12vdc to 6vdc

Discussion in 'General Electronics Chat' started by bsfrye, Feb 28, 2009.

  1. bsfrye

    Thread Starter Member

    Feb 20, 2009
    23
    0
    What is the easiest way to convert 12 vdc to 6 vdc. I need it to handle 5 amps continuous power. I am currently using a LM338 which IS rated to 5 amps. It just is getting REALLY hot. To the point where it shut down. I put a heat sink on it and that made a big difference. Am I just going to have to live with the heat? Should I use a bigger heat sink? I would really like to get something that didn't generate as much heat as this. Any Ideas?
    Brandon
     
    Last edited: Feb 28, 2009
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Linear regulators are the most simple way to provide a constant regulated voltage or current. They are also notoriously inefficient.

    If your output is 6v @ 5A from a 12v source, then the regulator also has to dissipate 6v x 5A = 30 Watts of power. That's a lot of heat. If you're using the circuit in an automotive application, your input voltage will be more like 14v, so you're actually dissipating 8v x 5A = 40 Watts of power in the regulator. That much heat will be tough to dissipate for a TO-220 package device.

    If you want to eliminate most of the heat and greatly increase the efficiency, you will need to go to a switching-type regulator.

    Check out National Semiconductor's web page for the LM138/LM338; look at the "Also Recommended" IC's.
    http://www.national.com/mpf/LM/LM338.html
    LM2677 - No Heatsink Required, On/off Pin
    LM2678 - No Heatsink Required
    LM2679 - No Heatsink Required, Adjustable Current Limit

    Their "webbench" software will even design it for you, and tell you what components you need to buy.
     
    Last edited: Feb 28, 2009
  3. bsfrye

    Thread Starter Member

    Feb 20, 2009
    23
    0
    SgtWookie - thanks so much for the input - I really appreciate it. The only problem there is price and complexity. I am a total noob at this stuff and I need to keep the price as low as possible.

    What about something like this? What is your opinion on this? Basically I need something that is really, really easy and inexpensive. Space is not an issue, so I can make it big if that makes thing any better...

    I am looking specifically at the section where he is talking about the 7806 with a transistor

    http://www.public.iastate.edu/~lhealey/vespa/regulator/
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Price is almost always an issue.

    He wasn't talking about a 7806 with a transistor; he was talking about a 7808 with a transistor, which would output 7.3v.

    You already have an LM338 regulator, which is rated for 5A. If you want better cooling for it, then use a copper heat sink that has a fan, like a CPU cooler (drill a hole for the mounting screw) - and use heat sink compound.
     
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    In the old days we used a dropping resistor. 5 amp at 6 volts about a 1.2 ohm resistor.
    Use a .7 ohm resistor or so and put it before the voltage regulator. Most of heat goes into the resistor. Get a little fan from an old computer and blow on it. Now you have a space heater and a 6 v power source. Make sure the resistor is good for at least 30 watts, maybe some sort of 6 v 6 amp light bulb, nicrome wire from an old toaster.

    The series resistor is a really simple and inefficient way of reducing heat in a regulator if it is know that the input voltage is too high.
     
  6. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    If you elect to use a resistor to drop a portion of the input voltage prior to the regulator then you will need size the resistor to only drop 4 volts at the maximum current since the 7806 needs a couple of volts at least maintain regulation. You will probably find it easier to use several resistors in parallel to spread the power dissipation across more than one resistor.

    hgmjr
     
  7. bsfrye

    Thread Starter Member

    Feb 20, 2009
    23
    0
    Thanks for all the help - I will try a giant resistor to take up some of the heat and see how that goes. Thanks again!
    Brandon
     
  8. bsfrye

    Thread Starter Member

    Feb 20, 2009
    23
    0
    Current I am using an adjustable regulator. If I use two of them - one to drop to 9 and the second to drop to 6 - will the heat dissipation be split between them? Or am I missing something?
     
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