https://en.wikipedia.org/wiki/Theory_of_solar_cells#Equivalent_circuit_of_a_solar_cell
This circuit is very easy to create with discrete circuit components. To get higher panel voltage, we can just connect a bunch of these circuits in series, or just put a bunch of diodes in series and adjust the other values as appropriate.
Here’s a simulation for it in Falstad circuit simulator: https://tinyurl.com/23x589dx
Let’s try and come up with a reasonable design a component selection for it.
Parameter | Component | Value | Reason |
|---|---|---|---|
IL | Power supply in CC mode | 10A min. | Isc of solar cells is about 7A. If the power supply is able to provide 10A then we are able to hit all operation points. |
ID | Diodes in series | Vf of diode string = 30V | 30V = max input voltage for Nomura MPPTs. We can connect others in series if we want to expand and test at higher voltages for other MPPTs. |
Rsh | Resistor | 100k Ohms or so. | Guess for shunt resistances in solar cell |
Rs | Resistor | 0.2 Ohms or so. | Guess for connection resistances in solar panel |
10A 30V programmable supply: https://www.digikey.ca/en/products/detail/b-k-precision/9103/7056819
We have one of these in the bay already. We could get more if we want to test series MPPTs. There are cheaper options for power supplies in this range, but they are not certified (we tried to get a KORAD 30V 30A unit previously but they wouldn’t let us keep it).
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.
Heatsink - diodes will get hot. Need a heatsink and fan to keep them cool.
2 of these heatsinks, with half of the diodes on each one, mounted so that it forms a channel for the fins in the middle and the flat base on the outside: https://www.amazon.ca/Awxlumv-Amplifier-Transistor-Semiconductor-300mm69mm36mm/dp/B07TJZ2MYZ/ref=sr_1_3?crid=BBU8JQU8V7FI&keywords=300*69*36&qid=1667273517&qu=eyJxc2MiOiIxLjM4IiwicXNhIjoiMC4wMCIsInFzcCI6IjAuMDAifQ%3D%3D&sprefix=300%2B69%2B36%2Caps%2C168&sr=8-3&th=1
Rough heat sink calculator: https://celsiainc.com/resources/calculators/heat-sink-size-calculator/#:~:text=Heat%20Sink%20Sizing%20Calculator%20%2D%20Rough,(Tjunction%20%2D%20maximum%20ambient%20temperature)
2 of these fans, one on each end of the heatsink module should be good: https://www.digikey.ca/en/products/detail/delta-electronics/AFB0824GHE/5799833
Fan power supply: https://www.digikey.ca/en/products/detail/mean-well-usa-inc/GST60A24-P1JR/10659909
Diode Temp rise
1.4V (diode datasheet) * 10A = 14W.
14W * 3C/W (diode datasheet) = 52C rise.
If we keep the heatsink under 100C, the diodes should be fine. A few fans for lots of forced air should be able to do this.
Adding a few thermocouples to monitor temperature of the diodes would be a good idea though.
A few forum threads on building these:
https://diysolarforum.com/threads/bench-power-supply-to-simulate-solar-panel.11564/page-2
Building and Testing the Circuit
Before jumping into a purchase to make one of these, we can make a small model of one with regular diodes and test the IV curve. Just need to make sure to keep the tests quick so the diodes don’t heat up too much.
I made the circuit with the following parts/parameters:
All tests were done with a python script available here: https://github.com/mbA2D/Test_Equipment_Control
I used Eload_cv_sweep.py to control the e-load and do all the measurements, and used Eload_cv_sweep_graph.py to make the graphs.
Test 1
Current Source: Digital power supply set to 1V, 0.1A (low current since I didn’t want to burn up the diode. Larger currents later)
Diode: Generic 10A-rated silicon diode (1 diode only for the first test).
RSH: 10k 1/8W resistor
RS: 5A glass fuse acting as the series resistance (about 30mOhm resistance).
I tested with an e-load connected to the output, set in constant voltage mode. I swept the constant voltage from 0 to 1.2V while measuring the voltage and current and plotted the following graph:
Test 2
Current Source: Digital power supply set to 10V, 0.5A
Diode: Generic 10A-rated silicon diode (10 diodes in series).
RSH: 10k 1/8W resistor
RS: 5A glass fuse acting as the series resistance (about 30mOhm resistance).
I tested with an e-load connected to the output, set in constant voltage mode. Also used remote sense on the e-load to get more accurate voltage measurements (not really needed at these low currents, but it was easy to do). I swept the constant voltage from 0 to 10V while measuring the voltage and current and plotted the following graph:
Test Results:
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