2x24V switchmode power supplies = 48V?

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** Should be possible to isolate the ground from the output of one supply and have it sit 24V above.

If there is any "devil" it is in details you have not supplied us.

.... Phil

Thanks Phil. How would I do this isolation?

You can start by supplying the maker and model number of the power supply so we can see if both output terminal are not connected to case ground and how any sense wires are arranged. If there is no model number, take an ohmmeter and check for continuity between each output terminal and the case ground. If there is continuity, see if there is a bridging clip on the output terminal strip to case ground. Remove it and see if you still have continuity. Lastly, apply AC power with no load, and see if there is any voltage between either output terminal and case ground. There might be some leakage voltage, but it shouldn't be the full 24VDC. If you see the full 24VDC between either output terminal and ground, it won't work.

--
Jeff Liebermann     jeffl@cruzio.com 
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That depends on the design of the power supplies as well as the characteristics of the load. Having said that, I pulled a pair of 70A 12VDC power supplies from a server and stacked them to form a 24V source for an electric wheelchair.

You'll likely have to isolate the "0V" output ("-" or "GND") from the chassis ("Earth") before you can do so. Otherwise, stacking them will directly short out one of the supplies!

XX X+.................... XX X-......... XE . . . . . . YY . . Y+.... . YY . Y-............... . YE . . ......

(forgive the crappy ASCIIART)

Note that Xe is internally tied to X-. Likewise, Ye is internally tied to Y-. X- is externally tied to Y+. Xe is tied to Ye in the power cord. So, Xe is tied to X- which is Y+ *and* Y-!

You'll have to disassemble the power supplies to determine where the bond between "gnd" and "earth" occurs and then physically sever that connection. In my case, the PCB was mounted to a METAL boss with a metal screw that tied some "gnd" land around the screw to the chassis.

I removed the metal boss(es) and replaced with nylon standoff(s). Then, passed nylon screw through the standoff to nylon nut to secure the board in place.

Once finished and reassembled, an ohmmeter should verify there is no longer a direct short from the earth pin on the power inlet to the 0V/gnd terminal on the power supply.

(note that the same reasoning applies if the "+" terminal had been bonded to earth)

What I would do is to wire them in series, like you want to do but add a bypass diode across each supply's output so it doesn't get a reverse voltage across it if they should be unbalanced.

Cathode to the positive and anode to the negative terminal. Of course the diode will have to be rated for at least 24V and be able to take the current you would be drawing at 48V

WAIT. That is only the NEXT STAGE!

FIRST he needs to make sure the output is 'floating', i.e. not connected to the case/ground. When he is not able to accomplish that for whatever reason, there is no need to go on to this step.

booB wrote drivel:

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** Not so smart if the outputs are both supply grounded. ** Makes no sense - WTF is " unbalanced".

However the diodes are a good idea in case one supply stops working.

** Drivel.

..... Phil

Any half decent AC line to DC output supply would be isolated

If not then it is just a stupid buck converter.

sure it'll be isolated but the output might be grounded

If it is, then unground it

In those cases, there is usually a strap that can be undone.

I don't think he said how much current this was supposed to supply ?

First find that out and make the mod. There are supplies where it is easy, and there are those where it is not so easy. It depends on the original purpose and design of the supply.

It is irrelevant. The same method works for 100mA and 100A.

The theory is the same but the bypass diode is not the same and would have to dissipate quite a bit if called upon at 100A

The reply already said "Of course the diode will have to be rated for at least 24V and be able to take the current you would be drawing at 48V". So that is well covered.

It is also dependent on the characteristics of the supplies and the load if this is at all required. For a resistive load it isn't required. For a load that can spike above the rated output of the supplies, or for supplies that have strange V/I characteristics like "foldback current limiting" (instead of a current source at max-I) it is certainly advisable.

However, again, all of it is irrelevant when the supplies cannot be made isolated to ground.