Author Topic: making a dual rail +/- power supply with two LM317 and no LM337  (Read 16978 times)

0 Members and 1 Guest are viewing this topic.

Offline dentaku

  • Frequent Contributor
  • **
  • Posts: 831
  • Country: ca
  • Country: ca
I know this is a naïve question but I'm going to ask it anyway :)
I've been looking through all my salvaged parts and lots of the stuff I recently got from an old man's collection of parts and I have two LM317 regulators. I wish there was an LM337 in there somewhere, but there isn't.

Question?
Is there there a way to make a dual rail +/- 15V power supply for powering dual supply op-amps with two LM317s? or is there no way around needing a negative voltage regulator?
 

Offline ElectroIrradiator

  • Frequent Contributor
  • **
  • Posts: 614
  • Country: dk
  • Country: dk
  • More analog than digital.
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #1 on: August 05, 2013, 10:21:54 pm »
There is, but you need a mains transformer with two isolated secondaries. Given that, then you can make two isolated/regulated LM317 supplies, and simply 'stack' them. If won't be as efficient as using an LM317+LM337, as you need 8 rectifier diodes instead of 4, but it will work.
 

Offline expertmax

  • Regular Contributor
  • *
  • Posts: 51
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #2 on: August 05, 2013, 10:54:14 pm »
It's not worth the trouble in my opinion. Just a quick drive to radioshack or eBay and you can get the LM337 for under a buck, free shipping.

eBay auction: #http://www.ebay.com/itm/2-x-LM337-LM337T-ADJ-Negative-Voltage-Regulator-IC-1-5-/260815926833?pt=LH_DefaultDomain_0&hash=item3cb9d73631
« Last Edit: August 05, 2013, 10:56:23 pm by expertmax »
 

Offline Paul Price

  • Super Contributor
  • ***
  • Posts: 1419
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #3 on: August 05, 2013, 11:04:55 pm »
All you need is a PNP power transistor, two small-power PNP  transistors, a current sense resistor,(.01 ohm 1-Watt) and a few 10K resistors, and an op-amp (LM358 will do) configured as an inverter with a gain of 1. You will need either two power transformers or else something like a 24V AC 70VA rated (3 Amp capable) center-tapped transformer and you can make a dual-tracking +- 15V 1.5 A power supply.
 
The following users thanked this post: jakeisprobably

Offline dentaku

  • Frequent Contributor
  • **
  • Posts: 831
  • Country: ca
  • Country: ca
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #4 on: August 06, 2013, 01:52:40 am »
As I suspected, it's rather impractical :)

By the way, kits like this http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_20626_-1 come with a transformer and everything is self contained but I'm assuming if you where to make one that uses a wall wart and no transformer inside your enclosure you would only be able to make a positive voltage adjustable power supply.

Basically it would be a plugged in version of this battery powered adjustable supply http://www.ladyada.net/library/equipt/diypsupp.html ?

I think I'm going to start simple and do that first before I try building a proper one.
 

Offline Paul Price

  • Super Contributor
  • ***
  • Posts: 1419
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #5 on: August 06, 2013, 02:27:24 am »
If you use two wallwarts that are isolated from each other and build two identical regulator circuits, then you can have two floating output supplies  and each one could be used either as a positive or a negative output supply, depending on whether you ground the positive or the negative output terminal.

The circuit I suggested in not that complicated! The Jameco dual output supply only goes down to 5V and it has 48 components. Mine has about half that many.
« Last Edit: August 06, 2013, 02:34:37 am by Paul Price »
 

Offline c4757p

  • Super Contributor
  • ***
  • Posts: 7805
  • Country: us
  • Country: us
  • adieu
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #6 on: August 06, 2013, 02:30:41 am »
On that note, old AC wallwarts are a decent source for line transformers. People throw them away all the time. If you don't want a big old wart hanging off your wall (or two! in this case), Dremel them open and take out the transformers.
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline TerminalJack505

  • Super Contributor
  • ***
  • Posts: 1205
  • Country: 00
  • Country: 00
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #7 on: August 06, 2013, 02:39:06 am »
If you use two wallwarts that are isolated from each other and build two identical regulator circuits, then you can have two floating output supplies  and each one could be used either as a positive or a negative output supply, depending on whether you ground the positive or the negative output terminal.

The circuit I suggested in not that complicated! The Jameco dual output supply only goes down to 5V and it has 48 components. Mine has about half that many.

Your schematic looks like a tracking supply.  I'm not sure if the OP wants the two outputs to be individually adjustable or not.
 

Offline dentaku

  • Frequent Contributor
  • **
  • Posts: 831
  • Country: ca
  • Country: ca
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #8 on: August 06, 2013, 11:42:47 am »
If I was to take two 12 volt wall warts and connect the positive of one to the negative of the other...
1 - is this safe, or will it just make one get really hot?
2 - it looks to me to be the same as snapping together two 9 volt batteries (- of one battery into the + of the other) right?
this is the easiest way to get +9/0/-9 out of 9V batteries I've ever used
 

alm

  • Guest
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #9 on: August 06, 2013, 11:51:20 am »
Yes, as long as the output is not connected to ground (eg. if it doesn't have a grounding pin), then you can safely connect them like this. Don't do it if both of the supplies are grounded, however, then you'd be shorting them.

The presence of a ground pin does not mean that the output is connected to ground. You can measure (while unplugged) the resistance between ground pin and the positive and negative output to see if there's a low resistance path between the output and ground.
 

Offline dfmischler

  • Frequent Contributor
  • **
  • Posts: 548
  • Country: us
  • Country: us
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #10 on: August 06, 2013, 01:57:04 pm »
Yes, as long as the output is not connected to ground (eg. if it doesn't have a grounding pin), then you can safely connect them like this.

Aren't you concerned about capacitive coupling to hot or neutral in a switching mode wall-wart?  I have an ungrounded  12V supply here that shows 48V AC from the DC return to AC hot or neutral, although I'm sure there wouldn't be much current.
 

alm

  • Guest
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #11 on: August 06, 2013, 02:26:51 pm »
Safety regulations usually dictate very low limits for this current. It will be shunted to ground during normal use (eg. plugged into a grounded computer), so I don't see any harm in shorting it. Grounded equipment will often have a very similar arrangement: one Y cap from hot to earth and one from neutral to earth. With even larger caps than permitted for double isolated equipment.
 

Offline dentaku

  • Frequent Contributor
  • **
  • Posts: 831
  • Country: ca
  • Country: ca
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #12 on: August 08, 2013, 11:01:05 pm »
I just built two identical LM317 circuits on a little breadboard an connected them together like snapping together two 9V batteries + to -.
One is powered by a 12V UPS battery and the other from a 13.8V wallwart. and it works. I set them both to 5V and one side shows 5V and the other -5V.

It's not terribly practical but for a beginner it's nice to see something work :)
 

Offline jakeisprobably

  • Regular Contributor
  • *
  • Posts: 145
  • Country: us
  • Country: us
    • Upcycle Electronics
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #13 on: December 21, 2016, 03:01:41 am »
All you need is a PNP power transistor, two small-power PNP  transistors, a current sense resistor,(.01 ohm 1-Watt) and a few 10K resistors, and an op-amp (LM358 will do) configured as an inverter with a gain of 1. You will need either two power transformers or else something like a 24V AC 70VA rated (3 Amp capable) center-tapped transformer and you can make a dual-tracking +- 15V 1.5 A power supply.
  Ancient Post, I know, but this thread came up on a google search for a dual tracking regulated power supply without a LM337. After seeing Mr Price's hand drawn schematic I knew I had the spares and junk parts to build his drawing. I put together a schematic on Fritzing to try out their software. I thought it was worth sharing with the next person to come across this thread. Thanks.
 

Online Alex Eisenhut

  • Super Contributor
  • ***
  • Posts: 2064
  • Country: ca
  • Country: ca
  • If you can buy it for 4$ on eBay, why design it?
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #14 on: December 21, 2016, 03:33:22 am »
Where in Canada are you? I might have a 337 to spare.
 

Offline jakeisprobably

  • Regular Contributor
  • *
  • Posts: 145
  • Country: us
  • Country: us
    • Upcycle Electronics
Re: making a dual rail +/- power supply with two LM317 and no LM337
« Reply #15 on: December 21, 2016, 03:44:32 am »
I like to see what I can recycle and build from junk I already have as much as possible. The second I go to order something online, a digital black hole develops in my shopping cart, I blackout, and somehow manage to spend 100 times more than the one little chip I needed. I'm just a beginner with more time than sense, or money.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf