An SMU or Source Measurement Unit works a bit like a power supply, in that it can source current into a load and a bit like an electronic load, in that it can sink current from a power source. It includes a crossover circuit, so that it cleanly and predictably swaps between sink and source modes automatically. This makes it terribly useful for testing all manner of power circuits, charging and characterizing batteries or just saving bench space by replacing two separate boxes.
This DIY-SMU from analog electronics guru [Dave Erikson] is a full four-quadrant design, meaning that it can operate with both positive and negative voltages. The design shows excellent performance, comparable to commercial instruments that cost serious money, which is testament to [Dave]’s skill and experience.
The quadrants can be understood if you imagine a graph with voltage on the horizontal axis, and current on the vertical. Both axes can swing to both polarities, with quadrants I & III indicating power delivered into a load and quadrants II & IV power absorbed from a source.
The very detailed project logs show every gory detail, every problem found and the work to solve it. Its a long read, which for those interested in such devices, will be time well spent in this scribe’s humble opinion.
The DIY-SMU is mostly analog in nature, with the control portion courtesy of a Teensy 3.2, with a Nextion TFT display with touch for the user interface. The firmware even supports SCPI over USB to allow remote control and data gathering, so its ready to drop right into your test and measurement stack. For more reading goodness, checkout JSMU, a related project, taking inspiration from the DIY-SMU. Details can be found on this project GitHub repo.
Many power supply projects have graced these pages over the years, like this 2015 Hackaday Prize Entry but this is one of the few four-quadrant designs to be found, so hats off!
Thanks [David Gustafik] for the tip!
Ah nice one, it’s an interesting concept as specialised tool eg harmonics, power data logging, thanks for posting:-
Fwiw. I had an HP 6813A few years ago which is an AC source analyser ostensibly for US & European product testing 110/115 to 220/240V 1750W with frequency selection to suit or independent of volts. The unit could even go down to 10Hz up to about 100Hz though not necessarily within tolerance or power specs outside 45Hz to 65Hz or so. Can also supply varying DC via comms to test charge various batteries. I used it a lot for wide voltage range 4-20mA industrial device testing, burn in several products & 4-20mA loop comms testing.
That Hewlett Packard particular unit had a bug or damage from previous owner, on charging batteries at max current, it would pull large current back into device with No thermal warning of overload – so blew the sink side of output stage.
Got it fixed, after 2 years then sold it to a test lab that upgraded it to HP6813B for harmonic/flicker testing.
Owner of test lab rented it out to an “engineer” who connected the output to mains 240V ugh – blew it badly !
Oddly even though unlikely the unit didn’t have suitable protection against that – maybe output control timing as could control output as AC or DC easily & quickly across full range from front panel knobs or keypad.
If I find another will be in the market at right price, thanks
An interesting wrinkle with SMUs is that they can be WORSE for many purposes than a two-quadrant power supply. There’s a lot of applications where you might want to source or sink current, but don’t want the voltage to ever drop below zero. This is especially true if you’re supplying power to something that expects a rechargeable battery, but can also be true if you’re using it as a load. Many people press an SMU into service as a power sink and then blow up their circuit when it drives the supply negative to meet the current setpoint.
@Dave Rowntree said: “The DIY-SMU is mostly analog in nature, with the control portion courtesy of a Teensy 3.2, with a Nextion TFT display with touch for the user interface. The firmware even supports SCPI over USB to allow remote control and data gathering, so its ready to drop right into your test and measurement stack. Additional details can be found on the project GitHub repo.”
I don’t think the GitHub repo link you provided is for [Dave Erikson’s] DIY-SMU project, it is for something called the “jaromir-sukuba / J-SMU” which looks like a Keithley 236/7/8 clone. In fact it says this on the J-SMU GitHub page: “What is JSMU? JSMU is open source implementation of SMU, inspired by Keithley 236 and Dave Erickson’s DIY SMU.”
* Here’s a link to a pdf download for a venerable old Keithley document explaining how to use SMUs:
Keithley 236-904-01, Models 236/237 Source-Measure Units Applications Manual
{C) 1989, Fourth Printing June 1993
https://www.eserviceinfo.com/downloadsm/184229/Keithley_236%20904%2001D.html
* SMUs are expensive, even old used ones. The problem is once you have an SMU, it won’t take long before you realize you need a second one. An SMU that can’t be programmed and queried is almost useless IMO. Consider that before buying.
– Look at these prices for old used Keithley 236 SMUs:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=keithley+236
– Now look at these prices for old used one-Amp capable Keithley 238 SMUs:
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313&_nkw=Keithley+238
Yeah, ouch.
The github link presented is for another project derived from this one, but not for this project. Sloppy HaD editing as usual.
Yes, as you point out I made a mistake and mixed up couple of similar projects. Article updated appropriately to clarify. Thanks for your input.
Brilliant. DIY? This looks like something a team of engineers would normally produce. Hats off to anyone who builds a fully functional unit.
Thanks for the feedback. I’m real late to the party, almost 3 years. This is the first time I’ve seen my own project on Hackaday. The design files, all the Youtube links and lots of gory details can be found on my web page at http://www.djerickson.com/diy_smu. Enjoy!