Stuck: 40A@19v->40A@12v DC/DC converter or stepdown advice

Discussion in 'The Projects Forum' started by osims, Nov 11, 2012.

  1. osims

    Thread Starter New Member

    Nov 11, 2012
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    Hello all. Ill-advisedly I purchased the following setup and would like to see if it can be made to work without any further large investments.

    Solar panels provide 18.9v 40A DC. I have a 400W AC water pump connected to a 12v DC input inverter, 240v AC output at a max 800w. I wanted to avoid having a charge controller and battery in the system and thought I could connect the solar panels directly to the inverter. It won't go on because the inverter accepts a max input voltage of 17v.

    Is it feasible to use a resistive load in series with the panels to drop the voltage? Are there other possible low-cost solutions. Any advice will be highly appreciated. Thanks.
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    That's a horrible idea even if it was feasible since you are just burning up all the power your panels are creating.

    I can't advise on your situation because I don't know what you are trying convert into what.
     
  3. osims

    Thread Starter New Member

    Nov 11, 2012
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    0
    @bountyhunter - Thank you for your reply. To clarify: I'd like to power the ac water pump for a fountain. I only need it to run if there is sunshine. I am willing to sacrifice some power to heat. But if resistance in the circuit is such a bad idea, is there any other way of cheaply achieving my objective? Thanks again
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I am not a solar expert, but I have doubts about:

    as to whether that's a solid voltage source, or (as I have been told) the panel voltage is a function of the current being delivered. Need to know the input profile to design a power converter.
     
  5. crutschow

    Expert

    Mar 14, 2008
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    You could use a low-dropout type linear regulator but, if the output was set to say 16.9V, the dissipation would be 2V x 40A =80W which would require a good sized heat sink.

    The problem with a resistor, is that it wastes power, even when the solar panel output is less than maximum. Edit: Also a resistor only reduces voltage when carrying current. And if your inverter won't turn on at the high voltage then there's no current to create the voltage drop needed to get the inverter to start.

    2nd Edit: A simple way to reduce the voltage would to shade part of the array (of course that also reduces your maximum output).
     
    Last edited: Nov 13, 2012
  6. crutschow

    Expert

    Mar 14, 2008
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    Another thought. What is the panel output under load? If it's below 17V, then all you have to do is reduce the voltage until the inverter starts. After that the load of the inverter will keep the voltage low enough so the inverter keeps working.

    One way to temporarily reduce the voltage 2V or so is to put three 40A silicon diodes in series with the panel output. Then when the inverter starts you could bypass the diodes with a power relay or large MOSFET. You could use a time delay or voltage comparator to control the switch.

    Edit: And another thought. If the system is connected with the load when the sun comes up then the voltage may never get high enough to be a problem.
     
    Last edited: Nov 14, 2012
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