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Thread: Dc to dc

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    Dc to dc

    Hello
    i have a electric Nike with a 48v battery and i was thinking to build an electronuc device to convert the 48v to several output, as an example a 12vdc to charge aa batteries and a 5vdc (USB) to' charge the iPod
    is there an easy way or something already built outside?
    Thanks

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    Seņior Member ItsJustMe's Avatar
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    A friend of mine runs several things including a pair of MagicShine lightheads off his eBike's 48 volt pack. He built the regulators himself using National Semiconductors "Simple Switcher" chips. They have a website where you can type in the input and output voltage, required current, amount of board space you will allow it to take up, and the desired efficiency, and it'll pretty much design the thing for you.
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    Senior Member Road Fan's Avatar
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    Quote Originally Posted by ItsJustMe View Post
    A friend of mine runs several things including a pair of MagicShine lightheads off his eBike's 48 volt pack. He built the regulators himself using National Semiconductors "Simple Switcher" chips. They have a website where you can type in the input and output voltage, required current, amount of board space you will allow it to take up, and the desired efficiency, and it'll pretty much design the thing for you.
    If you're going to build something, this is probably the easiest approach. I wonder if Mouser and some of the other DIY dealers sell modules that are already assembled on substrates, that you can just connect together in a box, to make a centralized multi-output converter?

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    They do, look up the powertrends stuff. $$$ though. A simple three terminal LM340t-5 and -12 wold be fine.

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    Senior Member Pig_Chaser's Avatar
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    Quote Originally Posted by enine View Post
    They do, look up the powertrends stuff. $$$ though. A simple three terminal LM340t-5 and -12 wold be fine.
    Careful with linear regulators in this application. 48V down to 5V is a 43V drop, even a small current will produce a significant amount of heat (and wasted power). There are switching modules designed as drop in replacements for linear regulators: V78XX-1000 Series Very ez to use, but not cheap. Although if you look at the parts to do it yourself using a simple switcher... it probably all works out in the wash especially if you don't have access to free components.

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    Seņior Member ItsJustMe's Avatar
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    Yes, you do NOT want to use a linear regulator for this application. You'll be throwing away almost all of the power used, and many linear regulators can't even handle this level of drop. If you want 500 mA charge current at 5 volts off a 48 volt feed, the linear regulator would have to be able to dump (48-5)*500mA = 21.5 watts of heat. That's a WHOLE LOT of heat. You're talking a fist sized heatsink getting too hot to touch. And the efficiency at best can't be better than 5/48= 10%. 90% of the energy will just be dumped as heat.

    I doubt most off-the-shelf linear regulators could even dump this much heat without cooking themselves.

    A switcher wouldn't be that much harder to build, and could easily be 90% efficient. Buck regulators are dirt simple.

    Keep in mind though, if your USB device is "well mannered" it won't draw more than 50mA, and in fact will probably refuse to charge if you just put power on the USB. You need to have a chip on board to "talk" with the device and tell it it's OK to pull more current and begin to charge.

    The easiest and best thing to do might be to get a 12V to USB adaptor that you know works - it will already have the USB chip built in, and you can get these for $5. Then all you need to do is to get the voltage down to somewhere in the 8 to 15 volt range; those adaptors already have regulation built in and are designed to take a range of voltages from a car. THEY are not likely to be very efficient, but they'll be more like 40% efficient, which is a damn sight better than 10%.

    You'd still want to build a switcher to feed that device though, or you'd be introducing a 25% efficient part in before you even got to the final regulation stage.

    Besides, having a 90+% efficient regulator that can deliver a couple of amps at 12 volts from your battery will be useful for many other things.
    Last edited by ItsJustMe; 03-24-10 at 09:43 AM.
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    Sorry, my brain was thinking 12v source when I typed linear, scratch that idea.
    I was able to a few years ago get a sample of a powertrends, I was using their 8-18v in to get 12v out for a car computer.

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    Sorry, my brain was thinking 12v source when I typed linear, scratch that idea.
    I was able to a few years ago get a sample of a powertrends, I was using their 8-18v in to get 12v out for a car computer.

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    If I was designing this for work I'd use a push-pull or full-bridge buck driving a step-down transformer, regulate it, then use LDO linear regulators on the lower current windings.

    Might use a SEPIC instead of a buck, but the transformers are fully custom.

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