I have two solar panels in series giving ~106 volts when open circuit and around 4 Amps when loaded.

My mppt controller could accept 150v on the input, but it blew up when the battery was disconnected while the panels were active :( My spare controller can only accept 100v on a good day and only 92v in certain conditions.

I tried putting a 1N5377B zener across the input to prevent the controller being overwhelmed, but it only worked for a while before it went dead short and started smoking.

I found a circuit using a mosfet and a zener that when fitted in series with the -ve lead from the panels, eats a few volts and stops the controller getting overvoltage, the problem is that it always eats those few volts, even on a cloudy day when the panels are only at ~75v.

How can I prevent the controller getting too much voltage without permanently losing 10-15v? It doesn't need to disipate the entirety of the 450W, just the first 40W or so, so that the controller is never presented with more voltage than it is designed to deal with.

Except under no-load conditions, the full OC panel voltage would never be present, so the device would spend most of the time doing nothing.

  • \$\begingroup\$ What does OC @ ~4A actually mean? \$\endgroup\$ – Andy aka Feb 14 at 15:24
  • \$\begingroup\$ Open Circuit, and 'about' 4 Amps. \$\endgroup\$ – BillyB Feb 14 at 15:33
  • \$\begingroup\$ I think you need to be clear about that in your question. At the moment it reads open circuit at about 4 amps and that doesn’t make sense. \$\endgroup\$ – Andy aka Feb 14 at 15:35
  • 1
    \$\begingroup\$ I changed it to avoid confusing anyone. \$\endgroup\$ – BillyB Feb 14 at 15:40

You can easily engineer an extra MOSFET circuit that “thoroughly” shunted the MOSFET-zener follower circuit when the voltage dropped below (say) 90 volts. You could probably do it with a relay instead of a MOSFET except you’d probably worry about it switching on and off too often.

Or just use a low drop out buck converter rated at greater than 4 amps and greater than 150 volts. There are certainly buck chips that could be engineered to do this.

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