# Series SMPS Modification

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#### solis365

##### New Member
I have come across two identical Samsung PSCF401601A(C) SMPS units. They are the power supplies from Power Mac G4 with the Mirrored Disc Drives (google Power Mac MDD power supply for results)

The interesting thing about them is that they have +25V rails in addition to the standard +/- 12, 5, and 3.3. I am currently trying to build a power supply for an audio power amp and 25V is a very workable voltage for audio power amps.

However I would need +25V and -25V rails, and these units only have +25V. So the basic electronically-oriented person would say to run them in series with the center point your ground reference.

But, this is mains-connected stuff, so it's no light task to go around wiring up things that are ground to things that are hot, as earth is... earth. And it will short together and most likely explode.

My inspiration for the project is **broken link removed** ... however he made some modifications to his SMPS to create a +/- supply. (the reason I do not use the -12 rail is that it does not have enough current rated.)

I do not posess the SMPS that was used on that website; if I did I could follow his isolation instructions and determine exactly what they did. However the datasheet/website are at these locations:

American Skynet Electronics

From reading it looks like he was just extremely careful to make sure that no part of the output was in any way connected to chassis ground. I believe I am capable of properly performing a similar isolation on another SMPS.

Assuming no part of a standard ATX power supply's output ground is connected to earth ground, then it should be "safe" to connect two of them in series, correct? I just want to confirm that connecting two switched-mode power supplies in series works the same as other power supplies such as batteries (which it should, in theory)

I put safe in quotes because I'm pretty sure disconnecting from earth ground makes it a fairly unsafe circuit that does not meet any safety standards.

This is the type of thing I will have on a breaker-ed circuit which I turn on from several feet away after clearing flammable objects from the vicinity

I want to confirm my theory is correct before I try it as these power supplies can be sold on ebay for about $80 each if I end up unable to use them. (they came from macs, thats why) I will most likely try the isolation on a cheaper ATX or AT power supply (which I get for$0-\$5 most places) to check the theory and practice the proper isolation techniques before I hook up my babies.
If all goes well I will post pictures and document the whole process so other people can get themselves killed.

#### marcbarker

##### New Member
I didn't look at datasheet properly. Assuming you decide that each of these (independently) provides the A and V you require, you will very likely run into the problem of output grounding for the - 25 V rail. Generally, the 0 V of an SMPS is electrically connected to chassis, it's also better for emc reasons. In short, you'll crowbar the "- 25 V supply" if you connect it's "+ out" to the other psu's "0V". A workaround is to modify the "- V" SMPS itself, to break the ground path inside it. Another workaround is not modify the SMPS itself, but isolate the chassis of the "- V" SMPS from ground, and deal with any emc issues that it causes.

#### solis365

##### New Member
I didn't look at datasheet properly. Assuming you decide that each of these (independently) provides the A and V you require, you will very likely run into the problem of output grounding for the - 25 V rail.
this is what i said in my post... (by the way the datasheets are for the SMPS used on the website from which I took inspiration, not for MY SMPS)

Generally, the 0 V of an SMPS is electrically connected to chassis, it's also better for emc reasons.
This is my main concern. I have confirmed that they are connected. However if I remove this connection the output ground should be "floating", and able to be connected in series, right?

In short, you'll crowbar the "- 25 V supply" if you connect it's "+ out" to the other psu's "0V".
okay so this does still work with SMPS type supplies, as I expected it would. thanks for confirming.

A workaround is to modify the "- V" SMPS itself, to break the ground path inside it.
not sure what you mean here... break the ground path from chassis ground to output ground?
isnt that what isolating the output ground from chassis ground means in the first place?

[quoteAnother workaround is not modify the SMPS itself, but isolate the chassis of the "- V" SMPS from ground, and deal with any emc issues that it causes.[/QUOTE]
I am confused by this and the sentence quoted above, to me they mean to do the same thing. can you explain please? thanks

what sort of EMC issues would arise? (by the way what do you use EMC to mean? Electro-Magnetic... something. I checked google and wikipedia for the abbreviation and couldnt find anything that seemed to fit.)

my thoughts are to isolate BOTH the SMPSs output grounds from chassis ground. then connect them in series and connect the "center" of them (where 25V of one and 0V of another are connected) to chassis ground. Then rebuild a chassis to house both boards. this way it would be electrically "safe", rather than having a device without a chassis ground.

#### marcbarker

##### New Member
The problem as I see it, is that the 0V output is linked to the chassis of the SMPS. This is normal, because it's good for ElectroMagneticCompatibilty.

Somehow, it's your call how you do this in the end, you need to break this connection on the - V supply. The +V supply needs no such modification to prevent shorting the -V supply.

You either hack into the -V supply and cut some tracks, or you somehow run it without any electrical connection between mains-protective ground and 0V output.

#### solis365

##### New Member
Will attempt tonight with crappy PSUs I just salvaged from a Dell 4400 server... i.e. pentium III xeon processor

if it works, on to the 25V smps!

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