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Forum Index : Solar : PV panel blocking diode
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| davef Guru  Joined: 14/05/2006 Location: New ZealandPosts: 499 |
For the last 6 months I have been managing battery charging myself. Today I decided to put my Tracer MPPT solar controller back in the system ... to take better care of my batteries. But first, as I also have about 150Watts of microhydro, I thought I would see how the Tracer handled that. The open circuit voltage of the microhydro is 90VDC (rectified 3 phase), so it is within the rating of the controller. Well, I seemed to end up with about the same current going into the battery. Happy with that. As I previously had been directly running the PV panel and the microhydro into the battery, without any controllers, I thought I would do the same with the Tracer in the system. I get closely to the sum of the currents going into the battery. Happy with that. However, at one stage I accidentally I touched the PV +ve to the microhydro +ve without the controller in the circuit. Oops, about 3Amps flowed INTO the PV panel (probably about 50VDC). Now, to my question about the internal blocking diode. If it had a breakdown rating less than 90VDC, then maybe momentarily sticking 90VDC on might break it down. But when the sun went down no current flows from the battery into the panel, so I conclude it has not been damaged. What is going on here? I could try putting in an external blocking diode, but didn't want to throw power away. Appreciate your comments. Dave |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Hi Dave Bit of a weird one. The scientist in me says try connecting the hydro to the panel, without the controller, like you did before, to confirm the results. But you may damage the panel this time, so dont. Your not going to loose much power by adding an extra blocking diode to a 48 volt system, so it may be good insurance to go ahead. Or you could add some sort of over voltage protection circuit, with something like a 70 volt zener and relay, so if the controller becomes disconnected, and the combined voltage rises over 70, the relay is operated and the solar panel is isolated. The solar panel would be connected through the relays normally closed contacts. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Thanks Glenn for your comments. One test is enough until I understand what the consequences are! The distributor said there are NO blocking diodes in the panels. I have read that the leakage current into a "unlit" panel" is quite small, so the present trend is to leave them out and design the PWM or MPPT controller to handle that (small) issue. Still doesn't explain what I see. I'll re-read the Wikipedia article which shows a model for the PV panel and try to understand this behaviour. Evidently, the panels have bypass diodes, maybe they are conducting current at higher than normal panel voltages and NOT going short circuit. It is a 24Volt system. I have paralleled two PV panels in slightly different directions (NE and NW) and I read that you are suppose to use blocking diodes. However, my tests seem to indicate that the total current is the sum of the individual (unequal) currents. Still lots to learn here. Cheers, Dave |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Night time leakage current measured as 0.05Amps. Looking at the basic model for a PV junction I now think what happens is ... if you place a DC voltage higher than the normal panel O/C output voltage then the PV junctions become forward-biased and start conducting current. In my case about 35Watts into each 190W panel and it appears that didn't damage the panels. As a safety measure I am putting in external blocking diodes. Thanks again for the prompts Glenn. |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
Also a MPPT controller can only try to optimise one power source at a time. So, trying to run the PV and the microhydro into one MPPT controller in a non-starter! |
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