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Help on Circuit for providing usable DC power, from solar batteryarray

  1. May 9, 2014 #1
    Hi everyone,

    I have not designed a circuit for, well, decades now, so I’m hoping you can provide me with a bit of guidance.

    I have a solar power system with a 48V battery array. (System also includes an inverter [AC->DC + DC->AC], backup generator, solar panels, charge controller, etc..)
    I have many DC appliances (network equipment mostly) that require various DC voltages for a power source (but luckily always less than 48V).
    These devices are CURRENTLY being powered by using their own AC power supplies. This power is provided by the rest of the solar power system, using a wasteful DC->AC->DC process.

    I would like to create a circuit that will take the battery input and provide an array of DC output’s at specific voltages. (Meaning a few outputs at 5v, a few at 12v, etc…)

    Reality Issues:
    -The total load powered by this circuit, will not be known at the time of design.
    -The battery array’s ACTUAL voltage varies over time, as the rest of the solar power system charges (panels & generator) and discharges (uses) it.
    -Regardless of the variables above, the output voltages must remain constant.
    -POWER! I’m on an off-grid solar power system: so I need to use minimal power. In fact, the primary purpose of this circuit is to save power. (avoid DC->AC->DC)

    Initial thoughts & questions:
    -I was thinking a combination of: a voltage-divider to generate the desired output voltage, and (an op-amp based) voltage-follower circuit to isolate the voltage-divider from the load. The issue with this circuit is the variation of the battery voltage will still affect the output of the voltage divider.
    -So, how can I ensure that the voltage-divider circuit will get 48v input, even if the battery array voltage varies between 48v and 54v? Do I just use a series of diodes (such that their breakdown-voltages add up to 48v, or something just under that) in series with a resistor? Will this suck up power if I use a large resistor? Is there a better way?
    -Also, I’m not quite sure how to select which op-amps to use for the voltage-follower. They need to be able to supply a few amps of current, but also don’t need any frequency response. High current op-amps look pretty expensive, should I create the isolating voltage-follower using different components, like transistors (if so how)?
    -How do I provide the particular voltage needed to power the op-amp itself? Can I use the 48v regulated input? Use another voltage divider, exclusively for the op-amp power supply? (Don’t think these will work: load will vary.) Use the output of the circuit itself?
     
  2. jcsd
  3. May 9, 2014 #2

    nsaspook

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    Science Advisor

    The easiest way is to use efficient DC/DC converters. If your power needs are low (30W with Q48 model 18-75vdc input) per device then something like this would work.

    http://www.murata-ps.com/data/power/mdc_uei-30w.pdf

    If you search the web I'm sure you can find similar modules that will work with several outputs or higher power ratings.
     
    Last edited: May 9, 2014
  4. May 9, 2014 #3
    Thanks for the suggestion NSAspook.
    Alas, that device costs ($46) about twice what I was hoping to spend on this circuit. It also looks like I would need to get a separate device for each output voltage I need. However, simply giving me the term "DC/DC converter" is VERY HELPFUL! I'm looking at the various commercially available products now.

    Though I will probably end up with one of those commercial devices, I would still love to make a project of this circuit, for learning purposes.
     
  5. May 10, 2014 #4

    nsaspook

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    Science Advisor

    Glad to help.

    You should be able to find good products at about $25 per module. Designing a system with all the other needed parts for wiring, switching and circuit protection (fusing) will still be a good project. The last thing you need is to smoke a expensive piece of equipment with poor voltage regulation from a DIY designed regulator.
     
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