I need a 42v DC adapter for charging a battery pack. I have this 54v DC brick from a broken PoE switch and want to change the output voltage.

I've taken it apart and tried to locate the voltage divider that sets the reference, but can't identify the controller and don't feel safe poking around 220V AC (and 54V DC).

The powerbrick is a FSP Group FSP090-DMBC1.

The two yellow/green spots on the are connected with piece of PCB shield. Down in the left corner there's a diode bridge with part number FEP16GT.

The IC looks like it ends with 75ST.


Closeup of IC, contrast adjusted for legibility.

  • \$\begingroup\$ You are looking from the wrong place. Most likely output voltage is set by resistors near U4 which could be a TL431. Also, how do you intend to charge a battery with a power supply? You can't connect a power supply directly to a battery, you need a charger, specifically suited to charging the batteries according to their chemistry. For your safety, I hope your battery is not a 10S lithium with 42V max voltage and I hope you did not plan to connect a 42V power supply directly to a 10S lithium battery. \$\endgroup\$ – Justme Apr 19 at 9:12
  • 1
    \$\begingroup\$ The battery pack is connected to a charge controller. The voltage maxes out at 42v and should have over-voltage protection, but I don't want to risk it. I think you are right about U4. R37 and R38 are 620k and 29.4k which gives a voltage of ~2.5 with an input of 54v \$\endgroup\$ – Jes Apr 19 at 9:19
  • \$\begingroup\$ By charge controller, do you mean a BMS? \$\endgroup\$ – Justme Apr 19 at 9:25
  • \$\begingroup\$ Yes I mean a BMS. It's an e-bike battery built by a battery refurbishing shop. The original charger was a no-name chinese brick that did not last very long. \$\endgroup\$ – Jes Apr 19 at 10:16
  • \$\begingroup\$ A BMS is not a charger either. You need a charger. You cannot connect a power supply to a battery with a BMS. It will not charge the battery. Battery will load down the powet supply output voltage and power supply will try to lift it back to the setpoint. No matter many amps the power supply puts out, it can't pull the voltage higher than battery voltage. Power supply will hit a current limit and shut down. It will not work without a charger between the power supply and battery pack. Do not damage the batteries by connecting a power supply. Damaged batteries can explode or start a fire. \$\endgroup\$ – Justme Apr 19 at 10:25

The output voltage can be modified by changing the resistances around the TL431.

Unfortunately what you are trying to do with it will not work.

Changing the output voltage of a power supply still does not make it a lithium battery charger.

It is still a power supply and power supplies are not lithium battery chargers.


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