how to wire old ARCO M51 panels

I am installing additional old Arco panels on a mission house in Central America.

I was perplexed by an extra terminal in the junction box. These terminals were daisy chained together with blue wires. Black and red were used for the positive and negative. I left the blue wire terminal unwired in my installation, and wonder if that was a mistake.

Here is a photo of the terminal box:

Anyone know what the blue wire terminal is for and how to wire it?

I found that on ONE of the panels the blue wire had been connected to the negative side of the diode. This is from an array of 27 panels on the previous installation. Almost every other panel had the blue wire attached to an unmarked terminal. Thanks!


  • BB.BB. Super Moderators, Administrators Posts: 32,609 admin
    Re: how to wire old ARCO M51 panels

    I could not find any datasheets online--There are two forms of diodes... Series and Shunt.

    Series diode is used to prevent current back flow at night--typically from an attached batter--or possibly when the panels are installed in parallel strings (prevents a short in one panel/string from grounding power from the other strings).

    If these panels are behind a modern battery charge controller--you should not need to wire in the series diode. Also, since each diode provides ~0.2-1.0 volts of drop--they may have chosen to only wire one of the panels in each series string. Saves the voltage drop from other diodes in the same series string.

    If it is a shunt diode--then if you had a large series string of panels, then if there was shade on one panel (or a panel failure), then the shunt diode will bypass current around the failed panel. In a small system, this sort of shunt diode probably will make no difference in your system.

    See this WindSun FAQ on solar panel diode for a clearer explanation.

    Sorry, don't have any more information...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: how to wire old ARCO M51 panels

    Thanks for the response. I suspect the diode in each panel terminal box is a bypass diode since there is one per panel. I added more panels and now have 3 circuits in parallel with each circuit having 6 panels in series. I will add the fourth circuit soon.

    These feed an OutBack MX60 charge controller which goes into a sleep mode at night and I assume prevents battery discharge, so a blocking diode would not be needed.

    I hope someone out there can tell me what those blue wired terminals are for.
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