μSMU - A credit card-sized SMU for measuring semiconductors - Page 1
Forum section: electronics / projects

author: joeltroughton
author_rank: Newbie
timestamp: January 10, 2022, 12:21:52 pm
content: Thought I'd post my SMU project here. It's a small (70x43mm), USB-powered, 4-quadrant device with a window of ±5 V and ±60 mA. I work in emerging photovoltaics research where the Keithley 2400 is the academic workhorse for running J-V measurements on solar cells. That said, for 90% of usage our devices' open-circuit voltages are less than 1V, with short-circuit currents of less than 5 mA (Our solar cells tend to be 0.1cm2 in area!). Hence, a tiny, low-cost SMU that hits the spot for this use case!The design is pretty simple and based around the very pricey LT1970 as an output driver. This chip has voltage controlled current limiting down to sub-mA levels, giving us some decent compliance control. Current is measured through a 50 Ohm shunt into a PGA281 programmable gain chopper instrumentation amp. Gain can be switched anywhere between 0.125 and 176, and auto-ranges in firmware to avoid swamping the ADC at high currents. ADC is a 16-bit ADS1115 and the DAC driving the output amplifier is 16-bit DAC8571. ±10V is generated from the USB bus voltage using an LCD bias supply (TPS65131). Finally, there's a STM32F072 running the show and communicating with a PC over a virtual com. port.In terms of performance, measured voltage and current agree with the Keithley down to around 0.5 mV and 0.5 uA, so pretty reasonable for what I need. Definitely keen to make improvements based on any suggestions from the wizards here though!Board files & firmwarehttps://github.com/joeltroughton/uSMU

author: brabus
author_rank: Frequent Contributor
timestamp: January 10, 2022, 12:30:58 pm
content: Brilliant! Thanks for sharing.

author: thm_w
author_rank: Super Contributor
timestamp: January 11, 2022, 12:26:49 am
content: Great work.It looks like LT1970 tops out at +/-18V 0.5A.TPS65131 is nice simple dual rail IC, +/-15V ~0.2A max.So I think the design could be scaled up slightly if needed, but obviously your use case is well within that range.Would probably be good for cycle testing small li-ion cells or supercaps too.

author: RichardM
author_rank: Frequent Contributor
timestamp: January 11, 2022, 09:34:35 pm
content: Thank for the post.This looks like a great hobbyist project. In you opinion could it be hand soldered ?Richard

author: joeltroughton
author_rank: Newbie
timestamp: January 12, 2022, 04:13:21 pm
content: Probably not with an iron as the MCU and power supply IC are QFN packages. I've made a fair few of these now using either a hotplate or a toaster oven with good success

author: RichardM
author_rank: Frequent Contributor
timestamp: January 13, 2022, 01:16:36 am
content: Quote from: joeltroughton on January 12, 2022, 04:13:21 pmOk, thanks. So a hot plate and stencils would be required? Through hole is so much easier for a novice like me but it appears only a matter of time before all we can buy are surface mount components. May be time to invest and skill upRichard

author: jbb
author_rank: Super Contributor
timestamp: January 13, 2022, 02:35:19 am
content: The LT1970 has a thermal pad underneath, so you’d best use a stencil for the solder paste, yes.It’s probably doable with a hot air tool, but a hot plate might be better. Not sure. QFN parts are not as nightmarish as they seem because the surface tension of melted solder pulls them into place.On through-hole; yeah, the big customers all prefer smaller parts so most new chips are surface-mount only. Also (not so relevant here) as the operating frequencies go up past a few tens of MHz (and gain bandwidths are a bit higher than that!) then the lead inductances of a through-hole package become a problem.

author: PartialDischarge
author_rank: Super Contributor
timestamp: January 13, 2022, 01:38:47 pm
content: Are you going to sell this ? I’d be interested

author: trophosphere
author_rank: Frequent Contributor
timestamp: January 13, 2022, 07:22:08 pm
content: Very nice project! Looks quite neat and professional. I have been working on something similar but will utilizing theLT8714instead.

author: jbb
author_rank: Super Contributor
timestamp: January 13, 2022, 09:46:01 pm
content: Quote from: trophosphere on January 13, 2022, 07:22:08 pmOh, that is an interesting chip.FYI if you power sinking with it, energy will be pushed backwards into the incoming rail. You’ve still got to get rid of it somewhere…

author: trophosphere
author_rank: Frequent Contributor
timestamp: January 14, 2022, 07:07:28 am
content: Quote from: jbb on January 13, 2022, 09:46:01 pmYou are quite correct. Unfortunately, the regulator maintains its output voltage regulation by power conversion rather than power dissipation. Currently, I have an op-amp with an external push-pull output stage feeding into the regulator. It's the output stage implementation as well as the compensation that I'm playing around with.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2022, 08:06:27 am
content: The usual ouput stages will not backfeed into the supplies. They will just produce more heat in the output transistors. This may need to have more cooling than just for a signal source. In many cases the amplifier would even compensate external current from a positive source with current from the negative supply and thus will dissipate power of external voltage + supply voltage times the current.

author: joeltroughton
author_rank: Newbie
timestamp: February 01, 2025, 09:05:19 am
content: Took a few years to get around to, but I've finally got a few up for sale on Tindie pre-calibrated and with some test leads included.https://www.tindie.com/products/undalogic/smu/

author: voltsandjolts
author_rank: Unknown
timestamp: February 01, 2025, 09:40:26 am
content: Welcome back!Can a "cal cert" of sorts be supplied with the unit...I don't mean full traceability, just a few cal/test point measuements?Just curious, what is the 7 digit smu you're using for calibration?Want to sell here on eevblog, payment via paypal, for UK sales... or do you prefer tindie for all?Thanks.

author: Warhawk
author_rank: Frequent Contributor
timestamp: February 01, 2025, 09:58:06 am
content: Hi there,it turned out that I have practically all parts already at home and now I would even have use for your uSMU. I was not aware of your project. A quick question - do you have somewehere the 3d data for the housing?It is not on the github.

author: jwet
author_rank: Frequent Contributor
timestamp: February 01, 2025, 02:43:51 pm
content: I've thought about something like this for a long time. I think you've done a nice job of it. Most of what I'd use something like this for fits in this +-15v/.5 amp envelope. I would imagine things getting warm at the extremes of V&I- any comments? The LT1970 is certainly a problem solver, worth every bit of $15!I have one question after looking at the unit on Tindie noting that its "headless". I saw you have a "demo" app and have documented the command set. How do you imagine someone using this- using a CLI in Teraterm to send commands?, using a python script or similar, using Lab View? One of things that makes the Digilent H/W so good is Waveforms software. I'd love to see a Digilent compatible box like this. (idea?).

author: voltsandjolts
author_rank: Unknown
timestamp: February 01, 2025, 08:48:07 pm
content: Quote from: jwet on February 01, 2025, 02:43:51 pmThe LT1970 SMD package is somewhat limiting. The datasheet suggests a maximum of about 3W dissipation on a 4L PCB with appropriate copper pours. So, if powering from +/-16V rails for the +/-15V ouput range, current would need derated from about 9V 500mA falling to 200mA at 0V. Bootstapping the supply is perhaps possible but adds complexity.

author: Warhawk
author_rank: Frequent Contributor
timestamp: February 07, 2025, 09:44:43 pm
content: Quote from: voltsandjolts on February 01, 2025, 08:48:07 pmLTC had somewhere an appnote for this part how to bootstrap it. I actually built a precise power supply with this part. However, never fully commisioned it. Now I found the prototype and will use the part in this little cutie uSMU.

author: Warhawk
author_rank: Frequent Contributor
timestamp: June 26, 2025, 06:42:13 pm
content: Is there please someone who would be able to build the project and share the binary? I just burnt hours on that and not moving forward. I am not an STM32 guy...Solved, compiled in windows sandbox using the -fcommon option for the compiler.