Linear PSU based on the µSupply - Page 1
Forum section: electronics / beginners

author: cu
author_rank: Newbie
timestamp: April 19, 2020, 04:27:34 pm
content: Background:As my first real project I want to build a psu. I have already done the schematic and the pcb layout but before I place the orders I want to make sure that things work like intended.I have not breadboarded anything of this yet because I have nearly no stock and the DACs/ADCs are SMDs (-> cost).Specs:2 isolated outputs (20.48V, 2.048A, 12-bit resolution)measurement of VOUT(12-bit chanA, 10-bit chanB), IOUT2x VIN=24VDC±2VLCD Display, 4 encoders, 6 buttons for fine/coarse, on/off, set200x200mm pcbseparate front panel pcbcase made of acrylic glassrocker switche at the back for each channelThese are by no means fixed. It's how it turned out. Accuracy remains to be seen. The priority is CV mode, so CC mode doesn't have to be that accurate.The case and the front panel will be made after the main pcb was finished and connected via pin headers. The schematic includes the LCD (U22) but it will be on the front panel (pin header footprint).Why am I posting all this?I'm new to electronics and I'm pretty sure it won't work first try. I'm here to ask someone with more experience than me to spot some obvious mistakes. I know that reviewing this costs a lot of time but I'd be grateful if you pointed out some of my mistakes.Concerns:stability of CC modecapacitor values, typespcb track (0.25mm or 9.84mils, 1.1mm or 43.31mils for power) / via (0,80/0,40mm or 31.5/15.7mils, 2.2/1.1mm or 86.6/43.3mils for power) widthrouting, spacingcheaper ICs available?Attachments:Schematic (pdf)Gerber FilesPCB PlotDigi-Key BOM (pdf)I didn't include the original KiCad files because I don't want to add the libraries but I'll do so if you want me to.Thank you in advance,cu

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2020, 05:28:27 pm
content: The old µsupply did not very well and never got really work well or got finished. Using a regulator chips is usually not a good starting point for a lab supply. It starts with the usually limited power handling capability and drift if the reference + regulator part get hot.It is usually easier to use separate reference + control loop (OPs) and power elements (transistors).For the start I would reduce it to a single channel, and if needed build a second identical supply: the cheap Chinese boards come in Qty 5 anyway. Having 2 supplies together is tricky, especially 2 positive supplies with a common ground have a limited use and add complications in feeding current from one channel to the other.Chances are it may not work the first time and even if it does, building a second identical one may be easier than one large board.

author: cu
author_rank: Newbie
timestamp: April 19, 2020, 06:26:28 pm
content: Smth like this?If I make the same board twice I'll have to get 2 atmegas, I/O expanders and displays because the channels should be isolated; hence they don't have a common ground.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 19, 2020, 06:45:40 pm
content: With 2 channels it could need 2 µCs, maybe even 3 with a 3rd one just for the display and the knobs.When ADC / DAC with serial interface (e.g. I2C or SPI) are used one may not need IO expanders. There could be just 1 display and isolated data transfer between the units, but 2 displays could be OK as well, as the 2 lines modules are not that expensive.The circuit shown is a constant current load circuit. There are plenty of lab supply circuits around, however with a large fraction of not so good ones. The choice of the primary power source is an important factor. This could be 1 transformer, 2 transformers or a something like an old lap-top supply (e.g. ready made fixed voltage SMPS). This may also set the voltage / current range. Small adjustments are usually not that difficult anyway.For the start I would build a version with just pots for the control, with maybe the option to later add a DAC.As the control is not so easy it may help to first build it on a breadboard and / or do a simulation, for the analog part.

author: RoadRunner
author_rank: Frequent Contributor
timestamp: April 21, 2020, 09:40:01 am
content: Just few notes from me, I have tried to make dave's uSupply quite a while back with partial success. Though you may have already addressed the problem, i have not looked into details of your design. But please be aware uSupply's method of current limiting sufferes from oscillation issues, and poor current regulation. This problem is discussed on the forum on multiple posts.Because my take on uSupply was not very sucessfull, i never reached point of publishing. But i was planning to do nice touch sceen UI on linux.here is new bie youtube video , Warning! really bad audio, Showing issues in current limmiting.Regards

author: mikerj
author_rank: Super Contributor
timestamp: April 22, 2020, 10:06:34 am
content: Quote from: RoadRunner on April 21, 2020, 09:40:01 amA touch screen seems like a poor choice for a PSU; unreliable, greasy marks/scratches on the screen and you often need to make adjustments whilst looking at other instruments. Just like Elon's joke dashboard on his cars, real buttons and knobs are the way to go.Running a full blown OS like Linux to achieve this takes it to a whole new level ofI guess running a cross complier for other embedded projects on your PSU might be fun though

author: exe
author_rank: Unknown
timestamp: April 22, 2020, 10:28:43 am
content: On my power supply I have two channels, each with dedicated CV and CC pots. They are also positioned under corresponding channels. I've been using it for the last four years, it works perfect for me.My next design is fully digital, with touch-screen, etc, but I'll preserve knobs, and there will be dedicated buttons too (some people like dial pads, but there is no room on front panel for this on my design). Frankly I'm not sure what is the use-case for touch screen, may be scrolling for logging function?

author: RoadRunner
author_rank: Frequent Contributor
timestamp: April 23, 2020, 10:12:32 am
content: Quote from: mikerj on April 22, 2020, 10:06:34 amI also believe Physical Rotary enoder is a must. But touch screen are becomming more common for ease of use. More and more insturment now runs Qt on linux have touch screens. Time of 7 segment display and metal toggle switch is over, if one like or not.With Linux capable hw being cheap, linux based system offer huge advantage is development time , Debugging and Maintaining Complex systems. Any Networked instrument or a instrument with Rich UI can be calssified as complex system even though primary function is just basic power supply.Lab power supply running on full blown linux not asas you may imagin.Instruments which have any commnucation capablity to talk to over LAN or networked, If implemented with linux are a class appart interms of security and stablity when compare to running some custom firmware on some microcontroller.Linux based system are a different class of product.

author: Dave
author_rank: Super Contributor
timestamp: April 24, 2020, 07:31:40 pm
content: I can see why some would like a full blown embedded OS for a power supply. It's just sad that the effort put into the software part far exceeds the effort put into the circuit design. The most important aspect of a power supply will always be the capability to supply power in a controlled manner.