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Weird op amp problem... LM324, feedback loop...

Discussion in 'General Electronics Chat' started by ARMDeveloper, Sep 28, 2007.

  1. ARMDeveloper

    Thread Starter New Member

    Aug 16, 2007
    I've using an LM324 to drive the gate of a MOSFET in a current controlled source.

    I'm feeding a 1MSec 2.5V pulse into the + terminal of one of the LM324 op amps. I've got a 0.1 ohm sampling resistor downstream of the MOSFET. It connects to the - terminal of the LM324 op amp. I've used this circuit many times and its always worked well.

    Right now though it doesn't work. When the current hits 25 amps the feedback voltage will be 2.5V and the op amp should lower its output. But for some reason it doesn't. If I put a scope on the current feedback I am seeing 5 volts or more ! 50 amps ! And the + input is 2.5V just like it should be. If I put a scope on the output of the op amp, its trailed to the positive supply voltage, 13V in this case. It never moves down from that, even when the feedback current voltage is way above the positive input voltage.

    I've swapped in several LM324s with no change in behavior.

    What is going on with my circuit ?

  2. ARMDeveloper

    Thread Starter New Member

    Aug 16, 2007
    How does an LM324 fail ? Is it possible that its blowing when I turn on the circuit and the output rails high ? Or maybe the current feedback blows the negative feedback input ?
  3. nanovate

    Distinguished Member

    May 7, 2007
    Could you post a schematic with the circuit and supplies?
  4. bloguetronica

    AAC Fanatic!

    Apr 27, 2007
    I'm guessing that the LM324 is doing what it should (or would be a strange malfunction). The feedback circuit might not be working properly. You can replace the LM324 and see if the circuit works. If not, you may conclude that is something wrong in the circuit. If it does, then I was wrong.

    Nevertheless, like nanovate just said, posting a diagram would help us help you.
  5. ARMDeveloper

    Thread Starter New Member

    Aug 16, 2007
    I found the problem. The op amp output trace was shorted to VCC. I removed the short and all works well.