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voltage regulator help!

Discussion in 'Electronic Basics' started by [email protected], Aug 21, 2007.

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  1. Guest

    I'm needing a voltage regulator or DC-DC converter or something
    similar. I'm not too much of an electrical engineer so I need help
    figuring out WHAT I need and WHERE to get it. If a part like this is
    unavailable, I'll need some circuit design ideas to help me make one.
    Here's the deal...

    I'm needing to run a monster servo (Tone Seiko PS-050 if you've seen
    it) off a 12V battery. I know, I know, just run it off the 12volts -
    but I'm sticking to the manufacturer's recommendation of 8.4V max.
    Max
    current draw is expected in the 4 to 5 amps range.


    So that's it. All I need is something to convert the 12V to 8.4V and
    handle 5 amps. Something efficient would be nice (switching?) I
    thought this would take about five minutes to find online but I'm
    having a helluva time.


    Any help appreciated!
    Carsyn
     
  2. gearhead

    gearhead Guest

    This link has a couple of circuits that will allow you to use a three-
    terminal voltage regulator and use a few added components to increase
    the current capability.
    http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm
    The second and third entries of the webpage show a couple of ways to
    do it. The circuit with a single pnp pass transistor is the simpler.
    You can use a smaller resistor in the emitter of the pass transistor
    than the one shown, for a higher current limit.
     
  3. Mike

    Mike Guest

    This may be a good starting place.
    http://www.national.com/appinfo/power/switcher.html
    It doesn't get much simpler than the "Simple Switcher" if your goal is efficency.


    Mike


    "The scientist is possessed by the sense of universal
    causation...His religious feeling takes the form of
    rapturous amazement at the harmony of natural law,
    which reveals the intelligence of such superiority
    that, compared with it, systematic thinking and acting
    of human beings is an utterly insignificant reflection."
    Albert Einstein (theoretical physicist)
     
  4. Guest

    When faced with something like this, I often just use several dropping
    diodes in series sized to handle the current. This provides
    unregulated voltage, but it's close enough for servos. Each diode
    drops the voltage by about 0.7V, so you would would need five 5A
    diodes in series for this. Note that diode drop voltage varies
    between diode types, so you have to check the specs.

    BRW
     
  5. If you sincerely want a switching supply for this, and I can
    understand wanting that, you will probably need to use an IC. (You
    can do it using discrete parts, too, but I don't have the time to try
    that now.)

    In case a linear, discrete design may help, I've this example below
    which I simplified just a little bit from other existing designs
    posted recently in the group. It might be useful to put a current
    limit, but that would add a couple more parts and I'm assuming fewer
    parts is better for now.
    I haven't given C1 much thought, by the way. Also, I think Q2 is
    going to need to dissipate close to a watt. Which probably too high
    for a TO-92 or TO-18 device. Might be able to get away with it,
    especially if you can place a heat sink around the TO-18, but I'm open
    to suggestions as to alternatives. (I think there are heat sinks for
    TO-92, too, but I've never used one.)

    There also might be a better choice for Q1, that would have much
    higher beta at high currents, can dissipate the heat, and won't need
    Q2 to help boost things. That could save a part. Suggestions?

    Jon
     
  6. Guest

    I was suggested this in another forum and it sounds like a simple and
    cheap solution and I do love the KISS principle. A couple followup
    questions...

    1) Is there any heat concern? 17.5 watts max dissipation according to
    my math, should I use a heatsink somehow? How would I accomplish that?

    2) Do I just wire the three diodes in series inbetween the power
    source and the component on the positive lead? A multimeter should
    verify that its 8.4v alright?

    3) I may also do this to run a 6v radio receiver off the 12v battery.
    Same deal? 9 .7v diodes (of smaller current) will do the trick or some
    similar configuration thereof?

    Many thanks! I don't use Usenet often but it's where I go when I've
    exhausted all my other options cuz I always know I'll get the answers
    I'm looking for!

    Carsyn
     
  7. Guest

    1) There's no heat concern if you use diodes that are rated for the
    current you need. They are designed to dissipate the heat for their
    rated current without heat sinks.

    2) That's right, but you'll need about 5 diodes in series to get
    approximately 8.4 volts from a 12 volt source.

    3) That's right. But for a small load like a radio receiver, I just
    use a 7806 voltage regulator. Much easier than 7 diodes. When
    powering the radio from the same battery as the servo, it's a good
    idea to add some filtering. For the servo load you mentioned, I would
    run the battery through one diode, then the 7806. Put a 1000 uF cap
    and 0.1 uF cap in parallel on the input side of the 7806 (between V+
    and ground), and a 0.1 uF cap on the output side of the 7806 (between V
    + and ground). This will give your radio nice clean 6V power. The
    diode / capacitor combination gives you good noise filtering. Without
    the filtering, a big servo like you mentioned can hose your radio.

    BRW
     
  8. ehsjr

    ehsjr Guest

    See page 17 of the datasheet at:
    http://cache.national.com/ds/LM/LM117.pdf

    It shows a 5A regulator that should work for you.
    You can simplify it if you don't want adjustable
    current, like this:

    MJ4502
    +12 ---+------- -------------+
    | e\ /c |
    | --- [.2R] 5Watt
    | | |
    +--[33R]--+---[LM317]-----+---> Vout 8.41V
    | | |
    | +-[240R]-+
    |+ | |+
    [1uF] [1375R] [10uF]
    | | |
    Gnd ---+----------------+--------+---> Gnd


    You can make the 1375 ohms with resistors in series;
    the 240 is a standard value. Put the IC & xsistor on
    heat sinks.

    Ed
     
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