On Sun, Apr 05, 2009 at 10:11:28PM -0400, Jim wrote: > Ben Okopnik wrote: > > 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. > > > Yes, but I was referring to two different voltage sources like a 9 volt > battery and a 12 volt battery. If you connect them together you will > have a voltage of something in between, but that doesn't change the fact > each have a voltage associated with them until you connect them. That's > what I was referring to as a voltage source.
The original voltage of each source matters just as much as the amount of current that each one puts out. E.g., if you have a 1V generator putting out 1000A in parallel with a 1000V generator putting out 1mA, the circuit voltage will be 1V to any practical level of measurement. > > 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. > > > If there is a differential when not connected, then current will either > flow from one to the other (and the load) or from the higher potential > to the load if the lower potential source will not allow incoming > current. The second part of this sentence is correct: a source will not allow incoming current, so it all goes to the load. Hence, additive currents. > That is true if the load is of a high enough resistance to > permit the voltage at the connection between the two generators to > remain above the potential of the lower potential source. Unfortunately, this isn't an accurate description: the resistance of the load does not make any difference to the interaction of the two sources. They are additive, regardless of what the load does. > Yes, if the controller has a low input resistance as I mentioned, both > sources will feed current to the controller. I was merely thinking out > loud about what that resistance would have to be. I used the term > impedance because I'm fairly certain that the circuit is not 100% > resistive in nature and will have some reactance but I'm not completely > certain of that. But that will not matter, since the input voltage is purely DC. > If so then my use of the term impedance is at least > tolerable. ;) I can run a simulation of various loads to demonstrate > my contention, if you wish. Given an average input current of 20A (as in my configuration) and the fact that I don't see a capacitor or inductor of any significant size on the controller board, I'd be fascinated to see any sort of analysis that supports that point. :) > That said, can anyone tell me what the input voltage does when the load > on the controller is removed? Does it go up or does the controller dump > the excess current into an internal load. Depends on the type, of course. I'm not familiar with any MPPT controllers that use an external load; assuming one without, the input voltage will, of course, rise. Incidentally, my best guess for how these things work is that they're very efficient DC-DC converters: chop the incoming power, convert it down to 14.4v (or whatever their nominal voltage is), and collect your ill-gotten gains in current. I recall doing this at the mW level while designing drivers for millimeter-wave radar at Hughes Aircraft, but this is certainly a very nice application of the principle (assuming my guess is correct.) -- * 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
