Dual ac/dc brick isolated power supply - Page 1
Forum section: electronics / beginners

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 21, 2019, 01:28:16 pm
content: Looking for advice on a cheap way to get dual rail voltage. My idea is to use 2 identical 12 volt 3 amp ac/dc power bricks that appears to be switch mode each one has been tested by ohm meter to be isolated output from input. Each will be connected to generic ebay buck boost converterDC-DC Step Up Down Boost buck Voltage Converter Module LM2577S LM2596S ,and have its own volt/ amp display. I have read multiple tutorials and most suggest using old style transformer isolated power bricks and lm317 or similar. Just curious if this would work since they both are isolated from mains and would reversing the leads on one of the ebay modules in this specific configuration give positive and negative adjustable rail? If it wouldn't is there a way to do this because I believe there has to be a common/ground between the 2 dc/dc converters. Could I use maybe the unearthed project box as a common ground?As a side note im not interested in being able to parallel the output for higher voltage as each module should make over 20v on its own in boost mode and not trying to use over 500ma on either output at any time. Would be fused output with 250ma slow blow glass fuse and diode protected.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 21, 2019, 05:54:18 pm
content: Quote from: Tj138waterboy on February 21, 2019, 01:28:16 pm

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 21, 2019, 05:56:54 pm
content: I am aware that pretty much all dc/dc modules -input and -output are tied together but this should be negated by having isolated input and output powering the modules themselves.

author: rstofer
author_rank: Super Contributor
timestamp: February 21, 2019, 07:38:21 pm
content: If the output are truly isolated from the mains voltage, and they probably are, of course it will work.Just wire it up and try it! Maybe plug the power packs into a switched multi-outlet strip, perhaps put 1A fuses in each secondary wire (really over the top) and have voltmeters on each output.You know what you expect to see, you have meters to look at, and if you don't get what you expect, turn off the switch.In my view, truly isolated outputs are no different than batteries. As long as I don't short one out, I can wire them any way I want.

author: lordvader88
author_rank: Frequent Contributor
timestamp: February 23, 2019, 12:08:46 am
content: IDK about making them adjustable yet, but with a single supply u could use an LM317 and the negative version the LM337, and just a few diodes, resistors, caps. It splits the input V, and balances ground in the middle between the 2. I made a little one add it worked good enough to run cheap op-amp oscillator circuits. So I guess it was fairly clean on the scope, I don't remember, but it was ok.LM317/LM337 are good for (+/-) 2-37V and can handle 1.5A, and can be used in parallel, and can be made dual tracking adjustable, I've seen that in the datasheet I'm sure.I bet there\s lot's of SMPS type chips that can make dual-rail with very few parts as well.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 23, 2019, 12:19:59 am
content: Thanks for the info. I am trying to stay away from lm317/337 for now if possibe as the only transformer I have is from microwave oven. I have the knowledge and tools to wind it myself but rather save it for a 1:1 isolaton transformer. I want my contraption to be semi portable maybe fit in lunchbox size enclosure. Thats why im leaning towards a more modular setup so if any single part fails, it can cheaply and easily be replaced. Currently waiting on more buck boost modules to arrive from the slow boat as well as 2.1mm dc plug jacks. Will update with pics amd info as parts arrive.

author: lordvader88
author_rank: Frequent Contributor
timestamp: February 23, 2019, 12:34:47 am
content: Well u can power them from anything, like a SMPS brick. What voltage did u need? For me I used a 12V brick, feeding a single LM317 so it's 2-11V , then feed it to the split rail maker. I also used a nice 24V SMPS, and also tried a 35V transformer. The SMPS's added more noise, but did still run the op-amp osc. circuit ok too.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: February 23, 2019, 12:53:45 am
content: No specific voltage but just the usual would be +/- 15v for opamp with option to charge lipo packs or whatever other general purpose use. If i were to use the LM series regulator it definately would be dual tracking, but from schematics i have seen this would require its own transformer. I like the idea of 2 ebay modules and being able to adjust each rail independantly for some odd reason. I would probably be better off buying 2 old analog panel power supplies but meh.