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-   -   Making Symmetric Power Supply from Single with 4x ATX power Supply? (http://www.diyaudio.com/forums/power-supplies/223893-making-symmetric-power-supply-single-4x-atx-power-supply.html)

lupetalo 19th November 2012 10:31 AM

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.

rikkitikkitavi 19th November 2012 03:22 PM

You meant.that you removed the earth (as in earth in the wall outlet) connection, not ground, on the supplies?

lupetalo 19th November 2012 03:50 PM

Quote:

Originally Posted by rikkitikkitavi (Post 3247606)
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...

DF96 19th November 2012 04:06 PM

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.

macboy 19th November 2012 04:58 PM

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.

lupetalo 19th November 2012 08:30 PM

Quote:

Originally Posted by macboy (Post 3247724)
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.

lupetalo 19th November 2012 08:32 PM

Quote:

Originally Posted by DF96 (Post 3247670)
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...

rikkitikkitavi 20th November 2012 07:45 AM

sorry i did not reply sooner after my first reply but macboy summarized what I had to say anyhow :)

lupetalo 20th November 2012 07:50 AM

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....

DF96 20th November 2012 10:28 AM

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.


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