Checking bypass diodes on solar panels: Part 1

Almost all solar panels include integrated bypass diodes. Crystalline panels generally have three of them, which are located in the junction box and can each bypass a third of the panel when necessary. The diodes’ main task is to protect the solar cells from overheating when partial shading occurs. When combined with the right inverter, they can also help minimize yield losses on partially shaded roofs, as I’ve already mentioned a few times on this blog (may be not translated yet…). In this post, I’ll describe how to check whether all of a solar power generator’s bypass diodes are still in working order, which diode faults could occur, and how to correctly detect them. The first part of the post is about missing diodes; in the second part, I’ll discuss short-circuited bypass diodes. This post is intended for specialists and might not be suitable – or interesting – for anyone who doesn’t particularly care for technology stories.

scheme of open bypass diode on solar panel

It’s not unheard of for a bypass diode in a solar power generator to be defective. Since bypass diodes only jump into action when a panel is shaded, defective ones tend to go undiscovered for a while. As I mentioned earlier, there are two types of problems that can befall a bypass diode, and they each present in different ways. In the first case, the bypass diode doesn’t conduct any electricity at all in either direction. This can occur if a diode was never installed, if the diode isn’t properly connected, or a strong current damaged the diode. In this case, the system operator wouldn’t notice anything at first. Only when the affected panel is shaded will the entire string’s current drop to that of the shaded cell; alternatively, the cell will even start consuming electricity if the inverter can reduce the voltage enough. If the bypass diode were working properly, it would only have to take on the voltage from the unshaded cells in its part of the string; as the negative voltage increased at the shaded cell, the bypass diode would become conductive. When the bypass diode is defective, however, it’s unable to intervene, and the cell receives more and more negative voltage until it eventually reaches a breaking point. When I say “breaking point” here, I’m not talking only about a mechanical malfunction, but also about the point on a diode’s curve when it also becomes conductive in the reverse direction. For some diodes (Zener diodes), this effect is used to achieve stable voltages in electronics circuits. For many others, though, the effect leads to damage because of the high power loss. Solar cells can withstand this breaking point for a short period of time, but they become so warm that damage can’t be ruled out over longer periods of time. The negative voltage that takes a solar cell to its breaking point is about 14 volts. Since 23 unshaded cells supply about 0.55 V * 23 = 12.77 V, more than 24 crystalline cells are never combined in a partial cell string. This way, a partially shaded cell can force the open-circuit voltage to the unshaded cells, thereby preventing too great of a current from flowing. If the bypass diode is working, that is…

To detect a defective bypass diode, you’ll need to send current through the solar power generator in the usual direction, preferably at night. Connect a power supply unit that will try to send electricity through the solar cells. Since the cells can’t supply power at night, the bypass diodes – if they’re working properly – will become active and guide the current past the solar cells. If a bypass diode is defective, no current will flow for the time being. Before conducting the test, you should know how many solar panels were connected in series in the string in question and how many bypass diodes each panel has. Multiply that number by 0.4 volts to find the amount of voltage that should cause the bypass diodes to become conductive. Above this voltage, the current should increase exponentially. If you conduct the test with our pvServe, set the current limit at about 50 percent of the nominal current for the panel being investigated.

For five-inch cells, for example, that level is about 2.5 amperes. Then, continue to increase the voltage until that current is reached. If you’re inspecting a string with 15 panels and each panel has three bypass diodes, then the diodes’ forward voltage is about 0.4 V * 15 * 3 = 18 V. The current should be reached when the voltage is slightly higher than that number. If no current is flowing, the cause is most likely a defective bypass diode.

The next step is to find out which panel has the defective diode. For this, you’ll need much higher voltages – and a lot of careful attention. If only one diode is defective, then in the above example only 17.6 volts (instead of 18 volts) will be needed for the undamaged diodes. The current will have to flow backwards through the solar cells at the damaged diode. For 24 cells, the “breaking point” voltage is about 336 volts, which means that about 400 volts is needed for a current of 2.5 amperes. This current will heat up the affected cells relatively quickly, and the warmth will be visible with a thermographic camera. Since operating at their breaking point is very wearing on cells, make sure the test doesn’t take too much time. The difference in temperatures compared to the unheated panels is sufficient to quickly find the affected panel. If multiple diodes are defective in a string, things get a little more complicated, since the voltage always increases by about 340 volts (in the example above). pvServe can be used to detect up to three diodes in panel strings that aren’t too long (maximum voltage: 1,000 volts). If even more diodes are defective, your only option is to split up the string into multiple partial strings and inspect them separately.

thermography of a solar panel with one damaged bypass diode


  1. Hi, i have a defective panel that shows 0 voc when measured.
    The panel has 3 bypass diode.
    Can i say i have 3 shorted diode?
    I did test with multimeter with continuity mode..i got a beep sound showing short circuit when connected at the module connector

    1. Hi,
      there are two possible reasons for zero voltage:
      1. 3 defective diodes (shorted)
      2. a broken connection

      If 2. would be true your multimeter wouldn’t beep, therefore it’s most likely case one.

      Best regards
      Matthias Diehl

      1. Hi Matthias,

        Thank you for your insight. Other than manufacturing problem, usually what causes the bypass diode to be shorted? Initially the string run normally than suddenly one day all of the module in the string does not producing any power..upon checking, all of the module have defective bypass diode (shorted). One abnormal that i found, one of the module connector are melting. Could the melting connector are the culprit? my hypothesis, once connector melt, the connector exposed to water ingression and cause a short spike of high current that would destroy the by pass diode.

        Another question is, if all of this happen because of short and high current pulse, are the cell of the module are still ok?

        1. Hello,
          we observed several cases where nearly all bypassdiodes were damaged (shorted). In all cases this was because of overvoltage after a
          lightning strike in the neighborhood. In most of these cases the inverters were also damaged.
          There is a rule of thumb that overvoltage leads to shorted bypassdiodes whilst high current leads to open bypassdiodes.
          Matthias Diehl

  2. Interesting conversation.
    I can confirm that we have seen lightning causing short circuited bypass diodes (due to high voltage).

    What would be the cause of high current?
    *A second lightning strike on already short-circuited bypass diodes?
    *Or something else?

    Does the cells get damaged with lightning strikes?

    1. Hello Riaan,
      it depends. When a direct strike hits a module the cell(s) are completely damaged. A lightning event in the neighbourhood may only lead to an overvoltage and shorted bypassdiodes.
      We had several cases where the modules could be repaired, by changing the diodes.
      best regards Matthias

  3. Hi Matthias,

    I suspect I have a missing diode in a module of a single string connected to a single inverter (I have 3 strings and 3 inverters in total). When at full sunlight nothing happens, but when a cell is partially shaded in that string, the power output is reduced by 90% roughly. Shading other cells of the other strings produce a drop in power but not even close to the first one.

    I have a dc clamp meter and a thermographic camera to use… however I only have inspected the string without shading and every thing looked fine.

    If I shade a cell on the problematic module I should see the current flow through the string drop to zero, because the missing diode won’t bypass the cell and this one will block the current, is this right?

    If I do this on any other (healthy) module of the string the current won’t drop to zero (because the bypass diode will be active) but the voltage will be reduced by 1/3 of the module voltage?


    1. Hi José,
      you are absolutely right. A missing diode will lead to a significant drop of the outputpower, when the substring of the module where the bypassdiode is missing, is shaded. Whithout shading you will see no effect. When you shade a part of a module with healthy bypassdiodes and the inverter reduces the voltage by one third of the module-voltage, you should see the typical checkerboard pattern with your thermography-camera. I mention this, because not all inverters have a proper mpp-tracking. Some keep operating on local maxima instead of reducing the voltage and shift to the absolut maximum.
      Nice to see, that our blog is even read in Uruquay 😉
      Greetings to South America
      Matthias Diehl

    2. Hi Matthias,

      Can we say that if we have all of 3 bypass diode damaged (shorted) we still have no reduction of energy production if we do not have shade ?
      If so what other problem can we have ?
      Best regards

      1. Hi Nicola, when the bypassdiodes are shorted, the module is completely dead. It is true what you say for an open bypassdiode path.
        If it remains undetected cells can be damaged when they are shaded because the power that is converted to heat in the cells is no longer limited.
        Matthias Diehl

  4. I have a 150 watts module, and something wrong happened with its 9 diodes (lined in threes)..

    I’m getting and approx 19.8 Voc and the Amperage is dropped to approx 3 (I should have 8, from what I know).

    Is changing the 9 diodes going to fix this Solar Panel?

    Please, help me.

  5. Matthias, you have explained this thread very well & im very grateful for your simple yet concise explanation without going into some hectic scientific formulas etc. I am doing missions work in Zambia & were totally off the grid in rural Zambia. I run a 48v 4kva hybrid inverter with 4 x 255w panels. I’ve had issues with one of the batteries for a while now. I couldn’t afford to replace with exact battery withsame internal resistance & found out the hard way that one cannot replace a battery with any old battery! Anyway we’ve been limping along suffering the earlier switch off times until we can afford a better battery pack. However, of late we’ve had a lot of cloud cover and my inverter has been disconnecting the panels (even in full sun!). Yesterday I tested my four panels. Off load (open circuit) they test fine when the sun was weak & in full sun, but when connected I had one panel reading 1.5v and the other three ranged between 24v-18v each. I removed the “faulty panel” showing 1.5v and saw the three diodes you speak of but was totally flawed by the voltages I saw!! Open circuit I had 10v from each string which I now know to be normal, but on load was a whole different story!!! I’m going to do some more tests today thanks to this blog that has given me lots to think about!!! Just a shout out though for this blog!! Thank you!

    1. Hi Darren,
      thanks for writing from Africa. Nice to hear that the blog is read all over the world. If the voltage breaks down under load there can be two reasons:
      defective cells or loose connections between the cells. Sometimes these effects are temerature dependent.
      Greets to Zambia

  6. Hello Mathias, one of my solar panels was not giving more then 9v. So I took the diode out and now measure up to 3v? Just wondering wether to replace the diode or do a check first short circuiting the diode connection,
    Or is my panel for some reason dead.?
    Thanks rob

  7. Hi Matthias,
    If there are bad diodes in a panel, would this be evident from an initial assessment with a thermal camera? Do you expect a method like shading certain strings to be a viable method of testing diode function? Can you recommend any alternative methods (other than use of a thermal camera or physical inspection of the J-box)?

    Thanks for your help.

  8. Hello,
    One of my friend running solar pump with 14 panels ..Suddenly the panels are showing only 50% of its nominal voltage ..
    What could be the reason ..?
    Does defective blocking diode in a single panel affects the whole string ….
    Need your advice..
    6 cell panels…
    265watts ..

  9. Dear Matthias,

    We put into power 300kW PV roof top plant, and one string on one inverter was showing 640V instead of 790V same strings on all 12, 25KW Fronius Eco inverter with 19 modules in series connected. When we open the string in the inverter there was a sparks occurring. We assume that we have at least three damaged modules in the string. Didn’t manage to check all the modules in the string. We leave the inverter powered on, everything is protected with gPV fuses, so the question goes. How long this string can operate if we have more than one damaged modules, are we in danger if the strings stay like this for a longer period before detecting the main problem? As I wrote above on the end of the solar cable there was a spark and the polarity of string “+” and “-” is connected properly this was double checked.
    Thanks in advance for Your help.



    1. Hello Goran,
      you probably have several shorted bypass diodes. You should remove the 640V-string because it will no longer contribute to the energy yield. You’re seeing sparks because a current flows from the strings with the 790V to the string with the lower voltage. If you have no string fuses I strongly recomment to remove the string. It’s not very difficult to find the affected modules with a thermography camera.
      Best regards Matthias pvBuero

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