PSU Design Feedback - Page 1
Forum section: electronics / projects

author: echen1024
author_rank: Super Contributor
timestamp: October 13, 2013, 04:29:57 am
content: I currently have this design:I need feedback on whether the system will work, and am a bit stumped on the following:I need help on deciding the gain for the op ampHow do we get the 317 to go down to 0v,and, what would be the easiest way to digitally add current regulation.Thanks. Sorry if these are noob questions, but digital is more of my domain...

author: tree
author_rank: Regular Contributor
timestamp: October 13, 2013, 05:01:26 am
content: Dave does a good job explaining why the 317 can't go down to 0V output.

author: minime72706
author_rank: Frequent Contributor
timestamp: October 13, 2013, 05:51:14 am
content: You would need a negative voltage (but not necessarily a completely bipolar power supply) to get the LM317 to go to zero. Dave fell madly in love with the Linear LT3080, which is a similar kind of regulator but it uses a current source to form it's voltage reference than a fixed 1.25V reference. It can get very very close to 0V. First thing that comes to mind to make a DIGITAL constant-current setting would be a digital potentiometer, but there might be better ways. I never got the impression that digital potentiometers were very good. . . often inaccurate, non-linear, not terribly robust.Dave's power supply series is rather informative - check it out.EDIT - If you want to try the LT3080, I have a fairly large number of them and I'd send a few to you at cost. I sold a few lots of 5 on ebay and through here a fair time ago because they were relatively hard to get your hands on. I just bought 100 direct from Linear and offloaded some of them to help folks obtain them. I still have plenty.

author: echen1024
author_rank: Super Contributor
timestamp: October 13, 2013, 05:05:11 pm
content: Hm. I might switch over to the LT3080 some time, but I think I will just use the 317 for now. I still have the ever remaining question, ofwill this thing work?. I know I will need to adjust the Op-Amp gain, but other than that, it should be fine, correct?. Thanks.

author: skrap
author_rank: Contributor
timestamp: October 13, 2013, 05:53:05 pm
content: A bit of a warning, the LT3080 might not always work the way you expect. When Dave made the videos it was still unknown (at least to most of this community) that it has a few quirks that make it less suitable for use in a bench power supply. There are quite a few threads and discussions in detail you might want to read before implementing it.

author: echen1024
author_rank: Super Contributor
timestamp: October 13, 2013, 06:51:02 pm
content: I am aware that it will blow up if you change it to zero to quickly. So i will stick with the 317. How does this design look though?

author: Christopher
author_rank: Frequent Contributor
timestamp: October 13, 2013, 06:57:53 pm
content: I'd use a better DAC, or at least a cheaper reference if you're insisting on that DAC.Local decoupling on the ICs & maybe a series resistor (470R) on the VCC to act as a low pass filterCan you not connect the VOUT from the dac straight to the set pin? Why are you going through an error amp?

author: minime72706
author_rank: Frequent Contributor
timestamp: October 13, 2013, 09:20:01 pm
content: Good to know about the glitches ... that's interesting.

author: kizzap
author_rank: Unknown
timestamp: October 13, 2013, 10:29:57 pm
content: you can get the LM317 to go down to "zero" volts DC without a negative rail. "Zero" is just an arbitrary level that you are referring to. All you would need to do is raise where you are going to measure your zero from. A couple of Diodes, etc are all you need.-kizzap