LM339 Comparator and capacitor discharge

Discussion in 'Analog & Mixed-Signal Design' started by marigold.maripol, Mar 5, 2018.

  1. marigold.maripol

    Thread Starter New Member

    Aug 19, 2015
    2
    0
    Hi-

    I'm having some on-going trouble and I can't quite figure out what the problem is.
    I'm running a fairly standard comparator circuit, as attached below.

    It is being driven with the signal from a full-wave rectified bipolar op amp.

    From my understanding of the datasheet, the LM339 should be able to be powered by +/- 12v. I am running it from that because I want -V and +V at the output, in order to discharge the capacitor.

    When I breadboarded it, it worked. When I built it worked, then it stopped working. The capacitors no longer seem to discharge effectively. Replacing the IC doesn't help. Rebuilding the whole circuit does work - again, just for a little while.

    Can someone suggest a way to solve this problem?
     
  2. crutschow

    Expert

    Mar 14, 2008
    22,482
    6,581
    Design error. :eek:
    You didn't look at the rest of the data sheet to see the output maximum sink current.
    Here is that value:
    upload_2018-3-4_23-22-11.png
    As you can see, it's typically 16mA with a worst-case value of only 6mA.
    You have a 1k output resistor going to (apparently) +12V with an LM339 negative voltage of -12V.
    This means the current through the resistor would be near 24mA when the output is at its minimum (near -12V), but that won't happen, of course, because that's much higher than the LM339 rating.

    So to keep the current no greater than the worst-case 6mA value, the resistor should be no smaller than 4kΩ.
     
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