741 Op Amp Output

Discussion in 'General Electronics Chat' started by ut1205, Apr 11, 2014.

  1. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
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    I've never used a 741 and decided to play with one today. Wired it up as a comparator like the circuits shown here:

    http://www.circuitstoday.com/voltage-comparator

    I am using 5 volts as my supply voltage and instead of the two fixed resistors as a voltage divider I am using a 100K pot to vary the voltage. I used a fixed voltage divider to give my reference voltage of 2.5 volts. The output (pin 6) goes through a 330 ohm resistor to an Led.

    My issue is that even though it does go high and low on the output, in the low state I am still getting 1.9 volts (4.8 volts at high). I tried a 10K pulldown resistor from pin 6 to ground but that didn't help. The Led still glows even in the low state.

    My intention was to use an NPN transistor on pin 6 but I don't think it would ever turn off with the 1.9 volts on the base.

    Any ideas?
     
  2. hexreader

    Active Member

    Apr 16, 2011
    250
    82
    Check the datasheet for the exact device that you are using. I suspect that 5 volts will be well below specification.
     
  3. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
    0
    Thanks for the reply. The chip is a UA741ETC which shows to be obsolete. That makes sense since I know it has been in the cabinet for at least 20 years. It may even have issues.

    I have some NE5532's on order and the supply voltage on them is 5-15 volts. I guess that is where I came up with the 5 volts. I'll put this project on hold until my new stock comes in.
     
  4. #12

    Expert

    Nov 30, 2010
    16,261
    6,770
    We used to have a guy on here that was nearly rabid about hating 741 chips. They will kind of work if you give them a nice dual supply voltage and don't expect anything in a high frequency, but there are modern op-amps that put them to shame for less than a dollar and work on 5 volts. Just drop them in the trash. You'll spend more than 50 cents worth of time trying to get them to work.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,523
    1,247
    But to answer your question, a 741 is a 2nd generation device, designed decades before today's "rail-to-rail" devices. Neither its inputs nor output function well within 1.5V to 2V of either rail. With a total power supply of 5V, you've got about 1V peak to peak of input signal range, and not much more for output. 1.9V for a saturated low output? Perfectly normal.

    The 5532 is a much newer design, way lower distortion, better input performance and output drive, but still a classical architecture. It is not designed for single supply operation, and it will not do much better as a comparator. Try the LM339 for something old yet better.

    ak
     
  6. ut1205

    Thread Starter New Member

    Mar 20, 2014
    18
    0
    Thanks. LM339 are cheap. I'll try them also. At this point I'm not trying to build any project. Just figuring out how everything works.
     
  7. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    For a 741 output connecting to a resistor and LED:

    741 Output → (+)1N4148(-) → 270Ω → LED.
    If the LED is still lighting when the output is low, then you can adding two 1N4148, and the resistor change to 200Ω.

    For a 741 output connecting to a NPN circuit and LED:

    741 Output → 4.7K → (+)1N4148(-) → Base of NPN bjt, C of NPN ← (-)LED(+) ← 330Ω ← +5V, E to GND.
    Some 741 needs to adding two diodes, or increaing the value of resistor.
     
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