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help for lm324

Discussion in 'General Electronics Discussion' started by dariush, Jul 1, 2013.

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  1. dariush


    Jun 26, 2013
    i want to test lm324 in this way:
    im using 1/4 op-amps. short circuit all other inputs(5,6,9,10,12,13) to the ground. connect 17 volt,.02 ampere to pin 4, 1k resistor to pin3 and 10 k resistor to pin2(with a same voltage sours 17 volt) and noe LED with a 1k resistor to pin one but LED doesnt lights on?why? i dont know.
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  2. BobK


    Jan 5, 2010
    Why do think it would be on? You are putting the same voltage on both the + and - inputs. This could cause the output to be anything.

    Did you expect the 2 different resistors to create different voltages at the inputs? Will, they will, but the typical current drawn by the inputs is 20nA, which will result in voltage drops of 20uV and 200uV at the two inputs. This is way below the input offset voltage of 3mV, so there is not telling what it will do.

    Try using 2 different voltage dividers to put different voltage on the two pins. For example, try putting a 10K resistor to ground on each input in addition to the resistors you have to V+. Now it will draw enough current to make the voltages on the inputs significantly different.

    Last edited: Jul 1, 2013
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    What BobK said.

    Also, the LM324 cannot compare voltages that are close to its positive supply rail. Inputs have to be at least 1.5~2.0V lower than the positive supply voltage (depending on manufacturer and temperature) for the LM324 to correctly compare the voltages.
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