Tight Space circuit help

Discussion in 'The Projects Forum' started by Chills24, Dec 2, 2008.

  1. Chills24

    Thread Starter Member

    Nov 10, 2008
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    I have a 12 to 14V DC power source I need to reduce to 3V DC @ 675mA but would like to have the head room for up to 1.5 amps. So I planned on using a basic zener diode voltage regulator, with a 10W 16ohm resistor to provide the voltage drop with a 3v zener in parallel with my load. The board I'm attempting to create needs to is 1.75" by 2", and the big limitation is between this board and the next I have just .25". So the resistor I was looking at getting is to tall. I thought of doing some sort of resistor network to reduce the size, but then it takes up valuable board space. I also was hoping to fit an led on the board, so space is tight. Any ideas on how to regulate this, or incorperate a resistor that is some what small that can handle the power?:eek:
     
  2. beenthere

    Retired Moderator

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  3. steveb

    Senior Member

    Jul 3, 2008
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    Especially at 3V!

    This seems like a perfect application of that LM317 regulator that has been discussed in several recent posts. It will be important to provide adequate heat sinking since the power dissipation on the regulator will be 11V time 1.5A, or about 17W, worst case.

    If you don't want to waste power, a 12V to 3V switching converter (as suggested above) can be used, but this may be noisier and may take more space.
     
    Last edited: Dec 2, 2008
  4. Chills24

    Thread Starter Member

    Nov 10, 2008
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    This is actually hilarious. When I first started the project I started a board on here and wanted to start it with a LM317 as my voltage regulator. I was told it was to inefficient and at that low of power and voltage to use the zener setup. I'm really glad both of you shot down the zener thing because I was not happy with the results I was getting from it. So I think I will look into the LM317 and the switching convertor. Could some one give me a quick run down on some pro's or cons of either both? steveb you mentioned noise, like the hum of the circuit, or what kind of noise? Also what types of heat sink can I get in there? Just for clarity if it helps its going to be running a motor speradically, like rapid pulses of .5 to 1 second but should be able to handle a solid 20 to 30 minutes of solid operation. Can both these handle the constant on and off?
     
  5. SgtWookie

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    Jul 17, 2007
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  6. steveb

    Senior Member

    Jul 3, 2008
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    The LM317 is inefficient, but so is the zener. Basically any type of linear regulation is inefficient if the difference between the main voltage and the regulated voltage is high compared to the regulated voltage, which is true in your case (11V compared to 3V !) The switching regulator has the advantage of good efficiency (maybe 90%) compared with about 25% with a linear regulator in your case.

    The noise tends to be high frequency in a switching converter. Nowadays it's good to use a higher switching frequency 50kHz to 1 MHz for example. These frequencies are too high to be heard as hum.

    For heat sinking, I'm not sure, and someone else can probably give you better guidance. I'm pretty sure that 17W requires at least a simple small air cooled fin type heat sink.
     
  7. SgtWookie

    Expert

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    You need a "buck" regulator.

    An LM317 is really old technology that is still widely used today, and may be appropriate in some circumstances - when extreme simplicity is desired over concern for efficiency.

    Have a look at the two links I posted. They're not as efficient as larger DC-DC converters, but they're extremely easy to use, far more efficient than LM317 regulators or Zener diodes, and just might meet your space budget.
     
  8. mik3

    Senior Member

    Feb 4, 2008
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    If the starting current of the motor is greater than 1.5 Amps the LM317 is not going to work. It will sense this excess current and go into the current limiting region. If the regulator is just to drive a motor use a single power transistor. Connect its collector to 12V and the motor between the emitter and ground. Then use the LM317 to drive the base of the transistor with a voltage of 3.7V. The voltage across the motor will be approximately 3V due to the 0.7V drop across the base-emitter junction.
     
  9. Chills24

    Thread Starter Member

    Nov 10, 2008
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    Yeah sgt wookie those look like great options but would definitely be to tall. If I bent the leads down and used it horizontally and freed up some space I might be able to use it. So I look into seeing if I can free up some space. I'm going to look into that switching regulator, seems like it might work out for me. If not I don't forsee efficiency being a huge concern and might just go with the LM317.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If you don't have efficiency, then you will either have to have a large heat sink, or your regulator will either shut down due to thermal overload, or will self-destruct.

    There are other DC-DC converters on the market in various voltage and current ratings; some pre-packaged and some in IC form. I merely presented you with a couple of ready-made solutions with a small footprint.

    You'll have to do some research on your own. This forum is very valuable in helping with ideas, but ultimately the project success or failure is your responsibility.
     
  11. Chills24

    Thread Starter Member

    Nov 10, 2008
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    Wookie I wasn't saying you had bad ideas, I just have a tight fit. I'm sorry if it rubbed you the wrong way. I appreciate your help and I understand that my project is my responsibility.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    Chills24, I'm not "steamed" at you, and didn't know I was coming off like that.

    My whole point is that you're either going to need space for a more efficient "buck" regulator or space for a big heat sink. You might consider locating your regulator circuit off the main board somewhere, and just use a couple of caps to suppress transients.

    Another possibility for you might be an LM2675 "Simple Switcher" from National Semiconductor. It requires 5 external components; three caps, a Schottky diode and an inductor.

    You might check Traco Power's website for compact DC-DC converter units.
    http://www.tracopower.com/
     
  13. Chills24

    Thread Starter Member

    Nov 10, 2008
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    I'm thinking of going with one of TI's swift regulator eval boards. TPS5430EVM-173. You guys heard of any problems with these? I realize I'll have to replace R2 to bring down the voltage to the 3V I want. I can't find out how tall the parts are off this chip but I think this would be a good option. Its relatively cheap and compact and can handle 3A. Anybody have experiance with these?
     
  14. Chills24

    Thread Starter Member

    Nov 10, 2008
    17
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    I'm going to research those traco parts some more also those look like a good size, will get the job done and I can place the on my own board, not fill a slot and have jumpers like I would with the TI.
     
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