I have two server PSUs from HP which can output 12V with 1200W (HP DPS 1200.) My goal is to obtain 24V by chaining the outputs of the two units.

I have modified one of the PSUs so that the negative output is no longer connected to earth by replacing one of the case screws with a nylon one, as recommended by threads in RC forums, where users use those supplies to power their battery chargers.

The metal case of both supplies is still grounded, so the only concern should be high voltages on the 24V output of the combined unit if something goes wrong inside the PSU.

One guy in these forums was concerned about the over voltage protection no longer working if this modification is done.

I want to know if such a modification disables safety features in events where the transformer is defective and shorts mains voltage to the low voltage side and if the order in which the supplies get connected changes this behavior.

(negative/earth side of the unmodified unit to 12v of the floating unit)

  • \$\begingroup\$ There is nothing called voltage. It is always voltage difference. If the PSU never sees a greater voltage difference on its output than it is rated for, you have nothing to worry. You may have to add a diode for reverse voltage though, in case the load shorts. \$\endgroup\$
    – Indraneel
    Jun 17, 2019 at 2:49
  • \$\begingroup\$ Verify that 0V actually is disconnected from mains earth before connecting them in series. Put the multimeter in beep mode and poke the metal chassis right near the heatsinks and such to make sure there are no paths to earth remaining. \$\endgroup\$
    – Oskar Skog
    Jun 17, 2019 at 3:18
  • \$\begingroup\$ @Oscar Skog, I have verified that the resistance between earth and the 0v output is now in the MOhms range... \$\endgroup\$
    – Beny Benz
    Jun 17, 2019 at 9:02

1 Answer 1


Question: Does this modification disable safety features?

NO. Not if done exactly as stated.

It enables a status resistor loop to enable the supply and only removes the secondary earth bond and not the primary AC safety ground to enable series DC connection of 12+12=24V @ 75A (850W at 120Vac, 1200W out at 230Vac rating high efficiency) making the two supplies unique with an unmodified one = Earth gnd = V- = 0V and modified one 12Vout to V- to 24V=V+. with AWG 8 or heavier cable. You can choose an external fuse for the charger cables you select if undersized. ( e.g. or use welding cable)

I understand all the safety requirements and fundamentals of this category of designs and none of the safety features are bypassed or disabled in the article link below. However, any modifications are not permitted for resale, but ok for personal use.

I assume this is the one you have read, (because it is written by a professional and popular with hobbyists. )

But if you make any mistakes with your fingers touching charged HVDC contact points inside, this is your doing. -48V PSU's are normally designed with an earth ground jumper option for V+ or V- earthing. But for Server communication V- is always earth grounded for noise immunity. Short, #8 or bigger solid or stranded welding wire for interconnect cables are necessary to prevent short circuit wire fire. Your application load needs some kind of soft start switch to prevent overcurrent trips then a bypass series load for high current. How you regulate your battery charger with a BMS is outside the scope of this question.

DO NOT attempt anything described herein without ensuring that you have taken all the necessary precautions to safeguard yourself and others from injury.

  • 1
    \$\begingroup\$ Whoever downvoted , please show some courtesy to explain why \$\endgroup\$ Jun 17, 2019 at 3:04

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