ccheng-ppppp / j-smu

DIY source-measure unit

What is SMU?

In case you don't know what is SMU, take a look here Basically, it's glorified power supply, able to both source voltage (and measure current, with current limit) or source current (and measure voltage, with voltage limit), usually in wide range of voltages and currents. It usually works in four quadrants, meaning it's able to both sink and source power. When set to zero output voltage, SMU acts as ammeter and when set to zero current, acts as voltmeter. It's very useful as general purpose PSU with precise setpoint adjustment, as a tool for measuring high resistances, for testing semiconductors in wide range of voltages or currents. You can do all this things using classic lab tools (PSU, ammeter, voltmeter, switches), SMU offers this all in single package and is designed to be interconnected with computer to automate the tasks.

What is in this repository?

• BOM - Bill Of Material. Contains list of components and materials used in this device. Separated by PCBs.
• firmware - firmware for two microcontrollers running in JSMU
• mechanical - mechanical parts I designed for this SMU
• media - minimal amount of pictures
• pcb
  • pcb_gerber - gerber data in zip files, ready to be manufactured
  • pcb_ibom - interactive BOM files, helping when populating the boards, also helpful to get familiar with the project
  • pcb_kicad - PCB source files, made in Kicad 5.1.10
  • schematics_pdf - all schematics in PDF format, for easy viewing or printing

What is JSMU?

JSMU is open source implementation of SMU, inspired by Keithley 236 and Dave Erickson's DIY SMU

How to use it?

User and calibration guide is in the process and will be released soon(TM).

Parameters:

• Voltage ranges 12V and 120V (bipolar)
• Current ranges 1uA, 10uA, 100uA, 1mA, 10mA, 100mA (bipolar)
• Minimal set voltage step - 1mV
• Minimal set current step - 100pA
• Voltage resolution - 100uV
• Current resolution - 1pA

JSMU is designed to work as standalone device, but can be connected to computer via USB, and controlled via simple commands in SCPI form, similar to Keithley 2xxx series SMUs.

More pictures can be found in imgur gallery

FAQ

Q: I noticed you used AT90S4434, made in 1999 as controller for your analog board.

• A: Yes, that is correct observation. Two reasons led me to this decision; first of all, I like to do weird design choices from a time to time. Let's call it design dissonances. Secondly, when this design was born (winter 2020/2021-summer 2021) component crisis was (and still is, in the time of writing this) quite strong and I was limited by what components were available. I tried to choose as much components as were available in my junkbox as possible. AT90S4434 can be replaced by AtMega8535 after minor source touchup and recompiling.

Q: dsPIC33FJ128GP306 is also weird choice here

• A: Yes, again, I was fishing in my junkbox. A lot of open-source project go with STM32, but at the time of designing, majority of component suppliers had more-less zero stock of more common STM32's. Once the situation changes, I can think of respinning the D1 board for STM32 and port the code. I already did porting from MC9S08BG60 to dsPIC; perhaps I'll manage another port too.

Q: What is the point of using 14-segment alphanumeric display in 2021, when many test instruments are using graphic LCDs with touchscreens?

• A: Apart from the fact I don't like touch screens on test instruments, I intentionally tried to keep the design in kind-of similar feel than Keithley 236. It's a thing of personal taste.