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  1. #1
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    Default Solar panel diodes and more.

    Acknowledgement: Part of the following diagrams and text is based on material obtained from the www.solar-facts.com website.

    There is some controversy about the functioning of diodes as applied to solar panels that are connected in series. This post is an attempt to provide some insight into the use and application of the diodes that we generally find on solar panel modules. These diodes are actually part of the panels and they are not always visible to the eye. They can be integrated on the panel or in the junction box on the back of panels. Do not ever remove these diodes for they serve an important role in the functioning, performance and longevity of a solar panel.

    Diodes for Solar Panels.

    There are two different types of diodes which play an important role in the functioning of solar panels. The diodes themselves may be identical, but they differ in the way in which they are applied. First let's confirm what a diode is.

    Diode - what is it?
    A diode is made of semiconductor material, usually silicon, with two terminals. It's function in it's simplest application is to allow electrical current to pass in one direction but not the other. It will pass current when forward biased, and block current when it is reverse biased.
    Eggie.

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  3. #2
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    Default Re: Solar panel diodes and more.

    Blocking Diodes
    Below is a picture with 3 components. Diagram 1 shows a simple setup with two panels in series and charging a battery. For simplicity no controller is shown. A blocking diode is in series with each of the 2 panels and the panels are wired in series. When the sun shines, as long as the combined voltage produced by the two panels is greater than that of the battery, the diodes will be forward biased and charging will take place.

    In darkness, no voltage or current is produced by the panels. Now the diodes will be reverse biased because the battery voltage is greater than the panel voltage, and no current flows. If not for the blocking diodes in the circuit, the voltage of the battery would have caused an unwanted current to flow in the opposite direction through the panels, thereby discharging the battery.

    Blocking diodes come into play to protect the panels in a simple solar system that charges a battery. Blocking diodes are usually included in the construction of solar panels, so no external or additional blocking diodes are required.

    By-Pass Diodes
    Diagram 2 is the same circuit, but shown with 2 bypass diodes in place.

    Blocking & by-pass diodes as in a typical solar panel
    In diagram 3 the circuit is shown with both blocking diodes and bypass diodes in place. These diodes are already included and integrated in the typical solar panels we buy.

    Click image for larger version. 

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    Last edited by Eggie; 2016/06/13 at 10:07 PM. Reason: pic added
    Eggie.

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    Default Re: Solar panel diodes and more.

    Practical example
    Now let's consider a practical example of 2 x 12V panels in series charging a 12V battery with a solar controller included. With this example we can analyze what would happen if one of the panels in the diagram 4 below is shaded. Shading of part of a panel may typically be caused by a tree branch, debris, snow, etc. A shaded panel may not be producing any significant power. It may also have a high resistance to current passing through it, thereby effectively blocking the flow of current generated by unshaded panels.

    Click image for larger version. 

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    The small arrows show the direction of current flow. A fully shaded panel does not support any forward current through the solar cells. Therfore the bypass diode, now forward biased, passes the current around the shaded panel with its inactive cells. Note that battery current can be the same as, or different to, the current through the panels. This is determined by the type of controller in use.

    With the lower panel shaded, the by-pass diode on that panel comes into play since it will be now become forward biased by the voltage produced by the good panel. The current produced by the good panel can now flow unhindered through the by-pass diode to avoid the high resistance path of the shaded panel.

    Note that by-pass diodes come into play when panels (or strings of cells within a panel) are connected in series to produce higher voltages. They serve to protect against reverse current flow in panels and cell strings, thus protecting against unwanted voltage losses, power losses and internal heating (burn-out) of cells in a panel. Some solar panels are constructed with the cells divided into groups, each group having a built-in by-pass diode. A typical 12V panel may have 1, 2 or more 36-cell strings internally. Each cell produces approximately 0,6V open circuit and 0,5V when operating at its max power level.

    Solar Controller type

    Consider the effect the type of solar controller would have on the currents in the above example.

    1. PWM solar controller: The current delivery to the battery remains unchanged regardless of whether there is partial or full shading on the lower panel. Also note that battery current = panel current. With a PWM controller in place and both panels in full sun, this particular circuit will work, but it is not capable of even nearly harvesting the full potential of the 2 panels.

    2. MPPT controller: Battery current is no longer equal to panel current due to the DC-DC conversion capabilities of the MPPT controller. The current through the top panel remains practically unchanged even when the lower panel becomes partially or fully shaded. In the lower panel this current can either flow through the solar cells or the bypass diode or partially through both - depending on the degree of shading. The voltage delivered to the MPPT controller varies non-linearly and quite a lot as the degree of shading on the lower panel changes.

    The battery shown in the diagrams is assumed to be big and only partially charged. In other words - it will accept all the available charge current.
    Last edited by Eggie; 2016/06/13 at 10:17 PM.
    Eggie.

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    Default Re: Solar panel diodes and more.

    Hi, just revisiting this one as I have a problem.

    My mate and myself were checking over his system for a trip he has coming up. When we plugged in the 100 w solar panel into the WRND, the fuse would blow. After tracing everything back we opened the little black box on the back of the panel and saw that one of the diodes was broken.

    Can anyone please assist with advice on

    What type of diode required

    Where can we get ?

    They appear to be soldered onto lugs, anything we should be aware of when we attempt the repair ?

    Thanks
    Stranger

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    Default Re: Solar panel diodes and more.

    Quote Originally Posted by Stranger View Post
    Hi, just revisiting this one as I have a problem.

    My mate and myself were checking over his system for a trip he has coming up. When we plugged in the 100 w solar panel into the WRND, the fuse would blow. After tracing everything back we opened the little black box on the back of the panel and saw that one of the diodes was broken.

    Can anyone please assist with advice on

    What type of diode required

    Where can we get ?

    They appear to be soldered onto lugs, anything we should be aware of when we attempt the repair ?

    Thanks
    Maybe try Mantech for a new diode? https://www.mantech.co.za/

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  10. #6
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    Default Re: Solar panel diodes and more.

    You can use almost any typical silicon diode or Schottky diode.
    Choose one that can handle some 50% more than the current that the panel can produce.
    Almost any electronics parts shop can help with the diode.

    I think you probably have a faulty WRND unit and suggest you have it checked out by someone who is expert enough to do this. Otherwise get a replacement for the your WRND controller.
    Eggie.

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    Default Re: Solar panel diodes and more.

    Deleted double post
    Last edited by Stranger; 2021/09/13 at 09:14 AM.
    Stranger

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    Default Re: Solar panel diodes and more.

    I canít help but think that pos and neg are swapped.
    Cheers

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