HilldaFlyer August 2019
This article details how to modify a Delta Electronics DPS-750XB power supply to make it suitable for use as a power supply for RC battery charging.
For many years I have been using a deep cycle battery to charge flight batteries at the field. I have now reached a point where the battery doesn’t store enough energy to recharge all the batteries I use. Furthermore, when AC electricity is available, like at home, I use a repurposed computer ATX power supply that provides 12v DC. The downside to this is the ATX power supply only provides 10A, so charging a lot of batteries takes a bit of time. My goal was to create a power supply that would have no trouble dishing out enough power to feed both of my 300 watt charger, 600 watts total. The last part of this equation will be to provide AC electricity at the field, which will be done with an inverter/generator.
In searching out options for AC/DC converters, the commercially available supplies producing 600 or more watts seemed to be very expensive, ranging from $150 to over $300. I have heard many hobbyists converting server power supplies into RC charging stations, so I looked into it. In my search I found a really powerful compact unit the Delta Electronics Switching Power Supply Model: DPS-750XB A Rev 05F. At the time I am writing this, these power supplies are found on ebay from $15 to $20 US. This is very compact measuring 20 cm x 7.5 cm x 4 cm and dishing out 750 watts or 62 amps at 12v. This is very economical when comparing it to commercially available AC/DC converters. And, it was really a simple DIY project to make it safe to use as a power supply.
If you have access to a different power supply, the following RC groups article is a great reference and reading for those attempting this modification.
RC Groups: How to convert Server Power Supplies
Power supplies like this DPS-750XB will not turn on unless properly mounted in the intended equipment. Typically there is on pin or pad that needs to be grounded indicating to the power supply that it is installed. From what I’ve read, this pad is typically shorter than the rest so that it is disconnected first when pulling it and it is connected last when inserting. Below I describe which pad/pin that is. If you have aspirations to perform a project like this, I highly advise you read the RC Groups thread and get one that is already documented. The DPS-750XB was not in their list so I detail the process below.
Using server power supplies comes with a safety hazard.
The back end of this hot swappable power supply are engineered to be slid into a slot in the server. Protruding tabs are not safe because there is a risk of accidentally shorting them by touch or dropping something metal on them. To make these safer to use, I will remove this exposed tab.
The front of the unit has a fan with a handle to assist in removal and a securing clip that can only be depressed if the power cord is detached. This clip is unnecessary for applications outside the server, so to save space, it will be removed. The handle can be removed if desired, but it doesn’t add that much length and protects against blocking the fan from air flow.
To open the case, remove the two screws that secure the case.
Lift up on the case near the plug to disengage the case from the plug housing.
Slide the cover rearward and rotate off the main case.
On the rear, remove the wire tie securing the LED and fan wires.
Unclip the fan and LED wires from the back.
Unscrew the grounding wire.
Lift the plug housing out of the unit.
There are three screws securing the circuit board to the case, one is located underneath the plug housing that was just removed.
The other two are in the rear. Remove the three screws. The LED and Fan wires are attached to the case, so pull them upward so they are out of the way.
Lift up on the rear circuit board to disengage the board from the case standoffs. The board will slide rearward and can be lifted off.
For safety, turn the board over and ensure the capacitor is discharged by shorting its leads shown in arrows.
The circuit board with the protruding pads are secured to the main board with two small rivets. I used a rotary grinder to remove the expanded end pointed to above. Then the rivets will push through.
Once the rivets are removed, a small black plastic divider that separates the main board from the protruding pad board can be removed. Save it for reuse.
The extension board is still attached to the main board through the main positive and negative leads and an 18 pin connector. If you have the capacity to desolder the connectors, then that would be the way to proceed. I don’t have the capacity so I used a rotary tool.
Saw through all the connections with a rotary cutter making sure that none of the underlying electronics on the main board are disturbed.
The protruding board will lift off after all the connections are cut.
The cut off leftover posts can be removed by heating with a soldering iron and pushed them through.
The 12v and GND where the connectors will be attached are clearly marked on the board.
To get the power supply to turn on, the pad labeled SGND on the tab that was removed needs to be connected to ground.
Note that the SGND pad is shorter than the other 6 pads. I traced it to the middle pin of the back row.
With the extended tab removed, the main board will need a connection between ground and the middle pin of the back row to turn on. I installed a rocker switch between these two locations to allow turning the unit off while still plugged in.
Power out was achieved by soldering 12AWG silicon wire into the 12v and GND holes. Connect a small wire from the middle top row of headers to the ground. I reinstalled the black spacer. although I did flip it over so the solid part was behind the wires.
I used a three position rocker switch (because I didn’t have any two position switches) with one end snipped off and the other leads bent over so it would fit.
To mount the switch, cut one of the cross members from the grating.
The bottom of the case,where the tab extruded, is now open, so for additional safety I 3D printed a “L” shape cover which is held in place by sliding over the risers. A piece of tape I think would have worked fine.
This should keep accidental touches and metal objects from finding their way into the case.
I didn’t see a reason to keep the retention clip.
I used a 5/32” drill and drill out the two rivets that held it to the case.
The final touches are to reassemble the case. Stick the wires through the grating and add female banana connectors to the ends of the leads, and a switch to the switch leads.
Floating the Ground
Some individuals need more than 12v DC to power their power hungry charging systems. To get more than 12v, two power supplies can be connected in series.There are some real dangers with attempting these kind of modifications, so be sure you understand the dangers and perform the modification at your own risk. To connect a 12v lead to the negative lead of the second unit, the ground of the second unit needs to be floated, in other words, the connection between ground to the board needs to be removed. Most electronics ground the board to the case.
Turns out that this board is grounded to the case in only two locations, the two standoffs in the rear. I used a rotary tool to sand off the ground traces on the bottom of the board to expose the fiberglass. The inside of the holes are not connected to the trace and the trace on the top side will be insulated with the black plastic space that was removed earlier. This should only be done if the power supply is the second unit in series with the first unit having proper grounding.