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Diode string - 30 of these in series should be reasonable: https://www.digikey.ca/en/products/detail/vishay-general-semiconductor-diodes-division/VS-E4TU2006FP-N3/8269295
We can connect all the diodes in series, but have a movable connection so that the string length can be adjusted for different numbers of solar panels.
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These tests clearly show a maximum power point is reached with a smooth curve to the top of it, in contrast to the IV curve of a power supply that has a sharp drop at the MPP. This simulates solar panels very well and can be used to thoroughly test the MPPTs, removing the solar panels from the equation.
More Design Points
1 Diode is roughly 1 or 2 solar cells equivalent depending on forward voltage. We want to be able to do up to 30V output voltage even at low currents, so we need 30/0.7 = 42. We’ll go with 40 diodes since the ones we’re using have slightly higher voltage drop. 20 diodes on each heatsink.
Diode spacing: 200mm x 70mm heatsink
Diodes about 20mm tall, 10mm wide. 20mm horizontal spacing.
Heatsink can dissipate 300W - user should be responsible for obeying this. Safety features could be added in the future (Arduino-controlled MOSFET with a bunch of temperature sensors, voltage sensor, and current sensor).
Rshunt: 0.1Ohm should be good.
Rseries: 1k should be good.
Power Input and output connectors: Banana jacks