How to properly connect two 12V supplies in series

Discussion in 'Power Electronics' started by kmpres, May 15, 2017.

  1. kmpres

    Thread Starter New Member

    Jan 24, 2016
    Hi Guys,

    I have two HP server power supplies rated at 12V/75A at 115V, or 12V/100A at 240V that I have been using in series as input to a boost style RC battery charger. I was careful to isolate the DC ground in one of them and they have been working fine together charging my 36V e-bike battery pack at 6A with no sparks or shocks at all.

    Now I would like to use them to build a lab style bench power supply using two of these 50V/15A Chinese-made Programmable Power Supply Modules As I understand it, real bench supplies typically have two or sometimes three separate built-in supplies wired in a master-slave arrangement where the master is configured for CV and the slave(s) are configured for CC. My question is, other than the series output connections, are there any other connections between the two supplies that should be made? For example, is it necessary to design a circuit to power off both supplies in the event that one should fail or drop its voltage below a set point in order to protect the device to which they are attached? I'm told that boost chargers typically don't like sudden drops in input voltage.

    Also how should I fuse them given that the control modules can only deliver 15 amps but the supplies are capable of much higher currents? I would like to make this device as safe as possible.

  2. DickCappels


    Aug 21, 2008
    Make sure to isolate the DC return as you did on your charger setup and make sure that either the leakage current to ground from exposed metal parts and your outputs is safely low. See Table 4 on page 22,, pages 21, 22 and 23 of the document at the link below, paying special attention to Figure 17 on page 22 which shows you how to measure leakage with a voltmeter, a 1.5k resistor, and 0.15 uf capacitor.

    When I put two power supplies in series I usually remember to put big diodes across the outputs of each supply to protect the outputs from seeing reverse voltage so that in the case of one outputs accidentally being grounded to called upon to charge a large capacitance.
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    As long as they have separate DC primary input voltages, it would be ok to put them in series, looks like they monitor the Negative output for current limit.,( so maybe the positive is fed straight through.)

    As for fusing, i would go with a DC breaker used in solar panels rated for 30-45 amp.