diodes always in juction box (back of PV panel)?

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stevehull
stevehull Registered Users Posts: 20 ✭
in Solar Beginners Corner #1
Are diodes always in the junction box that is typically on the back of a PV panel?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: diodes always in juction box (back of PV panel)?

    Do you mean "are there always diodes in the junction box" or do you mean "if there are diodes are they always in the junction box"?

    From my experience better than 99% of the time if there's a junction box there's bypass diodes in it. Never seen them elsewhere, but have seen (small) panels without them.
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  • stevehull
    stevehull Registered Users Posts: 20 ✭
    #3
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    Re: diodes always in juction box (back of PV panel)?
    Cariboocoot wrote: »
    Do you mean "are there always diodes in the junction box" or do you mean "if there are diodes are they always in the junction box"?

    From my experience better than 99% of the time if there's a junction box there's bypass diodes in it. Never seen them elsewhere, but have seen (small) panels without them.

    I am asking if the juction box is there are there always blocking diodes in it? How would you test for this? I know how to test an isolated diode with ohmmeter, but not in series with a PV panel.

    I was just given a few 20 W panels WITH junction boxes, but there were for preproduction in house panel testing (not preproduction for outside evaluation - important difference). These were supposed to be "destroyed" so I can't call the company to ask . . .
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
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    Re: diodes always in juction box (back of PV panel)?

    Blocking diodes: no. They are common to small panels (sometimes) but rare in large ones because for most installs they aren't needed.

    For anyone reading this and not understanding the difference:
    A blocking diode prevents current from flowing backwards through the panel. Most often this will happen at night when the panels aren't producing and the batteries are directly connected. In-line charge controllers usually prevent this anyway.
    A bypass diode is integral to the panel design and allows current to flow around a shaded section of the panel so that it does not act as a resistor to the whole.

    You can test a blocking diode by locating it and taking an Ohm reading across it in either direction: one way you get continuity the other way you don't. No continuity either way = open diode, continuity both ways = shorted diode. Testing a bypass diode is more difficult as it is parallel to the solar cells and as such it's not a good idea to touch an Ohm meter to it as the cells can be producing more Voltage than the meter will take. It's best to disconnect the diode before checking it.
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  • stevehull
    stevehull Registered Users Posts: 20 ✭
    #5
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    Re: diodes always in juction box (back of PV panel)?

    How would I determine if there is a blocking diode in the glued on junction box on the back of the PV panel?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
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    Re: diodes always in juction box (back of PV panel)?
    stevehull wrote: »
    How would I determine if there is a blocking diode in the glued on junction box on the back of the PV panel?

    Easiest way: open it. :D
    Otherwise you try to measure continuity across the whole panel with the panel completely covered so that it doesn't produce any current. Instead of using an Ohm meter, use a battery and light bulb. The panel should conduct one way but not the other. Note that this is a generalized test that may not work with all panels, since you never know exactly how they are built.

    For the most part it usually does not matter if they have a blocking diode or not. If one is needed for the application it can be added externally. It will probably shave half a Volt off the output, but if the panel output is high enough to begin with (as in the 17 to 18 Vmp normally expected on a "12 Volt" panel) it won't make much difference.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #7
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    Re: diodes always in juction box (back of PV panel)?
    stevehull wrote: »
    How would I determine if there is a blocking diode in the glued on junction box on the back of the PV panel?
    If you see a diode and want to know whether it is a blocking diode or a bypass diode, look at how it is wired into the circuit. Bypass diodes are in parallel with sections of the panel or the entire panel. If you see a stack of diodes in series, connected at one end of the diode string to the + of the panel and at the other end to the - of the panel, and with additional panel leads connected to the junction points of the diode string, then you are looking at bypass diodes.
    If you see either a single diode or several diodes paralleled together, wired in series with one of the panel leads (so that all current must flow through the diode(s), then you are looking at a blocking diode.
    SMA SB 3000, old BP panels.
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  • stevehull
    stevehull Registered Users Posts: 20 ✭
    #8
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    Re: diodes always in juction box (back of PV panel)?
    inetdog wrote: »
    If you see a diode and want to know whether it is a blocking diode or a bypass diode, look at how it is wired into the circuit. Bypass diodes are in parallel with sections of the panel or the entire panel. If you see a stack of diodes in series, connected at one end of the diode string to the + of the panel and at the other end to the - of the panel, and with additional panel leads connected to the junction points of the diode string, then you are looking at bypass diodes.
    If you see either a single diode or several diodes paralleled together, wired in series with one of the panel leads (so that all current must flow through the diode(s), then you are looking at a blocking diode.

    Here is the problem. I can't see the diodes as they are glued into the junction box (if they are there) and opening the junction box destroys it.

    Can't see the diodes! So, I ask again, what test can I use to determine if a diode is in there . . . .
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #9
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    Re: diodes always in juction box (back of PV panel)?

    You can back drive the panel (in the dark) with 12-48 volts... If there is a blocking diode, then the leakage current will remain low. If there is no blocking diode, then as the voltage increases from 12 to 48 volts, you will (probably) see an increase in leakage current.

    Make sure you are connect panel + to power supply +... if you you connect the panel backwards, it will conduct electricity just like any forward biased diode--And if connected to a high amperage source (like a 12-48 volt lead acid battery), it will ruin the panel.

    Why do you want to know if there is a blocking diode? My guess is that there is no blocking diode (costs money, waste heat to dissipate, loses a few extra watts of energy)--So unless this panel was designed to connect to a battery bank (instead of through a charge controller), there will be no blocking diode.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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