Dual rail power adaptor - Page 1
Forum section: electronics / projects

author: calexanian
author_rank: Super Contributor
timestamp: November 12, 2015, 10:42:03 pm
content: Has anybody come acros a good source for +/- 12 volt brick type power adapters? I would like to design one into a project that is intended for hobby use and don't want inexperienced people playing around with line voltage and transformers or external supplies jumpered together or a pair of 9 volt batteries, etc.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 13, 2015, 03:07:05 am
content: Could you consider using a DC/DC converter inside the device?

author: calexanian
author_rank: Super Contributor
timestamp: November 13, 2015, 06:17:54 am
content: Quote from: NiHaoMike on November 13, 2015, 03:07:05 amI had considered it, however I am attracted to the simplicity of finding a ready made supply.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 13, 2015, 06:31:54 am
content: Maybe you can find some simplicity in a common supply, such as a 12Vdc wall wart, and then use dc/dc bricks to create +/- 12. Instead of some rare dual rail supply.I mean, farnell only hasone+12/-12V adapters in the catalog, against 164 +12V adapters.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2015, 06:35:27 am
content: Quote from: Jeroen3 on November 13, 2015, 06:31:54 amYes, but how many of those are ISOLATED? Any of them?Whatever is generating the negative bus must be isolated.

author: Ian.M
author_rank: Super Contributor
timestamp: November 13, 2015, 06:54:46 am
content: The classic way to do this was to start with a 15 or 16V RMS AC output adaptor, then use a capacitor coupled bridge rectifier to provide the negative rail. However, that means real copper and iron, not switched mode, which makes the adaptors rare and expensive by today's standards.Without knowing the current you need from the + and - 12V rails, its hard to be more specific, but there is almost certainly an affordable off-the-shelf switching DC-DC converter module that will meet your needs, or possibly two such modules, one for each rail. Depending on the power requirements, and the modules used, the DC input could be anywhere between the ubiquitous 5V 2A USB chargers and a 19V 4A laptop power brick.

author: Kjelt
author_rank: Super Contributor
timestamp: November 13, 2015, 10:27:19 am
content: Not brick type as in plug and play. But tme.eu has some Aimtec powersupply modules in 3 different wattages.Put them in a housing with AC inlet and DC bananas out and you are ready to go. Outputs are 4000VAC isolated from the inputs so should be ok.http://www.tme.eu/en/details/amec40-12dmaz/industrial-power-supplies/aimtec/

author: onlooker
author_rank: Frequent Contributor
timestamp: November 13, 2015, 03:25:50 pm
content: "for hobby use", you may use two 12Vdc wall warts for the +- rails respectively. The secondaries are isolated, so there should be no problem to serial the 2 rails.

author: ajb
author_rank: Super Contributor
timestamp: November 13, 2015, 03:38:15 pm
content: Quote from: Richard Crowley on November 13, 2015, 06:35:27 amThere are a number of isolated DC-DC converters on the market that will take 12V in and 12V out that would do the job, anywhere from 1W to 400W. But you actually don't need anything isolated or exotic to generate the negative rail--a standard buck converter will do the job just fine, just use what would normally be the positive output as ground and what would normally be ground becomes your negative output. Voila, inverting buck/boost converter. You could even use one of those off-the-shelf buck converter modules, provided its UVLO isn't set too low for it to start, or you're okay with bumping your positive rail up a little.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2015, 04:02:04 pm
content: Quote from: ajb on November 13, 2015, 03:38:15 pmBut that's just the POINT. You CAN'T do that with most buck converters because they have common ground between the input and the output by the very nature of their circuit topology. You need essentially a SMPS with a fullly-isolating transformer. Not just a buck inductor.

author: Ian.M
author_rank: Super Contributor
timestamp: November 13, 2015, 04:08:36 pm
content: Quote from: Richard Crowley on November 13, 2015, 04:02:04 pmI beg to differ:https://iw0ffk.wordpress.com/2014/09/19/negative-voltage-from-lm2596-dc-dc-ebay-module/, although as per datasheet (http://www.ti.com/lit/ds/symlink/lm2596.pdf, you should add a normally reverse biassed diode between -Vout and Gnd to aid startup.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 13, 2015, 04:21:40 pm
content: Brilliant! I didn't know it could bootstrap the ground pin. The explanation on p. 27 and the circuit on p. 29 (Figure 37) explains how to do that. Now if I can only get +/- 12V for my audio circuits.http://www.ti.com/lit/ds/symlink/lm2596.pdfI know that those things run at very high frequencies so I don't need to worry about leakage into audio (<20KHz). However, if I am using TWO random switchers I am wondering whether there might be a problem with them beating/hetrodyning against each other and producing a resulting audio-frequency tone.

author: Ian.M
author_rank: Super Contributor
timestamp: November 13, 2015, 04:57:49 pm
content: Its fairly easy to sync to a -ve output LM2596 if Vin < Vout, but the positive regulator needs to be a LM2586 or LM2588:Application Note 1082 Adjust or Synchronize LM2586/88which rules out the cheap ebay/shenzen modules. I'm sure there are other solutions though.

author: calexanian
author_rank: Super Contributor
timestamp: November 13, 2015, 05:15:23 pm
content: Quote from: Ian.M on November 13, 2015, 06:54:46 amThis has been my backup plan.

author: langwadt
author_rank: Super Contributor
timestamp: November 13, 2015, 05:16:21 pm
content: you can get a negative supply from both a buck and a boosthttp://i.cmpnet.com/planetanalog/2008/10/C0345-Figure1.gifhttps://www.maximintegrated.com/en/images/appnotes/3740/3740Fig05.gif

author: calexanian
author_rank: Super Contributor
timestamp: November 13, 2015, 05:21:25 pm
content: Quote from: NiHaoMike on November 13, 2015, 03:07:05 amI am looking at some. As I am typing this I am starting to thing maybe I should just do a preliminary project entitled. Mains power supply construction entitled "The only thing you need to fear, is yourself!" Perhaps a flat pack PCB mount transformer, bridge, caps, etc in a small enclosure or something may actually be cheaper than a off the shelf supply, however i still worry about the safety factor.

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 22, 2017, 02:05:46 pm
content: Relating to this, I'm looking for a general purpose module / circuit I can plug into my Farnell L30-2 (0-30v @ 2A) bench power supply to give a me a stable +/-0-15v output that can be used for different loads and situations. The output voltage and current limiting would be done by the Farnell unit.A link to such a low cost module in the UK would be appreciated.

author: Ian.M
author_rank: Super Contributor
timestamp: January 22, 2017, 02:29:58 pm
content: You need a high power rail splitter. Conceptually its quite simple - an OPAMP (optionally with a separate power supply so you don't run into problems at low supply voltages), a precision divider to set the 0V point midway between the input rails, connected to the OPAMP's +in, a complimentary Darlington emitter follower with bias carefully set just below turnon, driven by the OPAMP output, and feedback from the final output to the OPAMP's -in. However the implementation details are rather tricky, especially if you want good transient response behaviour, and worst case you have 30W of heat to dissipate if the load is maximally unbalanced. You'd probably be better off looking for a second bench PSU . . .

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 22, 2017, 02:37:35 pm
content: Quote from: Ian.M on January 22, 2017, 02:29:58 pmCan any two bench PSU's be connected in series for this or do I have to make sure they are or are not floating?

author: Ian.M
author_rank: Super Contributor
timestamp: January 22, 2017, 02:53:37 pm
content: Quote from: ziplock9000 on January 22, 2017, 02:37:35 pmThe negative side supply *MUST* be floatable as its positive terminal will be connected as 0V, which is likely to be grounded in the application circuit. The positive side doesn't have to be, as its negative terminal will be 0V, although its preferable to have a floating positive supply to avoid ground loops via your scope or other test equipment.

author: ziplock9000
author_rank: Regular Contributor
timestamp: January 22, 2017, 03:19:12 pm
content: Quote from: Lee Leduc on January 22, 2017, 03:10:53 pmThey wont be voltage adjustable or current limit adjustable which I'll need for my generic bench PSU

author: Lee Leduc
author_rank: Regular Contributor
timestamp: January 22, 2017, 03:26:52 pm
content: Ignore the linkhttp://musicfromouterspace.com/index.php?MAINTAB=SYNTHDIY&PROJARG=ELECTRONICS/MakingMoneyWithMFOS.php&VPW=1910&VPH=813For some reason I can't grab the URL.Sorry for that.

author: Ian.M
author_rank: Super Contributor
timestamp: January 22, 2017, 04:20:30 pm
content: If you *really* want to DIY, build a floating single output PSU using a LM723 based regulator with a 50Hz/60Hz transformer, (not a direct from mains SMPSU) so it doesn't have a lot of HF noise or excessive mains leakage current, and use it for your negative rail. Use a switched resistor network for the current limit: 30mA, 60mA, 120mA, 250mA, 500mA You usually don't need more than that for the negative rail of most analog circuits you'd have on your bench - higher power circuits should really have a dedicated PSU. Its helpful if it can go up to 18V rather than just 15V.Don't skimp on the transformer - remember: the average output voltage of a full wave rectifier under load is only 90% of the RMS AC input and although the reservoir cap will charge to the peak voltage (sqrt(2)*Vin), maintaining >Vin on the output under load requires a *VERY* large cap. Also the max DC output current is only 62% of the secondary RMS current rating. See:http://www.hammondmfg.com/pdf/5c007.pdfDon't forget the transformer's regulation drop between no load and full load, the possibility of the mains supply being at its upper or lower legal tolerance limits (proportionately affecting the secondary voltage), the rectifier diode drop, and the regulator's max input voltage and required headroom at max output current.You've also got quite a bit of heat to get rid of at minimum output and maximum current. The most compact solution would be a fan-cooled heatsink (e.g. repurpose an old CPU heatsnk/fan assembly), but then you need to provide 12V for the fan which isn't so easy unless the transformer has a center-tapped secondary.However, by the time you've done that its probably more cost-effective to buy a used 'fixer-upper' bench supply off EBAY and restore it.