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I have a small multi-junction solar cell that I will be taking measurements of short-circuit current and open-circuit voltage periodically in order to approximate an IV curve for the cell.

I have decided that the best method to measure the current is a shunt with an op-amp providing amplification to a microprocessor. I have previously used a hall effect current sensor, but interference from a nearby source of magnetic fields contaminated my readings. A shunt is more tolerant to this sort of issue.

The solar cell will be providing a voltage of between 1v and 2.5v, and a current of 5~20mA. What resistance should I use for the shunt resistor, and how should I consider a resistor for this application?

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    \$\begingroup\$ As small as you can find a reasonable in amp for. \$\endgroup\$
    – Ignacio Vazquez-Abrams
    Jan 17, 2016 at 5:42
  • \$\begingroup\$ Depends on the Power rating too. Check how much current will flow through it and voltage across it. Select the power rating with smallest package available online for the cheapest price (Mouser/Element14 is a good place to filter such results) and buy it. \$\endgroup\$
    – ammar.cma
    Jan 17, 2016 at 10:50

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The solar cell's current won't change much if it is driving into 0 V or 0.5 V in your case, so, if you expect 20 mA max, you could use a R of 0.5V/20mA = 25 ohm. These are not easily available -- use 22 ohm. Total power is 12 mW, so any resistor would work.

With 0.5 V signal, you can convert directly with your ADC; you don't need an amplifier.

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