μSMU is a small source-measure unit designed for the very low-cost electrical characterisation of photovoltaic cells.
SMUs are "4-quadrant" devices, meaning they can both source and sink current at both positive and negative voltages. This makes them very useful for semiconductor device characterisation - including LEDs, transistors and solar cells.
In photovoltaic research laboratories, a SMU is typically used to vary the voltage applied to an illuminated solar cell, whilst simultaneously measuring the current. This voltage sweep allows us to plot the solar cell's I-V characteristics, and calculate its light-to-power conversion efficiency.
SMUs are generalised pieces of test equipment, designed to be highly sensitive over vast current & voltage ranges. For example, the workhorse Keithley 2400 has a voltage range between 100 nV and 200 V, and a current range between 1 pA to 10 A. This is likely overkill for most research and education applications concerning solar cells, which tend to operate between 0-5 V and μA to mA. The μSMU doesn't intend to replace precision SMUs, rather to supplement them in cost-sensitive areas where such precision is not required.
The μSMU is largely inspired by Linear Technology's DC2591A evaluation board, which demonstrates an I2C address translator IC to interface up to 8 modules containing several I2C devices with an Arduino-style board. Someone consequentially, these boards also contain fantastic SMU circuits!
The voltage applied to the device-under-test (DUT) is supplied by a LT1970 opamp driven by a 12-bit DAC on the non-inverting input and a 2.048V reference on the inverting input. The current flowing through the DUT is measured by amplifying the voltage drop through a high-side 10 Ohm shunt resistor. Both the DUT voltage and shunt resistor voltage drop are measured using a 16-bit ADC. The whole system is controlled using a STM32F072 microcontroller, which presents a USB virtual communications port for interfacing.
| Parameter | |
|---|---|
| Voltage range | -5 to +5 V |
| Voltage measure resolution | ~0.6 mV |
| Minimum voltage step size | ~10 mV |
| Current limit | -40 to +40 mA |
| Current resolution | ~0.005 mA |