Multiple DC source feeds to a single load

Discussion in 'General Electronics Chat' started by jlester14, Aug 12, 2014.

  1. jlester14

    Thread Starter New Member

    Aug 12, 2014

    Let me start by saying I am new to the forum and joined for others expertise on a rather urgent topic I have for work.

    I have a scenario where I am DC powering a load (-48 VDC power supply). The load draws 250 Watts at nominal operation. In order to get the power draw on the line under 100 Watts (NEC code) the OEM said to tie 3 lines (-48 VDC each) to the "single" load terminal. Is the voltage/current/power draw able to differentiate independent lines? i.e. would each line then draw 83.3 watts per line?

    It seems to me like all it has done is created redundancy and not evenly split the power across all three feeds.

    I have attached an illustration for example. Any feedback is greatly appreciated.

  2. BobTPH

    Active Member

    Jun 5, 2013
    I am not clear on what you are trying to do. Are you trying to use 3 separate power supplies to supply power to one device? Or do you have one power supply and 3 sets of wires going to the device?

  3. jlester14

    Thread Starter New Member

    Aug 12, 2014
    sorry for confusion.

    Yes, 3 "separate" power supplies. They are technically 3 separate fused positions off of a DC power plant to feed 1 device with a single power input/connector...
  4. crutschow


    Mar 14, 2008
    The power supplies will not share the load equally because of the small, unavoidable differences in output voltage among the supplies. Typically to insure that the power supplies more equally share the load a small resistor in places in series with each output. A resistor value that will drop a few tenths of a volt at the operating current for each supply is a good value to start with, depending upon the unloaded initial difference in output voltages.

    If the output voltages are adjustable, then adjust them initially to as close as possible to the same voltage. An easy way to accurately check that that is to use a voltmeter to measure the difference voltage between the outputs, not the output voltage to ground.
  5. jlester14

    Thread Starter New Member

    Aug 12, 2014
    So with that said. Will all 3 lines draw power although they may not be equal in draw?
  6. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Typically the one with the highest voltage will deliver the most power up until it hits its current limit then its output voltage will drop and others will take up the slack. Some systems will increase the output impedance of the power supplies or use other methods to attempt to balance the load..
    Its very common in the telecom world (-48VDC) to have power supply systems with tons of rectifiers in parallel but many also feature fairly advances controllers.

    Since the OEM recommended this and assuming you didn't just talk to a "sales person" over there.. I'd assume they have the required protection built in to safely be paralleled.

    Other times you might use Diode OR'ing.. which can be accomplished with low vf diodes (schottky) or mosfets and ORing controllers.
  7. jlester14

    Thread Starter New Member

    Aug 12, 2014
    Thanks mcgyvr.

    We have kicked it back to the OEM to verify that this is in fact their "recommended" solution in this scenario
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    What equipment is the OEM delivering? The supply or the load?

    Redundant supplies such as this seems a way to back into a code class for low voltage/low power wiring, which is fairly open to applications. Things like outdoor lighting systems that observe that limit need not even be grounded.

    Once you get over that limit you need three wire grounded systems, enclosed cable runs, rated cables, DC rated switched and breakers... can be a much more of a mess then using 3 supplies.
  9. profbuxton


    Feb 21, 2014
    There are power supplies/Battery chargers which have a load sharing connection via a RJ45 cable. They allow up to 6 units to be paralleled and loadshare.