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Old 03-08-2018, 12:53   #1
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What is inside a solar junction box

I recently lost all voltage from one of my 100W flexible solar panels. Between the + and - terminals it actually reads 0.000V. This tells me there is a short somewhere as even the float voltage without the multimeter connected to anything reads a few mV.

I started taking apart the junction box and noticed that there are two leads coming out of the panel surface and they also read 0.000V. I am about to scrape the sealent inside the junction panel and see if I can dig out the circuitry.

Does anyone know or can guess what might be in there? Diodes?

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Old 03-08-2018, 13:18   #2
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Re: What is inside a solar junction box

Is your meter good? And on the correct setting.

Is the load disconnected?
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Old 03-08-2018, 13:28   #3
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Re: What is inside a solar junction box

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Is your meter good? And on the correct setting.

Is the load disconnected?


Good questions! Yes, the meter is good and I am measuring Voc. I get the same reading whether in the shade or lutside in the sun.
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Old 03-08-2018, 13:35   #4
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Re: What is inside a solar junction box

Okay, just asking the basic questions first.

Recently I checked one of my unused 20 watt panels outside and it read near 22 volts at the junction box . no load
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Old 04-08-2018, 13:03   #5
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Re: What is inside a solar junction box

A typical junction box contains bypass diodes. A 60 cell solar module is created with 3 strings of 20 cells in series. Each string has a bypass diode which can bypass the sting in case of a fault. In some older solar modules used for off grid applications, the diodes were replaceable in case they were damaged. Most modern solar panels use a potting material to fully encapsulate the diodes, so no user replacable components.
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Old 04-08-2018, 21:20   #6
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Re: What is inside a solar junction box

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[...]
Does anyone know or can guess what might be in there? Diodes?
You can easily pop the lid open with a screwdriver and you'll see the tabs from the cell strings coming in and connect in series to the wires.

If one of the tabs is broken you can try and bridge that gap with some wire. If it's stainless you can't solder to it though. A temporary fix would then be a crocodile clip across the gap.
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Old 05-08-2018, 12:56   #7
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Re: What is inside a solar junction box

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A typical junction box contains bypass diodes. A 60 cell solar module is created with 3 strings of 20 cells in series. Each string has a bypass diode which can bypass the sting in case of a fault. In some older solar modules used for off grid applications, the diodes were replaceable in case they were damaged. Most modern solar panels use a potting material to fully encapsulate the diodes, so no user replacable components.


I pryed the sealent encapsulated junction box off the panel and saw only two leads coming off the panel into the box.

If a diode fails, would the failure result in a short across the leads? (Since I am reading 0.000V, there must be a short.)

Since I have only two leads coming out of the panel, what kind of a diode do you suppose is in there?

I am taking the whole thing apart and hoping that I can seal it with sealent once I am done. My guess at this point is that the plastic laminated semi-flexible panel is OK but the junction box is faulty.

Thank you for your help!
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Old 05-08-2018, 12:57   #8
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Re: What is inside a solar junction box

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You can easily pop the lid open with a screwdriver and you'll see the tabs from the cell strings coming in and connect in series to the wires.

If one of the tabs is broken you can try and bridge that gap with some wire. If it's stainless you can't solder to it though. A temporary fix would then be a crocodile clip across the gap.


Its one of those flexible panels and the junction box is encased with sealent.
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Old 05-08-2018, 16:36   #9
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Re: What is inside a solar junction box

You can also measure 0.000V if you have an open circuit. Use your ohmmeter to see if it's an open or a short.

The junction boxes I've looked at have a terminal strip (or equivalent), bypass diodes, and ribbon leads from the panel sections. You may be able to re-solder an open connection.
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