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Negative-Rail SMPS

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solis365

New Member
I have two ATX power supplies with +12 rails. However the -12 rails do not have enough current rating for my application.

Thus I would like to combine two supplies in series. One will have the +12 referenced to the GND of the other unit.

This requires isolating the GND of the unit used for the NEGATIVE supply from Chassis ground. This is easy enough, but

I am worried about any ICs that are used... will they have the correct voltage to operate? Things like heatsinks are also grounded. It is just kind of odd.

Someone has done this with other types of SMPS, just not computer supplies. For example, I know it can be done with this unit: http://www.electro-tech-online.com/custompdfs/2009/08/PBA150F.pdf

However I have seen a few pictures of the PCB on that unit and cannot see any ICs except for one TO-220 package, not sure what it is.


anyone have thoughts on this? thanks.
 

mvs sarma

Well-Known Member
I have two ATX power supplies with +12 rails. However the -12 rails do not have enough current rating for my application.

Thus I would like to combine two supplies in series. One will have the +12 referenced to the GND of the other unit.

This requires isolating the GND of the unit used for the NEGATIVE supply from Chassis ground. This is easy enough, but

I am worried about any ICs that are used... will they have the correct voltage to operate? Things like heatsinks are also grounded. It is just kind of odd.

Someone has done this with other types of SMPS, just not computer supplies. For example, I know it can be done with this unit: http://www.electro-tech-online.com/custompdfs/2009/08/PBA150F-1.pdf

However I have seen a few pictures of the PCB on that unit and cannot see any ICs except for one TO-220 package, not sure what it is.


anyone have thoughts on this? thanks.
That could be a Power MosFet, if it is on the primary side. Don't tamper it, it would be live and Lethal.

If it is On secondary it would be a dual diode with common cathode. Generally such devices are used and both diodes paralleled to get higher load current.
 

Hero999

Banned
I have two ATX power supplies with +12 rails. However the -12 rails do not have enough current rating for my application.

Thus I would like to combine two supplies in series. One will have the +12 referenced to the GND of the other unit.
You've got that backwards, if you want more current, you need to parallel them.

Unfortunately, you won't be able to safety parallel them because they will fight each other.
 

solis365

New Member
You've got that backwards, if you want more current, you need to parallel them.

Unfortunately, you won't be able to safety parallel them because they will fight each other.
ATX supplies generally have a +12 and -12 rail, however the +12 is rated at a much higher current than the -12. My supplies are 16A on the +12 and .3A on the -12.

I need a +/- 12V supply with ground in the middle capable of supplying more than .3A. Hence, connect TWO in series.

The problem is that the GND connection on BOTH supplies is to Chassis. If I break this connection on one, I should be able to tie the +12 of that supply to Chassis ground of the other supply and get a -12 : 0 : +12 setup. I am worried about the ICs in the power supply. the one I linked above was a supply that someone has already done EXACTLY this with. My supply is slightly different; it is more complicated due to it being a computer power supply with more voltage rails, etc.


I have a way to safely work with mains voltages without starting fires and/or killing myself.
 
Last edited:

solis365

New Member
It worked. I took two ATX power supplies.

One was a 24-pin and the other was a 20 pin proprietary dell that I had to empirically determine the pinout of. However they worked swimmingly in series, producing a +12 and -12 output for a total measured voltage swing of 24 volts!

These are computer power supplies so whatever ICs were on the boards were not affected by the change in polarity. I am going to wire these up more safely and put them in an earthed chassis and perform some longevity tests - I dont want any of the ICs blowing up after 10 hours of use.

If anyone in the future wants to try this trick it works and you can get a 0-24V switched supply or a +12 : 0 : -12 supply... or anywhere in between.
 
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