Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 19th November 2012, 10:31 AM   #1
diyAudio Member
 
Join Date: Aug 2012
Default Making Symmetric Power Supply from Single with 4x ATX power Supply?

Hi i have 4 ATX power supplies that i connected in series and effectively got 48v DC. Removed ground connection from them and it is working. on first 0 is negative and +12 is connected to another and so on, last +12 is positive.

Question is can i use connection between second and third supply to get virtual ground for supplying AMP that needs +-25V. Voltages are good and it seams like +-24 V, but not sure if that can cause problems. All Supply's are rated to give 20A per 12v lin.
  Reply With Quote
Old 19th November 2012, 03:22 PM   #2
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
You meant.that you removed the earth (as in earth in the wall outlet) connection, not ground, on the supplies?
__________________
listen! Can you hear that ...
  Reply With Quote
Old 19th November 2012, 03:50 PM   #3
diyAudio Member
 
Join Date: Aug 2012
Quote:
Originally Posted by rikkitikkitavi View Post
You meant.that you removed the earth (as in earth in the wall outlet) connection, not ground, on the supplies?
That is right, disconected at input to power supply, sorry for bad english...
  Reply With Quote
Old 19th November 2012, 04:06 PM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
You may have introduced a safety problem and possibly an EMC problem too. To get an isolated output you have to remove the output ground connection, not the input ground connection. This might mean cutting tracks on a PCB. You may need to learn more about PSUs and safe working before you modify them.
  Reply With Quote
Old 19th November 2012, 04:58 PM   #5
macboy is offline macboy  Canada
diyAudio Member
 
Join Date: Oct 2003
Location: Ottawa, Canada
That is not safe. The metal chassis of the power supply must remain earthed for safety and for certain power line noise filtering ("Y" capacitors) to work correctly.

Inside your power supply there is a pair (or two) of "Y" line filtering capacitors, one from line to ground, and one from neutral to ground, intended to shunt noise from line/neutral into ground. By disabling the earth connection, you may have created a capacitive voltage divider between line and neutral with the power supply case in the middle. In other words, you power supply's metal case may be "live" with 1/2 of your mains voltage (240/2 = 120 VAC!).

To enable the low voltage output to float, you need to isolate the output ground (black wires) from the power supply's chassis, which must remain earthed. Most often, this connection is done on one or two of the mounting screws of the PCB. You will need to figure out how to isolate the power supply case from the output ground, while also ensuring that the PCB remains securely fastened. Be aware that there is a limitation of how far the output can float above or below earth without potentially exceeding the isolation capability of the power supply's transformer, which would result in risk of electrocution.
  Reply With Quote
Old 19th November 2012, 08:30 PM   #6
diyAudio Member
 
Join Date: Aug 2012
Quote:
Originally Posted by macboy View Post
By disabling the earth connection, you may have created a capacitive voltage divider between line and neutral with the power supply case in the middle. In other words, you power supply's metal case may be "live" with 1/2 of your mains voltage (240/2 = 120 VAC!).
Thanks for quick reply, i am amazed how people want to help and share their experience and knowlenge. +1 to you.
I found out that chases are indeed hot, but not with half mains, but 12v. case is same as +12 on one power supply. There is a 12v difference between neibourgh modules and 48 between first and last. Also i have noticed that when switch off one of them voltage just drop by 12v... One strange thing, when conecting +5v in series i get exactly same thing but with 4x 5v, and 4x 3.3v when connect +3.3 in series.
  Reply With Quote
Old 19th November 2012, 08:32 PM   #7
diyAudio Member
 
Join Date: Aug 2012
Quote:
Originally Posted by DF96 View Post
You may have introduced a safety problem and possibly an EMC problem too. To get an isolated output you have to remove the output ground connection, not the input ground connection. This might mean cutting tracks on a PCB. You may need to learn more about PSUs and safe working before you modify them.
I did cut earth from the wall outlet from pcb...
  Reply With Quote
Old 20th November 2012, 07:45 AM   #8
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
sorry i did not reply sooner after my first reply but macboy summarized what I had to say anyhow
__________________
listen! Can you hear that ...
  Reply With Quote
Old 20th November 2012, 07:50 AM   #9
diyAudio Member
 
Join Date: Aug 2012
Thanks guys, i will make some kind of isolation between cases and outside world... Right now it is not possible to buy toroidal transformer with such power (and would need aditional circuitry) so i guess i will be testing this on old LM3886 and will report back....
  Reply With Quote
Old 20th November 2012, 10:28 AM   #10
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
I'm not certain you have understood what we say. Cases must be grounded, not isolated. You don't seem to have grasped the difference between the various grounds in a circuit. To be blunt, if you knew enough to do all this safely you probably would not need to ask us.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Power supply positive-negative-ground from a single power supply mome Power Supplies 8 9th September 2011 05:17 PM
ATX power supply noise. owenhamburg PC Based 9 16th November 2009 10:15 PM
Experiments with an ATX power supply martinab2 Solid State 0 29th March 2004 10:04 AM


New To Site? Need Help?

All times are GMT. The time now is 03:24 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2