On Sun, Apr 05, 2009 at 08:09:49PM -0400, Jim wrote: > > > Here's my thinking and I don't know how the controllers work so I may be > off base. If you have two voltage sources in parallel and one of them > is generating a higher voltage, current will flow from the higher > voltage source to the lower voltage source and the load.
Please correct me if I'm worng :) - but isn't "voltage is the same everywhere in a parallel circuit, current varies" part of Ohm's Law? I may be 20-some years out of practice, but I think I recall that one correctly. I suspect that you were thinking of internal resistance rather than voltages. In addition, current in a pair of paralleled generators is additive; current from a larger one will not flow through a smaller one, since there's no voltage differential, but will simply be added to the total. > So I'm guessing that the input impedance (resistance) of a controller is > very low such that the panels are almost short circuited and there is > almost no measurable voltage at the input of the controller. Then > current from both sources would be available. That's not exactly how internal resistance (not impedance - that would only matter in an AC circuit, and not "input resistance" - there's really no such thing) works. I'm sure that it _is_ low for these controllers - there'd be no reason for it to be otherwise, and the controller wouldn't be able to work if it was - but the input of that controller is, again, a parallel connection: .______.______________. | | | |+ |+ |+ --- --- ---------- - - |Controller| |- |- ---------- | | |- ----------------------- Hence, the voltage will be the same at the controller input as it is at the panel output, barring a tiny drop through the wires (which are a series connection, of course.) > That analysis didn't come to the conclusion I was aiming for, but that's > OK. It supports Lee's suggestion. If there is a marginal advantage to > two controllers, it's probably not worth it. My analysis is essentially the opposite of yours, and yet I agree with your conclusion. Given the cost of a controller, and the gain to be had for that cost, you'd be much better off buying more panels with that money, and just enough controller capacity (plus a safety margin) to handle that panel output. -- * Ben Okopnik * Editor-in-Chief, Linux Gazette * http://LinuxGazette.NET * _______________________________________________ Liveaboard mailing list [email protected] To adjust your membership settings over the web http://www.liveaboardnow.org/mailman/listinfo/liveaboard To subscribe send an email to [email protected] To unsubscribe send an email to [email protected] The archives are at http://www.liveaboardnow.org/pipermail/liveaboard/ To search the archives http://www.mail-archive.com/[email protected] The Mailman Users Guide can be found here http://www.gnu.org/software/mailman/mailman-member/index.html
