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. > 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. 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. > >> 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.) > 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. 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.
It's a moot point however since the controllers work. I was just having a problem getting my arms around how and I, as I said, was thinking out loud to come to the conclusion the controller has a low input impedance. 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. Thanks, Jim. > >> 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. > > > _______________________________________________ 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
