Solar Panel reverse polarity protection

[BALKRISHNA TULSYAN]

Hi,

Does solar panel too requires reverse polarity protection OR reverse polarity protection is only required in case of battery???

I tried to explore internet and gone through various post but nowhere i found Reverse polarity protection for solar panel, instead i got reverse current flow to solar panel protection using MOSFET at panel side. It may be possible that same MOSFET might be protecting from reverse polarity too along with reverse current flow protection to solar panel but i am NOT sure.
But i am sure that the MOSFET is used for reverse current protection.

Can anyone guide me on this.??? It is really important to me.

Thank you..!!!

 

[Audioguru]

Do not let anybody connect the polarity of your solar panels backwards.

 

[betwixt]

PV panels are a source of power so connecting them backwards, at least in daylight, is more likely to damage your circuits than damage the panel.
However, electrically they appear as lots of cells in series and each cell behaves a bit like a diode so if you exceed their turn on voltage in reverse and supply enough current they might be damaged. Although the risk is small, it would be sensible to wire a diode in series withthe PV output to protect it against reverse polarity.

Note that many PV panels have "bypass diodes" but these will not conduct if the polarity is reversed.

Brian.

 

[FvM]

Reverse breakdown of solar cells can only happen in series connection of multiple panels and is prevented by the mentioned bypass diodes. Solar panel reverse current (= solar cell forward current) is the more common problem if you connect a solar panel directly to a battery because it discharges the battery.

 

[BALKRISHNA TULSYAN]

Although the risk is small, it would be sensible to wire a diode in series withthe PV output to protect it against reverse polarity.

Normally diode in series at the output of the PV is used for reverse current flow. Will it also protect from reverse polarity of solar panel.

I am using N-MOSFET at the -ve output of solar panel and controlling the gate of MOSFET through microcontroller after sensing voltage of panel and battery. MOSFET will be switched ON only in case panel voltage is greater then battery voltage so that reverse current to panel is protected during night time or in low light condition. I am using MOSFET in place of diode to reduce the voltage drop and power loss, so that efficiency of controller is more.

So NOW my question is, does the same MOSFET, however i am using for protection from reverse current flow, will also provide protection from reverse polarity of solar panel by default.


Thank you..

 

[betwixt]

We have to be careful with the wording here. If you mean will it protect is someone reverses the wires to the PV panel, no it will not, the reason is there is effectively a reversed diode between the drain and source of almost all power MOSFETS and it will start to conduct.

If you are concerned that power can flow 'backwards' from your circuit towards the PV, it is unlikely but not impossible as the voltage has to be above a certain threshold before the PV will become conductive but the MOSFET alone may not be enough to prevent it. If there is such a risk, wiring a diode (use Schottky for lower Vf) in series with the PV is still advisable. We would have to see your schematic to be certain so possible current paths can be investigated.

Brian.

 

[Twisty]

MOSFET will be switched ON only in case panel voltage is greater then battery voltage so that reverse current to panel is protected during night time or in low light condition. I am using MOSFET in place of diode to reduce the voltage drop and power loss, so that efficiency of controller is more.


So NOW my question is, does the same MOSFET, however i am using for protection from reverse current flow, will also provide protection from reverse polarity of solar panel by default.


Thank you..

As you have described it then NO your system will not provide reverse polarity protection in the case of battery being connected the wrong way round.
If you change the system so that it only switches on the N-MOSFET after checking that:
1) Polarity of both the solar panel and battery are positive; and
2) The solar panel voltage is greater than that of the battery

As you described it then you are only checking for condition 2, this logic alone will not provide protection as a battery connected in reverse will be showing a negative voltage, so it will always be a lower voltage than your solar cells.

Note that you should add some hysteresis/switch-on delays in your logic too or you will get rapid switching of the FET when the sun sets.