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Posted by David Lesher on June 8, 2010, 6:24 pm
 




You have no way to ensure the voltage will divide equally across
the two MPPT's. That happens *only* if they always drew equal
currents, and such would a fiction soon exposed.

Then the greater voltage will be across one or the other MPPT,
it will suffer from escaped smoke, and when it does, the full
voltage will likely be across the remaining unit; which then
joins it in the "WAS a MPPT but is now a paperweight" catageory.

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Unless the host (that isn't close).........................pob 1433
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Posted by m II on June 9, 2010, 1:32 am
 
David Lesher wrote:

I would guess that the division would be closer to the half way point.
The input current in a series connection has to be the same through both
units.

It would probably depend on the closeness of the voltage reading
tolerances on the output sides. I do not know how badly they would have
to differ before you get, say, 90 volts drop on one and 160 on the other.

The circuitry may not even behave in the expected way, what with pulse
width modulation or whatever derivation of that they are using. I don't
know in what manner the secondary load reflects back the primary. Do
they behave as a wire wound transformer? There seems to be some pretty
serious capacitors in the unit. I got a very healthy spark on the
initial connection.

Today, on the single mx60, I noticed about 90 volts input with around 7
amps flowing. The 12 volt side was putting about 30+ amps into the
batteries at 12.8 volt charge. I wish I had a 24 or 36 volt setup, but
there is too much of an investment in 12 volt stuff to change. I just
keep everything close to the batteries.

I still wonder whether or not the factory has ever tried seriesing (sp
?) two of their units. They would be the ones to know the intricacies of
the in/out circuitry and what fireworks, if any, can be expected.




mike

Posted by Josepi on June 9, 2010, 2:17 am
 OMG! Anybody ever heard of input GFI?

My PV inputs are referenced to ground in order to detect faults in either
leg of the DC input.
Transistor inputs may be insulated for  a certain value ...***TO CASE
GROUND****.

This is one of the silliest things I have ever heard.

There is a brand of high voltage input MPPT and somebody here posts about
it. Can't remember who or what brand. Do some searches on this group back
about a year or two.


You have no way to ensure the voltage will divide equally across
the two MPPT's. That happens *only* if they always drew equal
currents, and such would a fiction soon exposed.

Then the greater voltage will be across one or the other MPPT,
it will suffer from escaped smoke, and when it does, the full
voltage will likely be across the remaining unit; which then
joins it in the "WAS a MPPT but is now a paperweight" catageory.







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