Using isolated DC-DC Converters for Lab Power Supply - Page 1
Forum section: electronics / projects

author: 1994timmeh
author_rank: Contributor
timestamp: November 25, 2014, 01:45:52 am
content: Hi guys!Looking to make a lab power supply (either USB powered or using a 12v power adapter, 240VAC), but i do a fair chuck of work with dual rail op amps needing plus/minus 12v input. I was wondering if i could use a isolated DC-DC converter to achieve this? Im planning on using an LM317 as the base to the power supply, but I'm not stuck on those really. Any ideas would be greatly appreciatedAlso sorry if this has already been postedI couldn't find anything that really answered my question.

author: TiN
author_rank: Super Contributor
timestamp: November 25, 2014, 01:34:22 pm
content: Pay attention to specs of those DC-DC convertors, as most of them rather noisy for any kind of precision/high gain stuff.Also USB powered might not be a good idea either, grounds and power from PC are really dirty.Get a nice transformer with +18/-18 outputs and you can get lots of useful voltages, like 15, 12, 9, 5 etc, and it will cost you much less than any other switching solution

author: nctnico
author_rank: Super Contributor
timestamp: November 26, 2014, 12:18:07 am
content: Besides that the small DC-DC converters may not be short circuit proof.

author: Kappes Buur
author_rank: Regular Contributor
timestamp: November 26, 2014, 12:36:39 am
content: The 2014 November issue of Elektor has an interesting design for a 40V/2A lab power supply (Varilab 402) using a switch mode power supply as a prestage instead of an expensive transformer. Might be worth checking out.

author: Yansi
author_rank: Super Contributor
timestamp: November 26, 2014, 12:46:33 am
content: Saying mains sucks, is little bit foolish.And also, how many watts (what output) did you think you can get from the USB? 2.5Watts? Maybe 5 before the protection kicks in. Silly idea. Better wait for Dave finishing his uSupply! I really look forward for that. It is very interesting idea. You can also get some inspiration, some schematics have been published.

author: 1994timmeh
author_rank: Contributor
timestamp: November 26, 2014, 01:27:00 am
content: Cheers for the replies guys!I really just don't want to have to touch any 240v if i can get away with it!Since my original post i had done some current calcs working out what current and voltage i actually need and i think its around 2A 18v, i don't think I've ever used the uni ones above that, however i do need the two channels to be able to use dual rail op amps for signals and things like that.I will have a look at that Elektor one but still not sure it completely solves my problem! If i could use the base design of the Elektor one but split the voltage into two channels that would work a treat! Somehow i doubt any rail splitter ICs will do that high of power and i might have to resort to using some high powered transistors to get a virtual ground. Any ideas would be awesome

author: IanB
author_rank: Super Contributor
timestamp: November 26, 2014, 02:03:48 am
content: Quote from: 1994timmeh on November 26, 2014, 01:27:00 amIf you want to be an electrical engineering student you should learn how to handle the mains safely. Running away from scary things doesn't help you in the long run.What you could do to help you learn is to go for a two box solution. Get a mains transformer, e.g. 12-0-12 or 18-0-18 V, and put it in an enclosure by itself. Put an IEC socket on the mains side with a fuse and a switch and maybe a pilot light. Learn how to make crimped connections, and how to attach the ground wire to the case with a machine screw, ring terminal and shake-proof washer.Install banana plug sockets on the low voltage side for your AC output. Get someone to check your work once it is all constructed. Now you have a safely enclosed low voltage AC power supply.With this AC power supply you can freely experiment with various DC regulated supplies on separate boards or in separate enclosures and never worry about touching the mains. You can mix and match with different DC voltages, dual rail or single rail, just by swapping them in and out and using the same transformer to power them.

author: 1994timmeh
author_rank: Contributor
timestamp: November 26, 2014, 06:34:54 am
content: Yeah this actually sounds like a great idea! This way i can do it once and then its set forever. I can then make multiple "power supplies" that can just plug into my 18v unit and always get what i want when i need itMight have to go with this or just bite the bullet and grab an expensive one :/

author: Jeroen3
author_rank: Super Contributor
timestamp: November 26, 2014, 07:10:28 am
content: QuoteBut you can outsource it by buying a AC-DC converter module. Make sure the isolation is something they advertise with.