Bypass diodes for solar panels reduce the risk of damage to the photovoltaic system. When solar panels are connected in series, and one panel of the series produces less power than the others, it is forced to carry the full current produced by the other panels. This can lead to overheating and failure. Shading of a panel, dirt on a panel or mechanical damage may all result in a panel producing less power. The bypass diodes carry the current produced by the other panels around the bad panel and prevent further damage.
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Check the maximum output voltages, the power of the solar panels and whether they have bypass diodes already installed. Small solar panels with output voltages less than 18V and panels that produce less than 10 watts don't need bypass diodes since there is not enough power available to cause damage, and the voltage drop for each diode would reduce the system's efficiency greatly. For larger panels and higher voltages, commercially available panels usually have bypass diodes already installed.
Check the current output of each solar panel. Select bypass diodes that can carry the output current at the high temperatures at which solar panels typically operate. The rated current of the diode at high temperatures may be only half of the normal rated current. Check the diodes' forward voltage drop and reverse leakage. Both should be as low as possible, and will reduce the efficiency of the solar panels if the forward voltage drop is more than 0.5Vs and the reverse leakage is more than 20 milliamps at 93.3 degrees C.
Install the bypass diodes across the solar panels or within larger solar panels. For smaller panels, the positive lead of the diode must be connected to the positive output terminal of the solar panel and the negative lead to the negative output terminal. This is usually done in the solar panel junction box. If the solar panel generates an output voltage over 24V, more than one bypass diode will be required for each panel, and a diode must be connected across each series of solar cells generating a maximum of 18V. This is normally about 36 cells. Look for the two vertical tabs running across the fronts and backs of the solar cells. Check that the cells are connected in series by verifying that the positive front tabs of the first cell are connected to the negative back tabs of the next cell. Connect the negative lead of the bypass diode to the negative output terminal of the solar panel and solder a wire to the positive lead. Count about 36 cells from the first cell, and solder the positive diode lead wire to one of the front tabs of the solar cell. Solder the negative lead of a second diode to the same front tab. Repeat the procedure until the negative diode lead of the final diode is connected to the negative output terminal of the solar panel. The exact number of cells being bridged by each diode is not important, but all cells must be included in one of the diode circuits.
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