bypass and blocking diodes

[satman49]

Here is a pic of my junction box showing the diodes.It's a 100 watt panel.My question is: Is the left one a bypass diode and the right one a blocking diode? Could either one cause low power output? diodes.JPG

[SunEagle]

If either has failed then the voltage and current output of that panel would be lower than nameplate rating.

More than likely a failed diode will create an open circuit and isolate a number of the pv cells depending on which one failed.

[Mike90250]

Blocking diodes are seldom. if ever, used in a PV panel. Blocking function is nearly always performed in the Charge Controller

Bypass diodes are often found in the J-box of the panel. They should be a Schottky type of diode, lower loss and heat generation than a plain diode.
use conventional means to test them

https://en.wikipedia.org/wiki/Schottky_diode

[bcroe]

Here is a pic of my junction box showing the diodes.It's a 100 watt panel.My question is: Is the left one a bypass diode and the right one a blocking diode? Could either one cause low power output?

I think your panel is divided into 2 sections, and each diode parallels a section, to serve as a bypass if that section
were shadowed. The diodes should run completely cold in sunny operation, carrying no current. A meter would
show half the panel output voltage across each. If a blocking diode were in use, it would be in series, not parallel,
and show something like 1V forward drop when current was output.

If a bypass diode were to open, it would have no effect on sunny operation; an open blocking diode would stop all
current. A shorted bypass diode would run hot and eliminate about half your panel output voltage. A shorted
blocking diode wouldn't prevent operation, but would allow back (battery) current at night. Bruce Roe

[satman49]

It's very odd that the Voc and Isc are within specs but under load I can only get about 30 watts out of this 100 watt panel.See my previous post : low solar panel output

Voltage under load is about 14V and 2A current The only thing I can think of might be a COLD solder joint on one or more of the tabbing wires or buss wires.

[bcroe]

It's very odd that the Voc and Isc are within specs but under load I can only get about 30 watts out of this 100 watt panel.See my previous post : low solar panel output

Voltage under load is about 14V and 2A current The only thing I can think of might be a COLD solder joint on one or more of the tabbing wires or buss wires.

Since you have access, check for equal voltage UNDER LOAD across the 2 diodes. Might be one section is weak.
That might not show up on a simple OC/SC test. Bruce Roe

[satman49]

ok thanks - will do tomorrow if sunny

[satman49]

OK It shows 10.6 across each diode, which is about right, but under load I still can't get over 31 watts .I guess I'll just give up on this panel.Anybody want it? May you will have better luck.