Blocking Diode

[Scott Stevens]

I have 1 48 v array now and want to expand, should i use blocking diodes in
series with each array?
As it enters the inverter.

 

[Gymy Bob]

Yes. Could some knowledgable person explain the purpose for these bypass and
blocking diodes and the problems without them, please?

I have been around so long with this stuff I believe I invented the diode in
1941 but I am not familiar with the solar panel usage requirements of them.
(no P & N substrate explanations please. I wrote the GE manual...LOL)

 

[Sylvan Butler]

Yes. Could some knowledgable person explain the purpose for these bypass and
blocking diodes and the problems without them, please?

Background for those who need it: A diode conducts electricity in only
one direction.

Blocking: If the positive line is more positive than the panel, a
blocking diode prevents panel draining power from the system. Connects
in series between the panel and battery. Causes a bit of power loss
(power=0.6v * amps, where 0.6v is a typical power-diode drop).

Bypass: A shaded cell or panel in a large series string (typically 72
or more cells in series, a 24v or greater nominal array) can be damaged
in the potential across it is too great and causes junction breakdown.
A bypass diode allows the current to bypass that panel instead of thru
it. Connects in anti-parallel to the panel so there is no conduction
and hence no loss in typical case. My 100W@24v panels came with bypass
diodes already wired across both 12v halves of each panel.

sdb

 

[Gymy Bob]

This seems to be what I thought but if a panel is in the shade then it's
series string has a lower voltage than any others (or not enough to charge
anything) and will produce no current anyway making the diodes
non-functional. Also these bypass diodes do not bypass each individual cell
so wouldn't the same thing happen to echa cell in the 12V block? Perhaps
122V can be tolerated by the cell junctions but more cannot?

Thanx for that.

 

[Sylvan Butler]

This seems to be what I thought but if a panel is in the shade then it's
series string has a lower voltage than any others (or not enough to charge
anything) and will produce no current anyway making the diodes
non-functional.

For example, I run a 48v system (nominal, actual battery voltage is
about 54v and panel open-circuit is about 80v). If one panel is shaded,
the other 36v might damage that shaded panel. The bypass diodes ensure
that the current is conducted around the panel, so the panel will only
see 0.6v of reverse polarity instead of 36v.

Also these bypass diodes do not bypass each individual cell
so wouldn't the same thing happen to echa cell in the 12V block? Perhaps
122V can be tolerated by the cell junctions but more cannot?

Exactly.

sdb