741 Op Amp follower -- time delay?

Discussion in 'General Electronics Chat' started by Tom1962, Sep 24, 2012.

  1. Tom1962

    Thread Starter New Member

    Sep 10, 2012
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    If I use a 741 op amp as a voltage follower for a 12vdc pulse, will the output pulse lag behind the input by a measurable amount of time?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,984
    745
    can you post a diagram?

    i would not use a 741 they are out of date...
     
  3. ramancini8

    Member

    Jul 18, 2012
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    The slew rate on a 741 is very low, so the pulse may be gone before 741 reacts. Please the pulse width you want too detect, and a schematic of the proposed circuit so we can comment.
     
  4. Tom1962

    Thread Starter New Member

    Sep 10, 2012
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    I have a quick occassional 12vdc pulse from an existing circuit which this circuit should change to 5vdc and hold a bit longer so it can be logged and light warning LED. See circuit below. All tips welcome, but in particular trying to figure out which op amp (or dedicated buffer) and inverter to use on the input line, since the pulses are only 3 or 4ms. I'm worried that a 741 or similar will be too slow to act as a good buffer for such a fast pulse. Also not sure which inverter to use on the input since the 555 triggers on low rather than high.
     
  5. ftham

    New Member

    Jan 18, 2008
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    If we look at the datasheet for the 741, we'll find that it has a minimum slew rate of 0.3V/uS. Which means that it will take up to 40uS to slew from 0V to +12V, then it will take another 40uS to slew from +12V to 0V. What this mean is that the minimum pulse width need to be at least 80uS.
    Please note that the output of the 741 can only swing 2V from the rail, so if supplies were connected to 0V and +12V, the output is limited to +2V to +10V.
     
  6. Tom1962

    Thread Starter New Member

    Sep 10, 2012
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    Thanks, is there an Op Amp or dedicated buffer that can act as a buffer/follower for such a fast pulse?

    In the meantime, I'll have to google "slew rate"...
     
  7. ftham

    New Member

    Jan 18, 2008
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    Since we are only trying to generate a square pulse it might be better to use a voltage comparator (even though they are slightly more expensive) rather than an opamp. There are several reasons for this you can find details at http://www.analog.com/library/analogdialogue/archives/37-04/comparator.html.

    Slew rate (Large signal) is similar of rise time (small signal). Though rise time is directly proportional to the Gain Bandwidth Product GBW, Slew Rate has no relation the GBW at all.

    As to which comparator to use, just use one that suffice for your application, eg LM311.
     
  8. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,984
    745
    Use a CD4069 as the inverter input to the 555.
    http://www.fairchildsemi.com/ds/CD/CD4069UBC.pdf

    and use a CA3240e dual op amp, which as a slew rate of 9V/uS
    http://users.rowan.edu/~jahan/ECT/ECT_2007/presentations/324-opamp.pdf
     
    Last edited: Sep 25, 2012
  9. Audioguru

    New Member

    Dec 20, 2007
    9,411
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    Look at the datasheet for the ordinary NE555 or are you using a Cmos 555?
    With a 12V supply, the output high of an ordinary NE555 is +10.65V, and lower if it has a load, not 12V. A Cmos 555's output goes to +12V if it has no load.
     
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