Comparator Questions

Discussion in 'The Projects Forum' started by DC_Kid, Jun 24, 2009.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    i need to learn more about comparators.

    is the output of a comparator a complimentary stage, can it source and sink current? what is this state of the output when no power is on the device, is it hi Z?

    the reason i ask is because i need to drive a fet switch and depending on how the comparator output functions i may or may not need a intermediate stage between comparator and fet, etc. one aspect of the circuit is if the comparator dies the fet gate gets pulled high via a pull-up resistor, etc. its a "fail-safe" feature, etc.
     
    Last edited: Jun 24, 2009
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Not to be smart, but some do and some don't. You really need to do parametric searches for devices, or at least get data sheets to see how the output of any comparator is handled.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    Beenthere's right.

    Most of the common comparators (like LM111/211/311, LM339, LM2903, etc) have open-collector outputs; you need a pull-up resistor in order to see any output from them.

    Comparators such as the LM339, LM2903 etc. won't sink a lot of current. You'll need to use a driver if you're trying to operate at more than a few hundred Hz, particularly if you're using a high-capacity MOSFET; as they can have hefty gate charge requirements.
     
  4. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
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    i thought some did and some did not, just thought i would check.

    the comparator will run real low Hz, probably 1/10 - 1/200 Hz. i have "adapted" the AD595 into a set point cooling system with hysteresis. the final switch is a IRF1404 and it has big gate charge.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    That's not a comparator; it's a thermocouple amplifier.

    Indeed it does. You're going to need to use a driver circuit, even operating at low frequencies. Otherwise, it'll spend much of the time in the linear region, dissipating lots of heat.
     
  6. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    638
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    the ad596/597 datasheet uses a comparator as the setpoint mechanism, etc. AD595 is the same unit. 596/597 are optimized for high temp sensing.
     
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