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2x HP server PS for 24V

The following article is nothing more than a project report. I take no responsibility for how others may use this information.
During my research for converting a server power supply (PS from here on out) for RC use, I also ran across how to tie 2 of these guys in series to get 24V. So of course I had to try it.

The power supplies
When I got my first DPS-600PB and converted it for RC use, I knew it would make a perfect candidate for a 24V PS. So I ordered another pair and now I am ready to convert them and wire them in series.

Before I could wire them in series
I had to convert them to RC use first. I simply followed the same procedure as the other unit. In fact I followed the exact same procedure and so I technically just had 3 identical PS units. I did this so that I would not be limiting myself by making these 2 units different somehow. That would allow me to reassign the use of each unit as needed.

Safety concerns
During my research, I found many safety concerns that people had about doing this sort of project. The biggest concern is a result of the simplest method of floating the DC output on the second power supply in order to wire them in series, and that is to remove the AC earth ground on one unit. Doing this removes the "safety net" from the PS, and so if the PS were to fail, the case could become electrified with 120VAC. There are several approaches to dealing with this concern from just ignoring it, to installing a 120VAC neon bulb between the cases to act as an indicator of a problem. I chose to ignore the concern but isolate myself from the units.

Making 24V
Actually wiring the PS units in series was simple, I just made a jumper to connect the 12V output on one unit to the ground output on the other. Then I just connect to the remaining outputs and poof, 24V. But of course it is not quite that simple. First I had to deal with the fact that I am floating the output of one unit and the DC ground is tied to the case. So if I wire the 2 units in series and they touch, it will cause a short to ground. But it gets worse. Not only is the DC ground tied to the case but the AC ground is also. That means that even if I physically isolate the cases from each other, connecting the 2 units to the wall will still cause a short. I considered my options; 1) disconnect the AC ground inside one unit, 2) disconnect the AC ground outside one unit, or 3) disconnect the DC ground from one case. I chose #2 and planned to make a custom cable that only connects the AC ground to one unit. That brings me to the next part ... the box.

Building it

1st generation: PS in a box
I have seen lots of people simply wrap the PS units in packing tape and then tape them together. Although that would work fine, I wanted to try out putting the units in a box and making it look a bit more finished. I spent about a week trying to find the perfect box to house the PS units. In the end I settled on an Align 450 parts box. I have a couple of them and although it's not a perfect fit, it is the closest thing I could find.

Surveying the box I figured out where I wanted to mount the power plug and banana plugs. Then I did a lot of thinking on how to get air to move through the box. In the end I decided to mount the power plug on the lower right side, the banana plugs on the upper left side, the air inlet vent on the lower left side and the air outlet vent on the upper right. For the vents, I settled on a several 3/8in holes for the right side vent and a 50mm fan grill for the left.

All work was done with either a drill or Dremel. The case is made from aluminum edging around a pressed paper board that is covered in an aluminum skin. The board material is easy to work with but the skin is very thin and very fragile.

I used a piece of very dense styrofoam to fill the space between the two units and to isolate them from each other.  Once the styrofoam was the correct size, I used packing tape to secure the two units and foam together. Lastly I used two foam strips to hold the PS units in place and to stop air from recirculating inside the case.

The wiring
After mounting the 3-pin AC plug housing, I worked on making some cables to connect to both PS units. I have an overabundance of computer power cords at work so a pair of them was sacrificed for the project.  I trimmed them down in order to make them as small as possible and then removed the black sheath to expose the wires. Next I trimmed them to the length I wanted and removed the ground from one connector. Lastly I soldered them to the housing.

Next the output side. I wired the output of each PS with 12awg wire and female 4mm bullets. I used 12awg because I had it but 10awg is probably the best choice for this use. Anything larger is a waste. This made them interchangeable, as well as being usable outside the case as a normal 12V PS. To connect the PS units together, I made a simple jumper with male 4mm bullet connectors on each end. One end goes to the 1st PS GND output and the other end goes to the 2nd PS 12V output. The other output of each PS was then connected to the banana plugs. The last step was to solder a wire to each of the banana plugs, terminate them in male 4mm bullets and then plug them into the remaining two outputs. 

The end result
All in all I am pleased with the end result. It is larger than I would have liked but I can't really complain, it is a 1100W+ power supply. The case did muffle the fans a bit but they are still very much present.

Inside the completed power supply.

The air outlets and AC port.

The 24V output and the air inlet.

The completed power supply :)

2nd generation: PS in cardboard
I had a big box sitting around doing nothing and I thought why not try to wrap the PS units in cardboard? I mean the important thing is to isolate the 2 units, and cardboard does this really well. Anyhow I have been using it for a while now and it works really well. As such I would consider this a pretty good solution and one that is easy to try, one that doesn't modify the units, so it is easy to undo it later if a better solution is found.

That's right, cardboard.

The input side.

Here is a quick cable I tossed together.

This is the output side. I used some plastic to retain
the series connection.

3rd generation: naked PS + other models
I have been toying with ideas for a while now but just couldn't come up with one that I liked for the long haul. Well I think I have found it and I am very happy with the end result. I used the 2 HP units from the case above and started by removing the hot-swap connector and board. I installed 2 banana jacks in one unit, disconnected the DC ground on the other one and installed a single banana jack in it. Next I figured out a way to easily screw the two units together and to hide the series connection. Below is the end result along with 2 other sets of units I have tried (see Convert other server power supply for RC use).

Left: HP 575W, Middle: Dell 700W, Right: Dell 550W

Photos of the power supplies
A pair of ESP135 power supplies in stock form.

Handle removed and side opened for catch removal.

Both power supplies ready for wiring.

All wiring completed.

Notes and updates

Making both 24V and 12V outputs
One thing I found useful about having (2) 12V units wired in series is that they are 12V units, and so connecting to just one will give you 12V. This was not lost on me and something to planned to add later.

Keeping the AC ground and ditching the DC ground
I can never leave anything alone and so why not tinker on the 24V PS some more? I was reading a thread about these units and the poster was talking about removing the DC ground instead of the AC ground. I hadn't considered that but it made sense to me, and the benefits would include retaining the AC ground on both units and the cases can touch without issue. But how to do it? Well I tore one of my units apart and found that the case is grounded to the DC output at both screw mounts on the DC side of the PS. So all I had to do is sever that connection and I would be golden. I used my pocket knife to score the board on both sides of one mount and snipped to jumper wires leading to the other. I tested the connection as I went with a multimeter and once there was no connection, I was done. Once I had the PS back together, I tested it and it worked as usual. The last thing I did was mark the case to make sure I knew that unit lacked DC case grounding. 

A note to those looking to create their own 24V power supply using a pair of server power supplies
I recommend you disconnect the DC ground from the case on one unit when connecting them together to make 24V. To find the grounding locations in your power supply, use a continuity tester to locate each place of contact, sever it the best way you can and test with the meter to confirm. Once you have completed the task, reassemble the power supply and test the connection between the DC ground and the case. If there is no connection, you have succeeded and can connect the units in series. 

My friend Greg's build
"Two Feathermerchant 47a supplies in series in the Harbor Freight black case. The assembly is held in place with 4small square 3M dual lock pads." -Greg

"12v supply using banana plugs and the 24v supply using the massive 75 amp Anderson Power Poles. The 12v supply is taken only off of the first supply so both outputs share the same ground. This is to prevent shorting of metal cased chargers." -Greg

"All connections made with 8 gauge wire. Not as easy to work with as the smaller stuff. Probably more gauge than is required. But now I don't have to worry about." -Greg

"I used the flat PC power supply wires because they are more flexible than the round ones. I made these a little longer than they need to be so its easier to attach them to the two supplies and gives me some flexibility if I want to change the config." -Greg

"The fans are pretty loud so when I don't need 24v disconnect the second supply. Also note the neon bulb and ungrounded case label for safety." -Greg

"Other side of the power supply input cables using standard PC power supply female plug." - Greg