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Checking a PV panel for open bypass diodes

RCinFLA

Solar Wizard
Joined
Jun 21, 2020
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Panels subjected to regular shading conditions are prone to have blown out open bypass diodes. Bypass diode can fail open or short but most often they get overheated and fail open. A shorted bypass diode is easy to detect as Voc of panel will be low due to the shorted bypass diode shorting across a series group of PV cells in the panel.

Especially when buying used panels, it is good idea to check bypass diodes before installing panels.

This is a method to check bypass diodes in a panel without intrusive testing requiring opening of junction box on panel.

Checking PV panel bypass diodes.png
 
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Nice! I think even I could figure that out with your info.

When applying a reverse current limited voltage with a darkened panel, the DVM probes would be applied where the power supply connects to the panel?
 
You can make some MC-4 (or like type) connector test leads for power supply to plug into PV panel leads.
 
If one finds failed bypass diodes, is there a solution to beef up a panel (better diodes?) that has regular partial shading?
 
I only use TO-220 packaged diodes and try to rig some metal mounting plates to help heat sink them. This is dependent on how much room you have in junction box. Keep in mind the TO220 tab is one of connections to diode so it has to be electrically isolated from heat sink with an electrically insulating heat sink spacer and plastic screw hole bushing. Don't drill holes in junction box as it needs to be well sealed from moisture intrusion. DC wiring and moisture do not mix well, ask Ma-Bell telephone system techs.

If you have diodes soldered in parallel, get rid of them and replace with a single diode of appropriate amperage rating.

There is also the issue of using Schottky diodes or regular silicon diodes. Schottky diodes have lower forward voltage drop but higher internal Rs than regular diodes, so their voltage drop compared to regular diodes depends on how much forward current is passing relative to their amperage rating. Most panels these days use Schottky diodes to get lower diode voltage drop therefore less heating on diodes.

There can be an issue with Schottky diodes as their breakdown voltage is generally lower so if you have a high total array voltage stack you can have a bypass diode reverse breakdown voltage problem. Each diode needs to take maximum array voltage. A panel spec that lists 600vdc or 1000vdc maximum array voltage should have at least 600v or 1000v diode breakdown voltage. A lot of panels cheat on this breakdown voltage rule.
 
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Good stuff. A hack-ish solution for non-critical installations:

1) If you only need to fix one, purchase at least 2, preferably more.

2) Measure each individually, and choose the ones closest to each other. Don't just replace one, replace them in closely matched pairs.

3) When purchasing, consider simply plus-sizing it. That is, if your panel originally used 5A diodes, see if you have the space for a larger one, like 10A in the junction box. Overkill, but we're just increasing surface area dissapation.

4) If you can be neat and tidy, and ensure good connections with the oversized diodes, don't clip the leads short. Leave extra length on the leads to help act as an additional heat sink.

Emphasis on neat, tidy, and good connections if you do this.
 
I wouldn't call it "hack-ish". Or for non-critical installations. It's just a simple parts replacement/ upgrade.
With the information from @RCinFLA ,anyone who can solder can achieve great success.
 
anyone who can solder can achieve great success.

Well crap, I'm out then.
swear2.gif
 
Panels subjected to regular shading conditions are prone to have blown out open bypass diodes. Bypass diode can fail open or short but most often they get overheated and fail open. A shorted bypass diode is easy to detect as Voc of panel will be low due to the shorted bypass diode shorting across a series group of PV cells in the panel.

Especially when buying used panels, it is good idea to check bypass diodes before installing panels.

This is a method to check bypass diodes in a panel without intrusive testing requiring opening of junction box on panel.

View attachment 94174
Nice right up. It' kind of funny because I think most of the people on the forum would just tear open the back plate box and check the diodes with a meter, but this method is much better for testing panels in bulk.
 
As I mentioned, due to the shunt resistance of solar cells it is highly likely you will have to unsolder a lead of the diode to check it with the diode check function on a DVM after breaking into junction box. My preference is not to break the seal of the box if you don't have to. The purpose of doing it this way is you only need to break into the junction boxes that actually have bad diodes that need replacing.

The DVM will likely show like diode conducts in both directions when diode is in circuit, maybe a little more in one direction than the other, both directions will have less voltage drop reading on DVM than actual diode forward conduction has.

Realize a DVM is only testing diode with something in range of 1 mA of current so shunt resistance of PV cells will overwhelm it.
 
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I made the portable one using 9V battery as the source and 22 Ohms resistor for testing the Vf of the bypass diodes, my Sunpower Vf of the 3 bypass diodes in series is about 3V.
 
Nice right up. It' kind of funny because I think most of the people on the forum would just tear open the back plate box and check the diodes with a meter, but this method is much better for testing panels in bulk.
PV cells have leakage shunt resistance that inhibits ability to 'ohm' out bypass diode when in circuit with PV cells. They usually need to be unsoldered from one end of diode to panel connection to check with ohm meter.

A total panel can have 100 ohms to over 1000 ohms (typically 300-400 ohm) of shunt leakage resistance. Each sub-string within panel is a fraction of that resistance across each bypass diode. This screws up the forward/reverse reading on ohm meter when attempting reading diodes while in circuit.
 
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There can be an issue with Schottky diodes as their breakdown voltage is generally lower so if you have a high total array voltage stack you can have a bypass diode reverse breakdown voltage problem. Each diode needs to take maximum array voltage. A panel spec that lists 600vdc or 1000vdc maximum array voltage should have at least 600v or 1000v diode breakdown voltage. A lot of panels cheat on this breakdown voltage rule.
No, no, no. Reverse voltage is the output of the section of cells the diode can bypass. If that section goes Dark, the diode is Forward conducting when other sections or panels have output.
 
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