op amp ic gets HOT when input leads are floating

Discussion in 'General Electronics Chat' started by tpny, Apr 8, 2013.

  1. tpny

    Thread Starter Member

    May 6, 2012
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    Hi I'm using TL074 op amp and supplying +/-12V to V+/V- pins repectively. When the input pins are unconnected and just the power supplies are connected the IC gets really hot! So I measure the floating input and output pins to discover one of the output pins meaures -12V while floating.

    The IC is cooled when I connect a sensor to the input pins. This is the normal condition but I still can't allow that the ic run super hot when the sensor is not plugged in.

    Any suggestions? Thanks!
     
  2. YokoTsuno

    Member

    Jan 1, 2013
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    Do you have a schematic drawing of your circuit? I am under the impression that the Opamp is oscillating.
     
  3. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    I think so, too. The unused inputs should be dealt with.
     
  4. wmodavis

    Well-Known Member

    Oct 23, 2010
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    It is not a good idea to leave inputs unconnected or floating because it can allow feedback via the high impedance inputs resulting in the op-amp oscillating. In that case it could overheat. When it is doing that an oscilloscope connected to the output pin would confirm if it is oscillating. Do you have the + & - V pins well bypassed? If not that could also allow this behavior.
     
  5. tpny

    Thread Starter Member

    May 6, 2012
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    hi, attached is schematics. of the 4 op amps in the quad tl074: 2 are 5x non-inverting amplifier; 1 is follower; 1 is unused. When no signal inputs are connected to this tl074, the ic heats up.

    Yes I agree with your assessments, so how do I "bypass" the input pins or otherwise deal with the unused pins? THank you!!
     
  6. tpny

    Thread Starter Member

    May 6, 2012
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    please see attached schematic
     
  7. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Unused inputs should likely be grounded not bypassed. The power supply pins should be bypassed using a high frequency capacitors such as a disc ceramic.
     
  8. tracecom

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  9. YokoTsuno

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    Jan 1, 2013
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    Even the unused Opamp can show weird behavior if you're unlucky. You may want to connect the output to the inverting input (Gain = 1) and ground the non-inverting input.

    EDIT: Just noticed the above post. The resistors at the + input are not really required since your power supply is symmetrical. Grounding is sufficient.
     
    Last edited: Apr 8, 2013
  10. tpny

    Thread Starter Member

    May 6, 2012
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    Yes, I tied unused 4th channel opamp to ground but it still heats up. I think because my other 3 channels are configured as non-inverting amplifiers and there's nothing between the opamp's input+ and signal source - when the signal source is removed (input+ is floating) the ic goes into "oscillation" or what not.. How do I remedy this? Thanks! I think this might be a common non-inverting op amp problem?
     
  11. YokoTsuno

    Member

    Jan 1, 2013
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    Try grounding (for testing) the used inputs, one by one, to isolate the culprit(s) and see if the heat-up stops.

    It helps if you have an oscilloscope.
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    You remind me of the old joke. Guy says "Doctor, my arm hurts when I hold it like this" Doctor says "Then don't hold it like that!"

    Don't leave you inputs floating.
     
    absf likes this.
  13. tpny

    Thread Starter Member

    May 6, 2012
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    Inputs come from sensors which are plug-ins. Yes, if it was up to me I'd keep them plugged in all the time. But it's conceivable that the user might unplug the sensor in which case the parts inside can't be heating up by design..

    If floating non-inverting inputs have a tendency to cause oscillation, which leads me to think: why use non-inverting in favor of inverting configuration? Or is there a way to remedy this without too much hassle? Thank you!
     
  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    How about adding some high value resistor from the inputs to ground? I have no idea what your sensors are, but for example 1meg resistors shouldn´t alter your readings by much.
     
  15. Ron H

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    Apr 14, 2005
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    What are the loads on your op amps? It appears that they are ADCs, but what kind? How are they powered?
     
  16. tpny

    Thread Starter Member

    May 6, 2012
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    Hi Ron, yes op amp outputs to adc. Adc is 5V. Please see attached schematic. Thank you!
     
  17. tpny

    Thread Starter Member

    May 6, 2012
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    oops, here's attachment
     
  18. Ron H

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    Apr 14, 2005
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    So you have op amps that can swing ±10V, feeding ADCs running off 5V? What could go wrong there?:eek:

    You need to protect your op amps and your ADCs from overvoltage and overcurrent. Almost all ICs have intrinsic diodes from the inputs to the supply rails. These diodes will conduct if the input voltage exceeds the rails, causing possible damage to your ADC and/or your op amp. See the attachment from the datasheet.
    Are you sure your op amp goes to -12V? TL074 won't swing to the rails. -10 or -11V, maybe? If it's one that is connected to the ADC, then the ADC may be toast.
     
  19. tpny

    Thread Starter Member

    May 6, 2012
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    yes that would be a problem, however I do have a diode pair at each adc input channel to protect from over-voltage as indicates in the schematic.
     
  20. ScottWang

    Moderator

    Aug 23, 2012
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    If the user want to unplug the Sensor, you can suggests him to use as 3 pins headphone jack, when the sensor is plug in the jack, then the false load(resistor) will be opened, and the false load(resistor) will be replacing the position of sensor when the sensior is unplug.

    You may using 100K~1M to try the false load(resistor).
     
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