Voltage controlled voltage source

Discussion in 'The Projects Forum' started by mawright89, Jul 3, 2012.

  1. mawright89

    Thread Starter New Member

    Jun 18, 2012
    19
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    A little out of my depths here...

    Need a DC circuit which when a 0-5v input (<200mA) signal is applied gives a voltage out of 0 - 12V (0-3A), is fairly stable and reasonably linear.

    Thanks for looking

    Matt
     
  2. mawright89

    Thread Starter New Member

    Jun 18, 2012
    19
    0
    should specify;

    a variable input giving a variable output. Will be going to electromagnets.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    You could use a rail-rail op amp with a gain of 2.4 driving an NPN darlington power transistor connected as an emitter follower powered from a 14V supply. The transistor will need a good heat sink since it will be dissipating a maximum of about 42W.
     
  4. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    Such a device is commonly called an amplifier.
     
  5. mawright89

    Thread Starter New Member

    Jun 18, 2012
    19
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    Thanks for that, do I need to use a capacitor in series on the load resistor, and will pullling the - input on the op amp to ground and one of the diferential inputs be ok. Most show as +/- and ground.

    Are there any standard parts (transistors/opamp chips etc..) which I should look for?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    From what you've said so far, you can use a single supply with no "-" side, just "+" and ground. As mentioned, you'll want an op-amp that can sense all the way to the ground rail. This will allow an input down to 0V to still give a controlled output voltage. I believe you could use an op-amp such as LM358 that does NOT include the positive rail, though. It's an old and slow op-amp, but cheap and easy to find.

    I have no idea what you're talking about with the capacitor and resistor. I suggest you post a schematic.

    You'll need a power supply for all of this. You haven't mentioned what you plan to use. An old computer PSU would do it.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    He wants high current, thus an LM358 would not be a good choice, since, to minimize power dissipation, you want the op amp output to go as close to the supply rail as possible. An op amp with both input and output rail-to-rail operation would be a better choice with an emitter-follower driver.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    I was missing that part of the plan. You're saying that, by using an op-amp that goes to the rail, he can achieve 12V from a supply with only 14V, whereas if it goes to rail-1.5 like the LM358, he'd need a 15.5V supply and therefore would be burning off that much more power all the time. Makes sense.
     
  9. mawright89

    Thread Starter New Member

    Jun 18, 2012
    19
    0
    and now im lost... could one of you possible post a little scematic for me to copy?
    Cheers
     
  10. markbool

    New Member

    Jul 5, 2012
    2
    1
    i think this device is known as Amplifier.
     
  11. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,056
    Go back to post #3 as your guide. Use the references on this site or Google op-amp circuits. You want a non-inverting amplifier configuration. Then learn what an emitter-follower transistor configuration is. You'll use your op-amp to drive the emitter-follower.

    Draw up your findings and post them here. You'll get plenty of help.
     
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