schottky
In general the diode will not be in bypass, that's true, but when it is, you have to realize that it will consume quite some power. A normal diode is not that much cheaper nor does it have much other advantages, but it will have a forward voltage drop of 1 to 2V. If you have a 2V voltage drop then you are not only loosing 2V (which is not that much perhaps) but the diode will also have to handle 2V*3.5A. That means you're burning 7W which will generate quite a lot of heat. The only advantage I can see of a regular diode is the current leakage, but that is in general in the uA.
Your calculation is correct. You will loose the voltage of the cells over which the bypass diode is connected and the forward voltage of the bypass diode.
regards,
Hante
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I`m not sure that a schottky diode is necessary when used as a bypass diode. Ofcourse schottky is good when used as a blocking diode but i would use regular diode in bypass. Those diodes doesn`t cost much and the voltage drop happens only when shadowed. Is the panel voltage drop calculated like (lost voltage of bypassed cells + voltage drop of bypass diode)? For example with 1diode/4 cells (4X0.5Vcells+0.3Vschottky) = 2.3V drop in panels output?Leave a comment:
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Number of diodes
Hello Anttistaatti,
you're right to say that it is better for performance reasons to add more diodes, but there are several problems with that. First of all it is quite expensive. A diode which can handle the AMPs is not cheap. Adding a diode every 4 cells (or so) will require a lot of routing via buswire, which is not cheap either, and requires a lot of work.
A diode which is in bypass because one (or more) cells of the string is blocked/shadowed will result in a voltage drop. That is the reason to choose a schottky diode. This drop is on average 0.3V (for schottky diode). Suppose you put a diode every cell and suppose 4 cells are in the shade then this will result in voltage drop of 1.2V, but if you have only one diode per 4 cells then this would only cost you .3V drop. The same applies ofcourse for 18 cells, though it is perhaps less likely that a large number of the cells of string of 18cells in the shadow. On the other hand if you have a diode every 18 cells and only one cell is shaded then you will not only have a voltage drop of .3V but also loose the power which would normally be produced by the cells which are not in the shade (17 * 0.5V).
With this I only want to indicate that even though it looks to be the best to have a diode every cell (something the industry would prefer if it were cheaply possible and preferably be integrated in the cell) but it is not always the best way. And for cost reasons only 1 diode every 18 cells is used.
regards,
HanteLeave a comment:
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That article is great. I still think it is better to use more diodes than 1 per 15 cells. If just small part of the panel is shaded the diode bypasses all 15 cells it reduces voltage of the panel to useless level (-15X0,5V). If we have for example 1 diode per 4 cells and one cell is shaded the voltage drop is only -4X0,5V and the panel is still working. But I am not sure did I understood that correct.Leave a comment:
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Just remember, diodes have their own liabilities. (another thing to fail)
I really like the article at http://pvcdrom.pveducation.org/MODULE/Bypass.htm :
"The maximum group size per diode, without causing damage, is about 15 cells/bypass diode, for silicon cells. For a normal 36 cell module, therefore, 2 bypass diodes are used to ensure the module will not be vulnerable to "hot-spot" damage."
MikeLeave a comment:
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Very interesting. I think I`m going to include those bypass diodes to my next DIY panel. If I uderstood correct it would be better to use as many diodes as you can. Only bad would be the cost of diodes and more work soldering them. When I tested my first panel I noticed dramatic amp drop when one cell was covered with hand. So that might help this also. Maybe 4 cells per diode would be reasonable?Leave a comment:
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Bypass Diodes
Has anyone hear incorporated bypass diodes into their DYI panels.
I have heard of a number of people using blocking diodes to prevent backwards current from draining batteries at night.
I have also read that comercial panels include bypass diodes. As I understand it if one cell is shaded or short circuits a bypass diode can prevents hot spots when the circuit tries to compensate.
Given DYI soldering I would suspect that one might expect a cell to short circuit or in some way fail over its lifetime and the inclusion of bypass diodes might minimize the impact of this failure on the whole system
below is a link to an explination
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