need help building a function generator

Discussion in 'The Projects Forum' started by mashimaro2k5, Sep 10, 2009.

  1. mashimaro2k5

    Thread Starter New Member

    Sep 10, 2009
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    I'm trying to build a function generator with a few limitations just for fun but I'm having a difficult time finding the right components. I built a function generator from this website:

    http://www.sparkfun.com/commerce/product_info.php?products_id=9002

    which was very easy to build but now I want to modify some things in it. On the website it says that it needs atleast 14V or so volts to run but I am pretty sure there are ways to power this device with less than 9 volts but I'm stuck and have no idea how to do so other than unsoddering some of the components and putting some new components in. If possible I would like to maintain an amplitude of 500 mA. Any help would be great Thank you.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    If you try to get 500mA from a 9v "transistor" battery it will be dead within a few minutes.

    The design uses a 741 opamp, which can't get closer to the rails than about 2v. It needs >= 10v to operate properly; with a 10v supply it will only have an output voltage swing of 6v. They are supplying the 741 opamp with 12v from a 7812 regulator. A 7812 regulator has a voltage drop of 2v, which explains why you need 14v to power the circuit.

    If you want a function generator that will run on a lower voltage, you will have to re-design it using (for starters) a different opamp that is of a rail-to-rail type.

    When the 741 was first introduced some 40+ years ago, it was the greatest thing since sliced bread. It was not very long that other designs were introduced that far surpassed it in performance. It's still used in academia and industry because it's so cheap, and it actually works OK for some low-speed applications.

    But if it were an automobile, it would be a Model A Ford with a hole in the muffler, ripped upholstery, and a busted headlight.
     
  3. SgtWookie

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    Jul 17, 2007
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    Here's a different kit that's designed to run from a 9v battery:
    http://www.electronickits.com/kit/complete/meas/ck102.htm

    It uses an LM348, which is a quad 741 opamp, so its' performance is pretty dismal.

    However, the regulators have been dispensed with so you don't need as much voltage to power the thing. The sine wave output is quite poor. The square wave output has a slow rise/fall time, and the triangle wave output - well, it certainly could stand improvement.

    However, it does work, within it's limited range.
     
  4. mashimaro2k5

    Thread Starter New Member

    Sep 10, 2009
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    cool alright. thanks for the help. i was also wondering if it's possible to make a dc to dc power converter for this would that work as well or not?
     
  5. SgtWookie

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    Jul 17, 2007
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    You mean so that you can power it from a 9v battery?
    I suppose that could be done, but you would have very short battery life, and those batteries aren't cheap.

    I don't know how much current your function generator kit draws when powered by 14v, but let's just make a ballpark stab at it and say 30mA. The two regulators alone will use 5mA each, so that's not unreasonable.

    DC-DC converters can be pretty efficient. Let's just say you built a good converter and were able to get 92% efficiency out of it.
    To get 14v at 30mA from a 9v supply, you're going to need more current from the 9v supply to keep up the higher voltage at that current. Unfortunately, 9v batteries have a lot of internal resistance, and when a heavy load is put on them, their output voltage drops. They also have a limited mAh rating, somewhere around 150mAh. Manufacturers usually rate them by testing the battery voltage over a number of hours; a 9v battery might be tested with a 7.5mA constant current drain over 20 hours to arrive at the 150mAh rating. If you put a heavier drain on the battery, it's life will be much shorter.

    But let's proceed anyway.
    If your DC-DC converter were 100% efficient, to get 14v@30mA from a 9v supply you would need 14v/9v x 30mA = 46.7mA from the 9v battery.
    However, under such a load, your 9v battery will likely drop down to 8v or less.
    14v/8v x 30mA = 52.5mA
    But remember, the DC-DC conversion is less than 100% efficient - so we'll need more current.
    52.5mA / 92% = 57.1mA.
    Now we're really taxing that 9v battery. As that current passes through it, the battery heats up, which increases it's resistance, which increases the current needed to power the DC-DC converter. You might get 30 minutes' use out of it.
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    It would be a lot easier to put 2 9V batteries in series for a total of 18V. They would still have short battery lifes, and their voltage would quickly drop, but it would work.
     
  7. mashimaro2k5

    Thread Starter New Member

    Sep 10, 2009
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    i'm not really worried about battery life because i have an actual power supply. i knew that if i was going to use a smaller voltage that the battery would be eaten alive within the hour. i'm wondering what i should use to design a dc to dc power converter. it'll be the first time for me to build one of these
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    You could go on National Semiconductor's site and use their Webbench tool.

    ST Microelectronics has recently put switching supply type tools on their site as well.

    Linear Technology has their LTSpice/SwitcherCad tool available for a free download.

    Google is your friend.
     
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