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Choice of Op-Amps

Discussion in 'General Electronics Discussion' started by GeoffC, Mar 6, 2015.

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  1. GeoffC

    GeoffC

    39
    4
    Mar 11, 2013
    Please can you help

    I need to use an Op-Amp with a FET input - ie TL071 - with a resistor array to create a:

    2.4 non inverted Op-Amp, (5v to 12v), but this amp would not be man enough to drive a 12v motor

    At 0.3amps – (300m/a) or more.

    So, can I use a - FAN3100CSX - MOFET – with the same resistor array as on TL071 or similar???

    The voltage in-put will be variable (1v to 5v).

    Or you might know of a different Op-Amp that can manage the power out-put.

    Further info

    It needs to have a high in-put impedance as the input is from an RC filter.

    I want to avoid using a Power Transistor. There will be 4 circuits for 4 motors.

    I am doing away with PWM to avoid motor hum


    Data sheet

    http://docs-europe.electrocomponents.com/webdocs/12cf/0900766b812cf692.pdf

    Kind regards, Geoff. (GeoffC)
     
  2. hevans1944

    hevans1944 Hop - AC8NS

    4,536
    2,112
    Jun 21, 2012
    The FAN3100CSX is a digital device, a gate driver for N-channel MOSFETs operating in switching mode. It will not work for your application which requires a continuously variable DC output. If the non-inverting gain of the TL071 is 2.4 and the input from the RC filter is 1 to 5 V DC, then the output can be expected to vary from 2.4 V to 12 V.

    You need to add a current-boost transistor to the TL071 output to obtain at least 300 mA output at 12 V. There are many of these circuits available on the web, so Google is your friend. The most simple solution is an emitter follower whose base is connected to the output of the TL0711, with the emitter connected to the inverting input of the TL071 through a non-inverting gain setting resistor, and collector connected to +Vcc. You also need a small-valued resistor (about 1 kΩ from emitter to ground) to provide the emitter follower load. The inverting input of the TL071 also has a resistor, Ri, connected to common. Gain at the output referred to the non-inverting input is 1 + Rf/Ri. You might also need a capacitor across Rf to roll off the frequency response to prevent oscillations. This is a good application for pSPICE simulation.

    Note that the emitter follower can have significant power dissipation: (Vcc-Vout)(0.3). So if you power the op-amp from +15 V DC supply, the power dissipation in the emitter follower at full output will be (3)(0.3)=0.9 W. This exceeds most small-signal transistor specs such as the 2N2222 in a plastic case. If you can find a 2N2222 in a metal case, you might be able to get by (for awhile) with a small tab heat sink attached to the case. There are plenty of power transistors in tabbed cases that you can bolt to a heat-spreader or heat sink to dissipate the four watts or so that your four motors will require.

    The venerable 2N3055 is a workhorse power transistor that will take a lot of abuse and keep on tickin' but it has the large TO-3 package. You might even want to try a tabbed power MOSFET in a source-follower configuration, but I have little experience operating MOSFETs in a linear mode. Perhaps @KrisBlueNZ can chime in here with one of his circuits.
     
    Last edited: Mar 6, 2015
  3. GeoffC

    GeoffC

    39
    4
    Mar 11, 2013
    Thank you very much for your prompt reply. I had the feeling the word ‘gate’ would not work

    l will use the emitter follower circuit, as you suggest, I’m somewhat more familiar with that type of circuit. I have a few of the 2N3055 in my stock.

    Once again thanks for the prompt reply. Regards, Geoff. (GeoffC)
     
  4. BobK

    BobK

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    1,686
    Jan 5, 2010
    But the real question is why you want to use a variable voltage to drive a motor? PWM is much superior and does not waste power like the solution you are proposing.

    Bob
     
  5. hevans1944

    hevans1944 Hop - AC8NS

    4,536
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    Jun 21, 2012
    Apparently PWM causes motor hum. Hmmm. I wonder if the PWM frequency is too low... isn't it supposed to be in the multi-kilohertz range, above human hearing? I know stepper motors push out a terrific scream, PWM or not, but that's because their stepping rates are in the audible frequency range, plus there are mechanical resonances at some stepping rates.
     
  6. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    Yep. I looked a couple of controllers on Ebay and they used 13KHz and 15KHz. Not out of audio range but close.

    On my robot car I used 10KHz and did not hear anything, but then my ears fall off rapidly at 4KHz in one and 8KHz in the other according to my ear doctor. My wife also did not complain about any high pitched whine, so there was probably not much sound coming out.

    I suspect that motors are pretty good sound transducers in the lower audio but not in the upper audio range.

    Bob
     
  7. Arouse1973

    Arouse1973 Adam

    5,164
    1,081
    Dec 18, 2013
    My misses whines all the time, how do I convert that to frequency. Is it 1/24h.:)
    Adam
     
  8. hevans1944

    hevans1944 Hop - AC8NS

    4,536
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    Jun 21, 2012
    If you have ever had the opportunity to observe industrial robots in action, the servo motor noise is incredible. Of course the noise in most factories that employ them is even louder. In the Air Force everything we used was powered from 400 Hz AC. This was bad enough, but add in two-phase AC servo motors driven with magnetic amplifiers, a nearby three-phase 20 HP hydraulic power supply motor, and jet engines winding up all around the flight line... no wonder the guv'ment issued us ear protection!

    Personally, I like a nice, quiet, DC motor. Brushless is even better.

    Induction motors are also quiet, but it is a real bitch kitty to make them vary their speed on demand. We did do just that on a bearing tester I helped to build, a copy of an existing test rig. We used a commercial variable-frequency drive manufactured in Germany. It cost in the tens of thousands of dollars range, but it worked as advertised.
     
    Last edited: Mar 6, 2015
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