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Trying to understand the purpose of diodes?

Bossrox

Solar tinkerer
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I understand a diode is a dc directional semiconductor. other than blocking a reverse current, I don't get the need for them. I saw this diagram below in another post & it doesn't make sense to me. As you see, the diodes are bridging each string of cells but why are the strings both solid bridged & diode bridged at the same time. It appears to me the diodes aren't affecting anything being it's already solid bridged & if taken out would still be fully live. Could someone explain the purpose these are needed in solar panels?Sova Solar SS265P configuration.png
 
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The purpose of bypass diodes are primarily for safety.

All PV cells have some spot shunt defects. Some not so bad, some worse.

When a panel is partially shaded the illuminated cells will drive the shaded cells into reverse bias voltage direction.

During normal illuminated operations a cell has less than 0.6v across it, so a relatively low shunt resistance will not pass much shunt current, but when shaded with a reverse bias from illuminated series of cells, the reverse bias can be quite high causing the shunt defect spot to get very hot.

Usually no more than 20-25 cells are spanned per bypass diode which limits maximum reverse bias voltage to 10-12 vdc.
 
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The purpose of bypass diodes are primarily for safety.

All PV cells have some spot shunt defects. Some not so bad, some worse.

When a panel is partially shaded the illuminated cells will drive the shaded cells into reverse bias voltage direction.

During normal illuminated operations a cell has less than 0.6v across it, so a relatively low shunt resistance will not pass much shunt current, but when shaded with a reverse bias from illuminated series of cells, the reverse bias can be quite high causing the shunt defect spot to get very hot.

Usually no more than 20-25 cells are spanned per bypass diode which limited maximum reverse bias voltage to 10-12 vdc.
Oh, so it's like if a string in the panel fails or gets knocked out by shade, it still passes some power out?
 
Oh, so it's like if a string in the panel fails or gets knocked out by shade, it still passes some power out?
Secondary to safety, if you have enough overhead voltage with a single string of panels to keep the MPPT controller above its needed overhead voltage you will still get some output from that string. If you have two or more strings connected in parallel, any partial shading on a string will cause contribution by that string to drop out.

For many shading situations the shaded area still has enough illumination to produce some current so the bypass diodes don't actually get activated. But that string will only produce what the minimum shaded cell can produce in illumination current.

The diagram below is intentionally drawn to show the amount of shading is not always proportional to the loss of output.

PV shading.png
 
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Without diodes, or with failed diodes, current pushed through shaded cells could cause localized heating and damage to the panel.
Some vendors recommend avoiding shade when panels have near full sun, only meant to be used at lower current when sun is off angle.

1670735784163.png


 
Diodes work in two ways in solar panels, first you have blocking diodes that stop any power from a battery flowing in the reverse direction, which will run down the battery and also possibly heat up the panel and damage it, next you have bypass diodes which help stop lower outputting cells discharging the higher outputting cells (pulling down), by basically disconnecting any string of cells that are not producing enough voltage. Think of if you paralleled two batteries together, one dead and one fully charged, the dead battery will pull the energy from the charged battery, this is what a lower power cell/string will do to a better preforming cell/string.
 
Don't confuse blocking diodes with bypass diodes.

A charge controller should disconnect panels at night (or a total solar eclipse).

99.9% of the time, blocking diodes should not be used as you will usually loose more net cumulative power during the day in forward conducting blocking diodes heating loss then you save in periods of shaded string shunt leakage loss.

A typical reasonable quality series string of panels will shunt bleed 100-300 mA's if significantly shaded. A typical 300-watt panel has about 300 ohms shunt resistance. Poor quality panels can be as low as 50 ohms, good quality panels can be over 1000 ohms. (This is also why you cannot check bypass diodes in circuit with an ohm meter)

Many shading conditions still have enough illumination current generation due to light dispersion to prevent any series blocking diode from actually stopping forward conduction.

You only need enough illumination generated current to overcome panel's shunt leakage loss. For a typical 300-watt panel with 300-ohm shunt resistance that is less than 2% of full sun intensity for shaded area light level. It is rare to get that severe of shading due to atmospheric diffraction of light and reflections from nearby objects. Usually only an object less than 6-8 feet away can cast that severe a shadow on panel.
 
Don't confuse blocking diodes with bypass diodes.

A charge controller should disconnect panels at night (or a total solar eclipse).

99.9% of the time, blocking diodes should not be used as you will usually loose more net cumulative power during the day in forward conducting blocking diodes heating loss then you save in periods of shaded string shunt leakage loss.

A typical reasonable quality series string of panels will shunt bleed 100-300 mA's if significantly shaded. A typical 300-watt panel has about 300 ohms shunt resistance. Poor quality panels can be as low as 50 ohms, good quality panels can be over 1000 ohms. (This is also why you cannot check bypass diodes in circuit with an ohm meter)

Many shading conditions still have enough illumination current generation due to light dispersion to prevent any series blocking diode from actually stopping forward conduction.

You only need enough illumination generated current to overcome panel's shunt leakage loss. For a typical 300-watt panel with 300-ohm shunt resistance that is less than 2% of full sun intensity for shaded area light level. It is rare to get that severe of shading due to atmospheric diffraction of light and reflections from nearby objects. Usually only an object less than 6-8 feet away can cast that severe a shadow on panel.
I haven't seen blocking diodes used in full sized panels in years, but I see them is in sub 50w panels that are meant for direct battery connection, but I have also seen inline blocking diodes on 100w flexible panels.
Any idea why they add them on some flexible panels? I was thinking for reverse polarity protection, maybe.
 
Secondary to safety, if you have enough overhead voltage with a single string of panels to keep the MPPT controller above its needed overhead voltage you will still get some output from that string. If you have two or more strings connected in parallel, any partial shading on a string will cause contribution by that string to drop out.

For many shading situations the shaded area still has enough illumination to produce some current so the bypass diodes don't actually get activated. But that string will only produce what the minimum shaded cell can produce in illumination current.

The diagram below is intentionally drawn to show the amount of shading is not always proportional to the loss of output.

View attachment 124151
That explains alot, thx
 
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