Ready made power supply needed, + & - 12vdc output, and inexpensive...(sigh) - Page 1
Forum section: electronics / projects

author: Skimask
author_rank: Super Contributor
timestamp: January 29, 2014, 05:35:58 am
content: Need a ready made (eg. wall wart, desktop box, etc) power supply.Mains input, +12 & -12 vdc output, 1 amp at the absolute maximum. Don't want to mess with virtual grounds from a single output 24vdc supply, or open-frame types where I have to design/build up the mains input, fusing, safety, etc. Just want plug-n-play +12v/-12v at the output jacks/wires/etc. Linear type preferable, but I can deal with filtering out a switch-mode output if it comes down to that.First attempts were using battery packs and the LM2588. That worked ok, but couldn't get rid of the noise completely and got too expensive quickly. Then I was planning on using something along the lines of a mini-ITX PC PSU, but don't want to waste PCB space on minimum load resistors (or even CC MOSFETs/opamps for each output), and using up a lot of space and 'capability' by using a miniITX vs a dual output box.Searching Digikey, Mouser, Jameco, the only thing I can find relatively cheap is the Meanwell RD3513 at anywhere from $20-$35 depending where you get it, but those require me to set up the mains input...which as noted earlier, I don't want to deal with.There's gotta be a wonhunglo box out there to fit the bill...and not be "shockingly" cheap (see what I did there?).

author: Po6ept
author_rank: Regular Contributor
timestamp: January 29, 2014, 07:33:12 am
content: Have you considered using a surplus ATX computer power supply? Since you need less than 1A on the -12V rail it might work for you.If that won't work, and if you only need one, you might try one of the surplus places.I found a surplus (non-ATX) supply in a quick search athttp://www.surplussales.com/PowerSupplies/PowerS-3.html.They have a model TM1D-4CBDD for $25 that offers +12V at 12A and -12V at 3A.Edit: The Intel ATX power supply specification calls for each unit to supply 0.8A of current on the -12V line. A 250-300W unit will supply about 10-12A on the +12V line and a 160-200W unit will supply about 6A on the +12V line. The regulation specification is +-5% on +12V and +_10% on -12V. Most units will be pretty close to the rated voltage.

author: mariush
author_rank: Super Contributor
timestamp: January 29, 2014, 07:56:38 am
content: Modern atx power supplies usually have just 0.1-0.3A on -12v ... and it's not regulated, it can be as low as -10v -9v because the computer won't care, as it's only used for serial ports.could just use a 12v lcd monitor adapter (usually rated for 2-4a) and a mc34063 in inverting configuration to get -13v at some current and then use a ldo to get -12v. Not sure how much current it would be able to do.

author: Skimask
author_rank: Super Contributor
timestamp: January 29, 2014, 07:59:01 pm
content: The outputs are being post-regulated to +/- 8v (more if I can find the right PSU).Yes, considered using ATX PSU's, surplus or not. Can get them for a dime a dozen in a lot of places. BUT, they're big and bulky. I'm looking for something a fair amount smaller.Yes, most ATX style PSU's have low amperage, unregulated -12v outputs. That's ok. 1 amp is the absolute max, more like 300mA normal max load.I'll double check the MC34063. I was under the impression that the max it could do was less than 100mA...

author: mariush
author_rank: Super Contributor
timestamp: January 29, 2014, 08:13:06 pm
content: It can probably do about 250-300mA :http://www.bobtech.ro/tutoriale/componente-electronice/43-calculator-online-mc34063a-mc34063-step-down-step-up-inverterMC34063 is indeed not very suitable for this but it should be a good start. There are other ICs that can do more current though... maybe something similar toLT1931but cheaper if budget is an issue (though such IC and the other required components would be small enough to fit inside one of those 12v power bricks.

author: Skimask
author_rank: Super Contributor
timestamp: January 29, 2014, 08:40:25 pm
content: That little calculator, while it may not be 100% accurate, says I can pull about 350mA at 1mV ripple (1mV! Ya right!)Thanks for the tip. I'll have to get my hands on a couple of these and see how much magic smoke I can make.

author: jwm
author_rank: Regular Contributor
timestamp: January 30, 2014, 11:07:10 pm
content: Since you are willing to use batteries, have you considered just using two battery packs in series with GND being the middle tap? I use that configuration for analog circuits a lot when prototyping, very simple and dead quiet noise wise. Your batteries will likely wear unevenly, but you can always periodically flip them and you usually don't need both polarities to be exactly equal anyway.

author: Skimask
author_rank: Super Contributor
timestamp: January 30, 2014, 11:53:17 pm
content: Yep, thought about that as well. Even thought about adding in some extra bits to balance them out a bit by doing as you suggest...flipping the pack halves once in awhile (even during use if needed), or adding in some more elaborate bits to do that...but I'm still going to revisit the chips described earlier. I've never used them before. It'll be a learning experience.In my circuit (and yes I know I haven't even posted so much as a block diagram), I'd be drawing a fair amount more + than - anyways, a bunch of digital bits along with the analog bits. I'll get around to that block diagram some day...sooner or later...

author: LukeW
author_rank: Frequent Contributor
timestamp: January 31, 2014, 03:32:51 am
content: It's not really an answer to what you asked for, not ready made, but since you only want 1A at +/-12V, how about a basic power supply like this sort of thing, using a 7812/7912?http://www.qrp.pops.net/images/2008/2008-af-power/split15.GIFYou can get an wall-wart transformer with AC output (i.e. no rectifier or other electronics built in, just a transformer inside the box). So you don't need to wire the 110VAC/220VAC side yourself at all, just add the diodes, caps and regulators on the secondary side.The other common way to do a linear bipolar supply like this is with a center tapped transformer connected to ground at the secondary center tap. However it is rare to find pre-manufactured (wall wart style etc) transformers with a center tap connection, usually if you go down that road you have to be willing to construct the primary-side wiring safely yourself.http://www.jaycar.com.au/productView.asp?ID=KC5501Is there something roughly equivalent to Jaycar in your country, where basic educational kit projects like that are sold?

author: The Electrician
author_rank: Frequent Contributor
timestamp: January 31, 2014, 05:49:00 am
content: Get two 12V, 1A wall warts.

author: Skimask
author_rank: Super Contributor
timestamp: January 31, 2014, 07:14:30 am
content: I kinda want something ready made in quantity, like an ATX PSU, but obviously not an ATX PSU.The kit places are a bit hit-and-miss from month to month whether or not they've got the same parts in or not.And I would build one myself no problem, transformer, mains connections, rectifiers, filters, etc.etc.etc., but don't want to worry about the safety factor of plugging it into the wall.Sure, batteries can be just as dangerous as a wall plug, but, well, not really....know what I mean?

author: Skimask
author_rank: Super Contributor
timestamp: February 04, 2014, 06:01:05 am
content: Just ordered a few MC34063's along with necessary pieces/parts to build a dual rail + and - 12v power supply fed by a +12-ish v psu.We'll see what happens...Thanks for the advice...

author: jwm
author_rank: Regular Contributor
timestamp: February 04, 2014, 06:26:03 am
content: Quote from: Skimask on February 04, 2014, 06:01:05 amCan you share your circuit? I have been meaning to make a small bipolar supply and have some mc34063s on hand.John

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 04, 2014, 06:44:45 am
content: Quote from: Skimask on January 31, 2014, 07:14:30 amthe trend for bench lab quality psu is not positive negative output in one piece, but one or two isolated positive outputs only, they in the holy grail sky expect you to connect them in serial to get the output you want. thats one of the reason i built myself one in the past, still work for my little need up to this point. a simple '2 wires output transformer + voltage doubler config + LRO chips" is elegant and to my surprise is proven to be used in $1000+ Agilent/HP "low level measurement and low noise" sensitive device.

author: mariush
author_rank: Super Contributor
timestamp: February 04, 2014, 03:27:21 pm
content: Quote from: jwm on February 04, 2014, 06:26:03 amAny MC34063 datasheet will have an inverting circuit example. The calculator i linked to above will also show the schematic and the component values.

author: jwm
author_rank: Regular Contributor
timestamp: February 05, 2014, 01:15:54 am
content: Quote from: mariush on February 04, 2014, 03:27:21 pmAh, okay. I was thinking you would have one chip providing both regulated voltages from some other unregulated input voltage. I have been looking for something like that. My current thinking is using one of linear's flyback regulators, like this one.http://www.linear.com/solutions/1527Which provides isolation for free, and in fact provides two independent supplies that can be in series or parallel. Although I somewhat understand how it can use the back EMF to regulate the output voltage without the need for an explicit opto feedback path, I am not sure of the mechanism in the split supply like this, if one rail is loaded more than the other, how does it transfer energy to just that one without raising the voltage in the other? or do they naturally voltage balance somehow?