrong panel from what i ordered, and the glass cover is even cracked(DHL's fault). They wont take it back because i waited too long to inspect it(it was shipped to my dads house, didn't pick it till 2 months later) . I was wondering the best way to step down the voltage so i can use the frigger. Its my first panel.

the panel is showing today 50v DC under full sun, this is the OCv, or Open Circuit voltage. I hear this panel has built in protection, it does have a good solid diode(didnt get numbers off it yet) and assume one reason for this is to keep the panel from frying when disconnected, im wondering if i need to fully cover it up until it gets connected rather than leaving it up there mounted with an open circuit for one thing.
according to this page http://www.siliconsolar.com/Solar-Electric-Panels-p-16208.html the 11-40WSEPM has a OCv of 40v. I specifically stated i was using it for a 12v applicaiton, and also told them to make sure the Xantrex C-12 charge controller was compatible. Well the charge controller takes a max array open circuit voltage of 25V. i spent almost $350 for the charge controller and panel.

Im thinking of stepping down the panels voltage, but i realize this will decrease its power output tremendously. I was also thinking, what if i just divided(split in half) the power into two seperate outputs, then paralleled them onto the input of the max 25vdc charge controller, thereby losing very little, but this prob isn't very possible either. what kind of options would anyone suggest

Is there any kind of common household item that i can modify to do the job, such as an old power supply for a PC, of some kind of ac/dc xformer? maybe i can use the parts from a pc power supply and reconfigure them? or maybe somebody can show a good schematic for creating one?

i am on a tight budget right now, hence the reason i got the panel a few months ago, so would like to make a few sacrifices just to get it up and running.


Last edited by wolfdogg on Fri Nov 16, 2007 10:29 pm; edited 2 times in total
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Oct 01, 2007 9:28 am    Post subject: Reply with quote

Usually, a solar panel works with maximum efficiency when loaded down to about half the open circuit voltage.
I would recommend that you build a step down regulator using a suitable switch mode IC, for example, you can look at www.national.com for datasheets, application notes and design help software.
Additionally, you should have a shunt regulator to keep the voltage down in cases of low load and strong sunlight. This can easily be accomplished by a big NPN power transistor, a zener diode to give it base current when the voltage rises too high, and possibly a power resistor in series with the transistor's collector to take some of the power loss. Of course, if you need heat somewhere, arrange the heat sink for the shunt regulator so that you can get use for the heat (i.e. for heating water).
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wolfdogg



Joined: 30 Sep 2007
Posts: 7

PostPosted: Mon Oct 01, 2007 7:35 pm    Post subject: Reply with quote

note to site admin, this site is very jumpy at 1650 x 1080 resolution full screen, i have to restore the browser to use it.

thanks for the reply,
i found these two possibilities
L4960 http://www.alldatasheet.com/view.jsp?Searchword=L4960
and the LM2576. i have to verify that teh panel isnt over 2.5 A to use the L4960 which i assume is cheaper. anyone have a good link to price sheets from a cheap internet source? i lost all my electronic links for parts purchase and forgot the good websites, its been a few years Surprised

Is that the type of Switch Mode IC you were referring to? if so, does the shunt regulator still apply? my impression on the heat usage was this "Now thats a thinking man" proud comment! I can use that heat in the RV, or waterline, and will be taking this thing a few states and provinces north in a year or two!! thanks for the idea.

any thoughts appreciated!
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Tue Oct 02, 2007 7:23 am    Post subject: Reply with quote

Yes, it is about the type i thought about. National has a lot of different regulators, i can recommend their design tools at http://www.national.com/appinfo/power/webench.html to help choosing a regulator and the surrounding components. But beware, the Webench tool often suggests unnecessarily expensive parts in order to "help" get as good efficiency, small size etc as possible. Often, the most over-all economical solution is found far down the table of suggested parts. Also, for the sake of efficiency, the coils are often oversized.
It is possible to order parts on-line directly from National, and this is usually the best way of buying smaller quantities. There is a risk of getting counterfeit parts when buying from cheap uncontrolled sources.

Regarding the regulator from ST, my experience is that it is often nearly impossible to buy small quantities of ST parts unless they are stocked by any of the large component distributors (Arrow, Avnet, Digi-key etc). They are clearly targeted to industrial customers buying tens of thousands a year of the same part number.

There are also many other companies producing step-down regulators, i.e. Linear, ON Semiconductor, Texas Instruments.
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wolfdogg



Joined: 30 Sep 2007
Posts: 7

PostPosted: Tue Oct 02, 2007 10:18 pm    Post subject: Reply with quote

its slowly coming back to me,
heres teh place in pasadena, ca i forgot about where i used to get parts from also. they have alot of good supply and sell small quantities http://marvac.com
it may have been called Marvac Dow Originally
a quick search for the ST part brings nothing up however.
hope that link is useful for someone.
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been thru electronic engineering in college, top of class grades, 10 years ago. some field exp.
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