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Thread: What is the downside to a 50% voltage drop?

  1. #1
    Join Date
    Sep 2013
    Seattle, WA
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    Default What is the downside to a 50% voltage drop?

    Let's say I have a 24V 40A (960W) power supply like this one:

    Now also say I run power from there using 14 AWG copper, for 155 ft, and draw 15 amps over it. (This is extreme for the purpose of getting a 50% loss to make calculations below easier).

    The AWG calculator on tells me I will end up with a 12V voltage drop, and end up with 12V at the end of the line. Neat.

    My intent is to use a buck converter like this one:

    which takes anything from 11V to 30VDC and convert it to 5V/30A. I want to stick 2 of them in parallel inside a controller box so that they provide 300W (12.5A at 24VDC).

    Will this work?

    How much power will that power supply be consuming? Will the 300W circuit now consume 600W at the power supply's end due to the 50% voltage drop? If I'm ok with that happening, is there any other technical or safety reason for large but planned voltage drops to be an issue?
    Last edited by deonb; 10-05-2019 at 02:39 PM.
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  2. #2
    Join Date
    Dec 2012
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    Default Re: What is the downside to a 50% voltage drop?

    Just a couple thoughts:

    Will the 300W circuit now consume 600W at the power supply's end due to the 50% voltage drop?
    I think you understand, but I'd use different wording.... Your converters will consume what they consume. They don't consume extra because of the long run. However, the load on the original power supply will be higher. That long run of wire where the theoretical 12V drop occurs IS A LOAD. It's literally acting like a resistor, and that's why there's a voltage drop. So it's like you've got 2 loads in series.

    Will it work? ....well, I didn't click any of the links, but it probably will. I don't know if it's the smartest solution for your situation, but let's face it. MANY of us do some iffy stuff when it comes to xmas lighting.

    Other thoughts/concerns/tidbits I have:

    -The calculations are not exact they're based on ideal parameters (i.e. don't account for variation, or weather). Humidity can make a difference in long runs. Just something to think about.
    -At full output you'll have an estimated power loss of 480W (12*40=480, that's a big 'if' though. Cause that 12V drop might not be exact) between the power supply and the converters. That's a lot, but leaves plenty of room for the 300W your proposing. Are you putting anything else on that power supply? I don't think I would. Note: I don't math, I just include lots of padding/cushion in the setup
    -Whatever calculation thing you're using, make sure it's for stranded wire. The wire in your house is solid. Properties are a little different.

    Biggest problem I see is that's your converters need a minimum of 11V. You gotta be positive you're getting them that voltage or weird stuff is gonna happen to your lights.

    But hell, try it out. Just remember the fuses.
    2012 - 12 Channels and an Arduino

  3. #3
    Join Date
    Nov 2009
    Brisbane - Australia
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    Default Re: What is the downside to a 50% voltage drop?

    Excessive voltage drop in the power feed to the buck converters can cause (sometimes wild) instability. You should still be aiming for minimal voltage drop.

    The current on the (higher voltage) input side of the converter will be lower which means you can use smaller cable, but I'd like to see no more than a 15% drop.

    Also, when you say you're going to run 2 converters in parallel, that's fine for their inputs, but don't link the outputs as they are not designed for that.


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