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Thread: Modify Sproggy for 12V 5A out?

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    Low Bitrate InvisiBill's Avatar
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    Question Modify Sproggy for 12V 5A out?

    Looks like some of the new Mini-ITX cases for the Eden boards have an internal DC-DC power supply, and an external AC adapter. Would it be possible to modify the Sproggy to supply 12V 5A for the internal DC-DC supply?

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    FLAC mp3z24's Avatar
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    you can make a simple circuit from 3 MAX1771 from maxim. it you put them in paralell with a common input from the battery, common output to the computer power supply, and a common ground, you will be able to supply 12v at up to 6A... if you want more design margin... just add another circuit in paralell and you will have 8A.

    if you look at the data sheet you will see sample circuits with parts listed... use the bootstrapped mode as it will give you higher output current (2A per chip) and will require higher input current, but with a car electrical system that wont be a problem.

    thats about as simple as i can see it... i couldnt find anything at 12v out with higher than 2A output (running on ~12V in)

    if im wrong... someone please correct me!!!! im far from an expert on electronics design...

    ~Mike
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    Low Bitrate InvisiBill's Avatar
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    Got a direct link to the datasheet? I didn't see anything on their page with that IC number...

    Like some others, I can put the stuff together and I have a basic understanding of how most components work, but that's about it. I'm assuming that for this simple circuit you mention, I'd need 12V (or more) input to keep the output at 12V. I'd like to have a design like Mk2.5, where I could run it off the battery too. I'm thinking I could use the bottom half of http://sproggy.mp3car.com/psumk25/MK26.jpg minus the -5V circuit to get just 12V out of it. Could I combine that with your idea to get it up over the 1.3A that the Sproggy speced at?

    Also, it's been a while since I've done electronics stuff. Will a higher amp rating on the PSU hurt the device, or is it just the output capacity of the PSU, and the device will "pull" however much it needs? Anyone got links to good sites with refresher material I could take a look at?

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    FLAC mp3z24's Avatar
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    you can type in MAX1771 at the top left of the first screen... (for future reference)

    here is the datasheet

    this thing can run from 2-16v on the input and still give you 12v output. you might want to incorporate part of the relay switch circuit from the sproggy so you can control when it comes on. i never really took the time to study the sproggy and learn all that is happening... especially on the input. so i am a pretty poor resource for that stuff. rob or rob whithey (probably spelled that wrong!) are damn good for circuit design (i aspire to be like them ).

    but like i said, the basic circuit components are in the data sheet and from that circuit you should be able to put something together, even if you use prototyping boards, you should be fine! so perhaps one of the rob's can shed some light on this a little later in the day.

    ~mike
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    Talking Re: Modify Sproggy for 12V 5A out?

    Originally posted by InvisiBill
    Looks like some of the new Mini-ITX cases for the Eden boards have an internal DC-DC power supply, and an external AC adapter. Would it be possible to modify the Sproggy to supply 12V 5A for the internal DC-DC supply?
    Do you have a URL to the case for the mini-itx containing a dc-dc PS?
    My search is turning tons of link most of them irrelevant.

    Thanks

  6. #6
    FLAC
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    here's the theread with the links for the mini-ITX cases with built in DC-DC PSU

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    Maximum Bitrate Raas's Avatar
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    I could be wrong, but I thought that MAX1771 Chips need an +5vdc input in order to produce 2 amp per chip....

    The datasheet says in also accepts inputs higher than the desired voltage, but the chip won't be that powerfull..

    So one would need 1 max 787 to step down the car-voltage to +5vdc and use this to power the MAX1771

    I know Dima has used a similar setup to acquire +12vdc on his supply (something like 4 amp)

    Just my 2 cents...

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    I'm using 5V (from MAX787) as input for MAX1771, bootstrapped.This setup gives me about 1.5-1.8A for each MAX1771, peak current is about 2.5A,but I changed a little the circuit that was suggested on datasheet.I used 20mOhm resistor (you can get more current if you use smaller value - but be careful) instead of 40mOhm,my FET is MTP3055 (3A),coil is 10uH (20 uH also works fine),2200uF capacitor.
    I would rather use flyback regulator as Sproggy did,it seems to be more efficient,but i know that is kinda hard to find all the components,so MAX1771 is may be pretty good substitute.

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    You may want to look at the LM2677 from national semiconductor. It is a simple switcher with high efficiency.
    input : 8 to 40V
    output current: 5A
    they have it in different fixed output voltage (3.3, 5, 12V and an adjustable output).

    it also require minimum parts.

    I had printed out the spec but did not bookmark the link.

    I have not tried it. I was just looking around for what was available.

  10. #10
    Low Bitrate InvisiBill's Avatar
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    http://www.national.com/pf/LM/LM2677.html
    http://www.national.com/ads-cgi/view.../LM/LM2677.pdf

    Now you smart people can figure out how to make it work. =)

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