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hope you all are well

The bypass diode is usually connected in parallel with PV cell to avoid hot spot and prevent more PV output power losses. The bypass diode is connected in opposite direction to the PV cells in such way that it conduct when the PV cell is shaded by (tree).

My question is that what prevent the DC current generated by a string of PV cells in a normal condition ( no shading) from passing through the bypass diode and not through PV cell ? and why the current can flow through the bypass diode only in the shaded PV cell.

is that because the resistance inside the diode is higher than the PV cell, and the current always prefer to choose the route which has a lower resistance ? Thank you ! enter image description here

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  • \$\begingroup\$ In normal operation, the bypass diode is (weakly) reverse biased. \$\endgroup\$ – Brian Drummond Jul 29 '19 at 21:36
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As the light fades to zero, the voltage in the cell drops. In the graph below, as the light fades (lower in the graph) the voltage also drops slightly. As the amount of light decreases the cell looks like a larger and larger load (the curves on the bottom of the graph are below the 100Ω load line). So the cell starts to dissipate energy, which is not a problem if there are a few cells in series, but if there are a lot of cells with a large amount of current, the cell can overheat or be damaged.

To solve this problem a diode is place in series, when the voltage on the cell goes low due to lack of light, then it can bypass the cell. In the picture below, it might be good to put a protection diode at 0.4V so if the light is that low the current would go through the protection diode.

enter image description here

Source: http://www.aurorasolarenergy.com/iv-curve-of-a-solar-panel/

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