Urgent op amp assistance

Discussion in 'The Projects Forum' started by hwengindude, Nov 9, 2012.

  1. hwengindude

    Thread Starter New Member

    Nov 9, 2012
    3
    0
    BACKGROUND INFORMATION IF DESIRED:

    I have a sensor (MQ7 Carbon monoxide sensor) that needs to be switched between 5v and 1.4v. What I did was create a voltage divider in series attached the source of an N-channel mosfet, with the source being pulled up to 5v. So, when the gate has 'off', I'll have my 5v, and when the gate 'on' current is allowed to flow through the mosfet to ground thus completing the voltage divider and giving me my 1.4v when desired.


    ASSISTANCE NEEDED:
    I have buffer between this voltage and my sensor. Currently in simulations, I'm using a basic 741 op amp. With no sensor attached, the output voltages are fine, but once I attach my sensor (internal resistance of 30 ohm) the op amp essentially shuts down and my voltage at the output of the buffer is in the milli volt range. A professor told me that the 741 doesnt have the power capability since 5 volts at the output and a resistance of 30 ohms gives .83 W (p = v^2/r), and I noticed the 741 can handle power dissipation of 500 mW. Would anyone have any suggestions on an op amp, ideally only powered with 0v to 5v and ground, that can handle this capability? I'm having trouble reading in datasheets this information as it was easily listed in the 741 datasheet.

    I realize this is extremely long so any assistance would be greatly appreciated. Long time lurker, first time posting!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Can you post a schematic of the current setup?
    Here is how to attach the schematic:
    Attachments and Images

    Also a datasheet of the sensor would be helpfull.

    Bertus
     
  3. wayneh

    Expert

    Sep 9, 2010
    12,103
    3,038
    It would help a lot if you could sketch a schematic including the resistor values of your divider and the part number of your MOSFET. I don't quite follow your description. A divider is not a great way to power something and could be part of your problem. I guess that's why you are using the op-amp.

    There are plenty of "power" op-amps that can handle higher power levels but you shouldn't need to resort to that, IMHO.
     
  4. hwengindude

    Thread Starter New Member

    Nov 9, 2012
    3
    0
    Here's the link to the MOSFET I'm planning on using:

    http://www.mouser.com/ds/1/200/irfml8244pbf-27061.pdf

    However, could not find the exact same model in my schematic capture software so that's why I'm hoping to find an op amp and order to bench test the whole set up.

    Here's the link to the sensor datasheet:
    http://www.sparkfun.com/datasheets/Sensors/Biometric/MQ-7.pdf


    Hopefully I did it correctly, but I attached the circuit. the resistor 'R6' of 30 ohms is simulating the internal resistance of the sensor.

    Thanks again!
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The output current will be to high for the LM741.
    The LM741 has a short circuit protection that will limit the current to ca. 25 mA.

    Better look for an opamp with higher current output capability.
    Here is a list from national with high current output opamps:
    http://www.national.com/products/tree/table.do?treeId=14

    Bertus
     
  6. hwengindude

    Thread Starter New Member

    Nov 9, 2012
    3
    0
    Thank you for the list!

    I selected the LMH6672

    http://www.ti.com/lit/ds/symlink/lmh6672.pdf

    And was able to find it in my schematic capture software. The op amp did not shut down in simulations this time, although I noticed another glitch , when I simulate it with the MOSFET switch off (meaning I should get 5v across my sensor) I am only reading 3.63 v. I went back and tried the 741 again (with no sensor) and it also didn't output the full 5v. So I was wondering if anyone had any type of insight on this or the parameter I am overlooking that could cause the op amp to drop the voltage.

    I've attached a picture, the bottom multimeter is measuring the voltage at the output of the buffer and the top multimeter is measuring the current.
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    If you want 5 Volts as output voltage with a 5 Volts power supply, you should look for Rail-to-Rail opamp capability.

    Bertus
     
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