48v@40A -> 12V @40A conversion

Discussion in 'The Projects Forum' started by wman88, Apr 5, 2009.

  1. wman88

    Thread Starter New Member

    Apr 5, 2009
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    0
    I'm working on a project and need to convert 48V DC @ 40A to 12V DC with the highest amperage conservation possible. I know conversion is a lossy process, but I need the power! :) Would switching be best or a voltage regulator chip...I can get a +-5V chip from Radioshack...do they make a +-24v???? Lol HELP MEEEEEEEEEEEE! What do I do? Pref. I'd want to DIY.
     
  2. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    No problem, just use a .9 ohm dropping resistor, that will drop 36 volts at 40amps flow. Oh by the way, the resistor wattage rating will have to be better then 1,440 watts. ;)

    Lefty
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    1,988.5 feet of AWG 6 copper wire would do it. At around $0.90/foot, that would be a tad pricey though. :eek: Send a big truck to pick it up.

    You wouldn't want to use a linear regulator, even if there were one available for that amount of power. A linear regulator would have to dissipate that 1,440 Watts across itself. If it could survive, it would make a great room heater.

    Offhand, I'd say that your solution would be something like a synchronous buck regulator, but it won't be easy to build.

    Can you explain in more detail why you're wanting to do this?
     
  4. wman88

    Thread Starter New Member

    Apr 5, 2009
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    I'm working on a school project that entails running several motors, cameras and other equipment that all run on 12V. But I will be provided 48V@40a, and want to convert it down to 12V, while keeping as much power as possible. I'd love to build it myself, and mistakenly went with a transformer,...and it only works for AC apparently. So yeah...need some help!
     
  5. leftyretro

    Active Member

    Nov 25, 2008
    394
    2
    A switch mode DC to DC power supply is what would be required for an efficient conversion. I would take some electronics education and experience to design and build such a supply. However you can purchase one.

    Lefty
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    If you go for a ready built switching power supply, there is an added bonus that the output amperage can be higher than the input current.

    Let assume the efficiency of the switching regulator is some 85%.

    Under this assumption, when your regulator is supplying a 12V 40A load, then it will only draw 11.8A from the 48V supply.

    Alternatively, another switching power supply can provide a maximum load current of 136A at 12V if the input current is 40A.

    You will need for the example above a 48V->12V 500W and 48V->12V 2000W rated switch mode regulator respectively.

    Unfortunately, those supplies don't come cheap.
     
  7. wman88

    Thread Starter New Member

    Apr 5, 2009
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    Yeah, I've looked up the pricing online, and they are NOT cheap, hence the DIY and plus, learning how to DIY is ftw. Looked up schematics on synchronous bucks, it seems reasonable enough to do...

    But- Are you saying its a 'can't be done-r'?
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    It is certainly not easy. I have found one using Google and the schematic looks reasonable enough. However, you will need to provide a separate 12V power for the controller IC, using perhaps the LM317HV.

    Don't be fool by the simplicity of the schematic, it is difficult to build and there are many hidden catches.

    Source: 48V to 12V 50A switching supply

    [​IMG]
     
  9. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    One of the catches you're probably going to have to build a coil yourself, using big hunken wire. The transistors have to be able to take the current, which isn't as big a deal as it sounds, like EBLC1388 said the current will actually drop some. Still, plan for 30A, which is significant (you want to overrate components to extend their life).

    A lot of it is off the shelf, some isn't, and you'll need to test it pretty throughly before connecting it to anything expensive. Think about 40 @ 12V, how would you provide a load to test with. My brother ran into something similar, his manager wanted to test a 1000W power supply. His solution was to run a length of wire between two posts back and forth about 10 times well spaced apart and make it glow. How he insulated it and how long it was I have no idea, but looking at that space heater at 500W the manager relented.

    Makes mine look piddling.
     
    Last edited: Apr 7, 2009
  10. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Good point by Bill. For this I offer the following picture to give you more information about the 10uH 50A inductor and why it is so hard to get one.

    [​IMG]
     
  11. wman88

    Thread Starter New Member

    Apr 5, 2009
    4
    0
    Thanks guys, I'm going to try it out and see if I can do it, will repost on problems/successes!
     
  12. brainiac

    New Member

    Oct 15, 2009
    1
    0
    wman88,
    There is another way of powering your 12 vdc devices from 48 vdc. No additional parts required! You said you had several cameras and small electric motors. Take four 12 vdc devices and connect them all in series, "+" to "-", and it will drop 48 volts across the four loads! If your lucky, your load that draws the most current won't burn up your other devices:eek:......
     
  13. Thav

    Member

    Oct 13, 2009
    82
    0
    I would guess this method is almost guaranteed to burn up your devices. Your motors will likely pull a lot more current than the cameras, but the current will HAVE to flow through the camera. They are in series, after all. That stands a good chance of frying the camera due to overcurrent or maybe there won't be enough current flowing for all loads so the motors may experience overvoltage.

    In short, do not attempt!
     
  14. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Just one question. I guess your power is labeled 48V@40A. But that does not mean you have to use 40A. Are you sure you need 40A for your 12V application? This is important for 40 to 12 volt application
     
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