Wheatstone's Bridge w/ thermistor, 741 op amp: output motor problems

Discussion in 'Digital Circuit Design' started by Mackatack8, Jan 23, 2018.

  1. Mackatack8

    Thread Starter New Member

    Jan 19, 2018
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    I'm designing a circuit to detect a slight change in temperature and activate a motor. I used a Wheatstone bridge, with one leg being a 10kohm thermistor shown in schematic below. The signal is then amplified with an 741 op amp. When I read the voltage output (delta V) from the op amp it is exactly as I want it when the temperature changes, about 0.6V. When I attach the motor (0.5V threshold) as shown, it does not work and the output voltage (delta V) now reads -7.2V. What is wrong? What should I try to fix the problem?

    Thanks!
    IMG_3326.jpeg
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    How much current does the motor draw?

    Are you sure a 741 opamp can supply that much current?
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A 741 can't source or sink enough current for driving a motor directly, unless the motor draws only a tiny current. You need a power output stage between the 741 and the motor. What that stage would be depends on the motor specification. What is the motor stall current at 3V?
     
  4. Mackatack8

    Thread Starter New Member

    Jan 19, 2018
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    At 3V the motor draws 0.5A but does not stall. I can raise the voltage even higher - but I'm afraid I'll burn it out. I bought the motors cheaply online and have no manufacturing stats on them.

    you mention a power output stage - Im not familiar with this term. Could I accomplish this through use of a relay?
     
  5. #12

    Expert

    Nov 30, 2010
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    Probably. I'd have to look up which 741, its current abilities, and the relay coil current requirement. Not a bad idea, but not completely baked. For instance, a 24VDC relay coil would require a lot less current than a 3VDC relay coil.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    5,630
    If the motor only runs in one direction you could use an NPN emitter follower at the 741 output, such a TIP31.
     
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  7. WBahn

    Moderator

    Mar 31, 2012
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    Which is a WEE bit more than the typical 0.025 A short circuit current spec on the 741.

    Your design implies that you want the motor speed to be related to the temperature difference. Is that what you want, or do you just want the motor to be switched on and off based on temperature?

    Also, your design implies you want the motor to turn one way above a certain temperature and the other way below it.

    It might help if you describe the problem you are trying to use this circuit to solve.
     
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  8. Mackatack8

    Thread Starter New Member

    Jan 19, 2018
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    0
    I used a NPN transistor to fix the problem. Thank you all!
     
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