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Blocking Diode

Discussion in 'Photovoltaics' started by Scott Stevens, Dec 18, 2004.

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  1. I have 1 48 v array now and want to expand, should i use blocking diodes in
    series with each array?
    As it enters the inverter.
     
  2. Gymy Bob

    Gymy Bob Guest

    Yes. Could some knowledgable person explain the purpose for these bypass and
    blocking diodes and the problems without them, please?

    I have been around so long with this stuff I believe I invented the diode in
    1941 but I am not familiar with the solar panel usage requirements of them.
    (no P & N substrate explanations please. I wrote the GE manual...LOL)
     
  3. Background for those who need it: A diode conducts electricity in only
    one direction.

    Blocking: If the positive line is more positive than the panel, a
    blocking diode prevents panel draining power from the system. Connects
    in series between the panel and battery. Causes a bit of power loss
    (power=0.6v * amps, where 0.6v is a typical power-diode drop).

    Bypass: A shaded cell or panel in a large series string (typically 72
    or more cells in series, a 24v or greater nominal array) can be damaged
    in the potential across it is too great and causes junction breakdown.
    A bypass diode allows the current to bypass that panel instead of thru
    it. Connects in anti-parallel to the panel so there is no conduction
    and hence no loss in typical case. My [email protected] panels came with bypass
    diodes already wired across both 12v halves of each panel.

    sdb
     
  4. Gymy Bob

    Gymy Bob Guest

    This seems to be what I thought but if a panel is in the shade then it's
    series string has a lower voltage than any others (or not enough to charge
    anything) and will produce no current anyway making the diodes
    non-functional. Also these bypass diodes do not bypass each individual cell
    so wouldn't the same thing happen to echa cell in the 12V block? Perhaps
    122V can be tolerated by the cell junctions but more cannot?

    Thanx for that.
     
  5. For example, I run a 48v system (nominal, actual battery voltage is
    about 54v and panel open-circuit is about 80v). If one panel is shaded,
    the other 36v might damage that shaded panel. The bypass diodes ensure
    that the current is conducted around the panel, so the panel will only
    see 0.6v of reverse polarity instead of 36v.
    Exactly.

    sdb
     
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