# What is the simplest method to simulate a solar panel on a current/ voltage regulated PSU?

Solar panels have a typical non-linear I/V relation. The figure below shows an example of such relation. What is the simplest method to simulate this relation with some current or voltage regulated power supplies and minimum component count?

The simulator will be used to test Maximum power point trackers. I found this circuit, but I'm looking for a simpler solution. I'm looking for a cheap solution.

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If you want to test the algorithms and not real MPPTs, you can buy a small panel for a relatively cheap price and try th principles on it...I've done such experiments, but on Matlab :) – clabacchio Mar 29 '12 at 17:18

Agilent makes a solar panel simulator. This may give you the best results at the highest cost.

The simplest circuit is actually mentioned at the beginning of the paper you linked. It isn't a very pretty circuit in the sense that it wastes power, but it will give a similar behavior to a solar cell. It uses a power supply that can operate as a constant current source, and a bunch of power diodes.

If you take a standard 60 cell panel, it should output at least 6-7 Amps at about 30 Volts. That means you would need a power supply capable of delivering more current than that at over 30V, and it should have a constant current mode.

The diode string needs to have enough diodes that the forward voltage drop at the short circuit current (Isc) is equal to the open circuit voltage (Voc). For example, this diode has a Vf of about 0.66V at a current of 6A. Ignoring thermal effects (which will probably play a part), you would need about 46 diodes in series to emulate the solar panel.

Note that the price you pay for such simplicity may be a big heat sink on the diodes, and lack of flexibility. Also, the performance will be dictated by the power supply - if your MPPT circuit behaves in certain ways, the power supply may not be able to keep up. I might set up a simple load testing circuit that can switch between different loads rapidly to verify that the power supply can keep up.

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(1) Required complexity depends on required fit to a "real" panel, but a moderately good result can be had from
a constant current source
feeding a voltage regulator,
with a series resistor in the output
plus a high resistance resistor bypassing the voltage regulator to provide a soft voltage knee.

This would require an eg LM317 + 1 resistor to provide constant current,
followed by an eg LM317 2 resistors as the voltage regulator
plus a series resistor to provide some "droop" as increasing current is drawn plus a resistor to output from Voc or a resistor divider to make Voc from a higher voltage.
throw in a few filter capacitirs
and start playing with the series resistor and output knee softener to get the shape you want.

If using an LM317 and wanting 0.5 or so volts out - ie 1 PV Si cell - a few diodes in series with the output will drop enough voltage to offset the otherwise 1.25V minimum.

(2) At low power levels - and you can test MPPT algorithms at low power and scale them directly - power level makes little difference - a small photovoltaic cell and a variable brightness LED light of a few Watts output - or a steady few Watts of light that can be moved, makes a very simple, relatively low cost near perfect simulation of itself.

Spectral output of light is quite different from standard AM1.5 1 kW/m^2 sunlight but this is reasonably irrelevant here.

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