# uSMU

**Repository Path**: Fly_zhou/uSMU

## Basic Information

- **Project Name**: uSMU
- **Description**: No description available
- **Primary Language**: Unknown
- **License**: CERN-OHL-P-2.0
- **Default Branch**: main
- **Homepage**: None
- **GVP Project**: No

## Statistics

- **Stars**: 0
- **Forks**: 1
- **Created**: 2026-08-18
- **Last Updated**: 2026-10-10

## Categories & Tags

**Categories**: Uncategorized

**Tags**: None

## README


<p align="center">
  <img src="https://github.com/joeltroughton/uSMU/raw/main/Hardware/Version%2010/uSMU_v10.png" width="650" title="μSMU">
</p>

# μSMU

μSMU is a small [source-measure unit](https://en.wikipedia.org/wiki/Source_measure_unit) designed for the very low-cost electrical characterisation of photovoltaic cells.

## Get a μSMU

Want to buy a pre-assembled and calibrated μSMU with USB and test leads? Grab one from [Tindie](https://www.tindie.com/products/undalogic/smu/) or [Undalogic](https://www.undalogic.com/usmu).

<a href="https://www.tindie.com/stores/undalogic/?ref=offsite_badges&utm_source=sellers_undalogic&utm_medium=badges&utm_campaign=badge_medium"><img src="https://d2ss6ovg47m0r5.cloudfront.net/badges/tindie-mediums.png" alt="I sell on Tindie" width="150" height="78"></a>

## Background

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](https://uk.tek.com/keithley-source-measure-units/keithley-smu-2400-series-sourcemeter) 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 a USB-powered SMU with a +/- 5 V voltage range and +/- 50 mA source/sink capability. The PCB is only 70mm x 43mm

## Function
The μSMU was originally inspired by [Linear Technology's DC2591A evaluation board](https://www.analog.com/media/en/technical-documentation/user-guides/855-dc2591a_rev01_demo_manual.pdf), which demonstrates an I2C address translator IC to interface up to 8 modules containing several I2C devices with an Arduino-style board. Somewhat 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 16-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 50 Ohm shunt resistor using a precision programmable gain amplifier. 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.


## Capabilities 

| Parameter                  |               |
| -------------------------- | ------------- |
| Voltage range              | -5 to +5 V    |
| Voltage measure resolution | ~0.6 mV       |
| Minimum voltage step size  | <1 mV        |
| Current limit              | -50 to +50 mA |
| Current resolution         | ~10 nA     |

## Free I-V Curve Tracer
A free, browser-based I-V curve tracer for the μSMU is available from [Undalogic here](https://www.undalogic.com/usmu/app) 



<p align="center">
  <img src="https://github.com/joeltroughton/uSMU/blob/2af5fc53fadd81d5408aaaa4daaa668cc5c6f108/usmu_tracer.png" width="650" title="μSMU">
</p>

## Usage
A simple [python package is available](https://github.com/Undalogic/usmu_py) to interface with the μSMU and perform basic measurements.

You can install this package directly from PyPI:
```bash
pip install usmu_py
```

Here is a minimal script demonstrating how to initialise the SMU, set a voltage, measure it and the current, and then close the session.
```python
from usmu_py.smu import USMU

def main():
    # Open SMU on the specified port (e.g., 'COM3' or '/dev/ttyUSB0')
    smu = USMU(port="COM3", baudrate=9600, command_delay=0.05)
    try:
        # Identify the SMU
        idn = smu.read_idn()
        print("IDN:", idn)

        # Enable output and configure current limit
        smu.enable_output()
        smu.set_current_limit(20.0)  # 20 mA current limit

        # Set voltage and measure
        voltage, current = smu.set_voltage_and_measure(1.0)
        print(f"Set voltage: 1.0 V | Measured Voltage: {voltage:.3f} V, Current: {current:.6f} A")

        # Disable output after testing
        smu.disable_output()
    finally:
        smu.close()

if __name__ == "__main__":
    main()
```


## Errata
### Version 10 (release 1.0)
- The board layout is missing grounding on the MCU for some reason. Make sure to place a couple of vias in the MCU's exposed pad to ensure proper grounding. Sorry!

## Changelog
### Version 10 (release 1.0)
- Voltage DAC changed from a 12-bit Microchip MCP4725 to a 16-bit TI DAC8571
    - Can now achieve sub-mV voltage steps
- Current sense amplifier changed from an Analog LT1991 to a TI PGA281
    - PGA281's gain can be programmed using 5 GPIOs ranging from 0.125 to 176. This allows low currents to be gained more than high currents, improving current resolution.
- Current shunt resistor increased from 10 to 50 Ohms.
    - The programmable current sense amplifier (PGA) means we can use a high value shunt resistor and decrease the gain when measuring high currents.
    - Voltage drop across a 10 Ohm shunt is sensed by the power amp (U10) to impart programmable current limiting. Measuring this across the main 50 Ohm shunt would limit the current output to ~10 mA due to limitations in the LT1970
    - The increase in current shunt value along with the new PGA means we can now sense currents on the order of ~10 nA
- Buffer amplifier changed from a quad Maxim MAX44252 to 4x Gainsill GS8331
    - Less expensive and lower V<sub>OS</sub>
- Electrostatic discharge protection added to USB port (U5)
- Isolated DC-DC converter replaced with bipolar switching regulator TI TPS65131 supplying ±9.7V
    - Extra headroom for power amplifier to push higher currents through 50 Ohm current shunt
- 4.5V LDO added for DAC and ADC
- USB-C port replaced with lower cost 2.0-pinned version

## License

#### Hardware
CERN Open Hardware License Version 2 - Permissive ([CERN-OHL-P-2.0](https://ohwr.org/cern_ohl_p_v2.txt)).

#### Software
GNU General Public [Licence v3.0](https://www.gnu.org/licenses/gpl-3.0.en.html).

#### Documentation:
<a rel="license" href="http://creativecommons.org/licenses/by/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by/4.0/88x31.png" /></a><br />This work is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by/4.0/">Creative Commons Attribution 4.0 International License</a>.