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power amplifier- pls help me

Discussion in 'General Electronics Discussion' started by nza0104, Dec 3, 2010.

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  1. nza0104

    nza0104

    4
    0
    Dec 3, 2010
    Hi!

    I'm currently trying to amplify the power so that the power from my digital to analog converter can turn/ drive my motor using LM675 as in the first page of this datasheet:
    http://www.national.com/ds/LM/LM675.pdf

    I just replaced the RL with my motor, but how can I:
    1. Set the gain of the power amplifier? Is it by changing the 20k resistor only?
    2. How can I know/ verify what is the power amplifier gain without having to measure the voltage & current?
    3. How to limit/reduce the amplified power so that it is not excessive or will not damage my motor?
    4. I'm trying to make the motor to turn in both cw and ccw directions, but after implementing this amplifier as in page 1 of the datasheet, it only turns cw. How can I make it turn in both direction? I have a dual power supply. Do I need to use the circuit as in page 8 "Servo Motor Control"? If yes, how can I set the amplifier gain? Is it by changing the 1M resistor only?

    Sorry if my questions sound stupid/ very basic.
    Thanks
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,301
    2,738
    Jan 21, 2010
    1) Changing the resistor in the feedback loop will alter the gain, but never to less than unity.

    Typically you set the gain to some suitable value and then use a pot as a variable potential divider to alter the amplitude of the incoming signal.

    Like this:

    [​IMG]

    2) in an op amp, the voltage gain is set by the feedback loop. In this case the 20K and 1K resistor give you a fixed gain of 20.

    3) The easiest way id to operate the amplifier from a supply that does not exceed the voltage rating for the motor.

    4) the motor should be connected between the output and 0V and you should be using a split power supply. (you are using a split power supply, right?)
     
  3. nza0104

    nza0104

    4
    0
    Dec 3, 2010
    thanks :):)
     
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