comparator

Discussion in 'The Projects Forum' started by chandu13, Dec 30, 2008.

  1. chandu13

    Thread Starter Member

    Oct 15, 2008
    22
    0
    Hay

    I need feed back from the solenoids (24V, 300mA) whether they are switched ON/OFF, I am switching the solenoids using micro controller & BDX33 Transistor
    I kept shunt resister 1ohm across the emitter and giving the voltage drop to the comparator (non inverting terminal of the LM324) and comparing with ground Point (inverting terminal of the LM324 connected to the ground)
    Whenever solenoid is switched ON there will be voltage drop and LM324 switching ON the LED but
    Whenever solenoid is switched off there is no voltage drop, both terminals of the LM324 at ground potential, at this point LM324 is comparing the ground potentials and giving the un proper outputs
    Please give me the some suggestions
    how to compare the volatge with ground point
    Regards
    chandu
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The comparator becomes unstable due to internal offset errors and because the LM324 is not designed as a comparator. You don't really need a comparator, just a connect the op-amp as a voltage follower.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    What is needed to advance the analysis of your design is a schematic of the circuit you are using.

    hgmjr
     
  4. Alberto

    Active Member

    Nov 7, 2008
    169
    36
    You are using a BDX33 which is NPN transistor, which means you will always have a voltage in the coil when the transistor is open. I think you should use a BDX34 (PNP transitor) so that the coil will be connected from ground to the transitor and when the PNP transistor is open then coil will be at ground level and any OP will respond correctly, specially if connected as voltage follower, as suggested in post # 2

    Alberto
     
  5. chandu13

    Thread Starter Member

    Oct 15, 2008
    22
    0
    Hay

    I am using IEF510 MOSFET instead of the BDX33

    In the voltage follower mode for which pin I have to use for input signal & reference signal (ground).
    In the circuit (LM324 as the comparator mode) when ever I connect inverting pin to the ground with large resister(56K) the circuit is working.
    What happened whenever I connect inverting pin to the ground with large resister.

    Thank you


    Regards
    chandu
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    As hgmjr said, if you want help, post your schematic.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Connect the output of the op amp directly to the inverting input via a wire and apply your input signal to the non-inverting input.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Do you have some insight that I missed? It's not clear to me how a voltage follower will solve his problem. In fact, I don't even know what his problem is.:confused:
     
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