Jump to content
Electronics-Lab.com Community


  • Posts

  • Joined

  • Last visited


Posts posted by walid

  1. Dear guru:

    D5 is needed if the opamps are ordinary ones that their output go down to only +1VDC.

    Give me an example (names) of that op-amps.

    The outputs of the LM324 and LM358 opamps go down to +10mV

    I Know the o/p high = Vcc – 1.2V, but don’t know the o/p Low = 10mV?

    and about 25mA for the LED if it had a C.

    I want to make a calculation,
    Let  R = 330 ohm
    12V – I * 330 – 1.8V – 0.2V =0
    I = 10/330 = 30 mA
    If we choose R = 470 ohm
    I = 10/470 = 21 mA
    If we choose R = 1K
    I = 10/1000 = 10 mA
    Can you please tell  Any value is the best? And why?

    Now we go to a general questions:
    (1) Is all of an op-amps suitable for use as a comparator or is that only certain types?
    (2) How can know the value of the relay coil resistance?

    Thank you guru for your patience
  2. Hi guru

    I understand from your words that there is no problem in the value of the R4 (=1K) resistance. And all of the problems resulting from a lack of a current limiting resistor in series with the LED.
    I am now comes over as I feel I did not understand anything. At the outset I thought I understood. But your last reply shows I did not come to the desired point yet. Therefore, I want to start again:
    (1) If we assume that the LED and its limiting current resistor are removed at all from the circuit as shown below.
    I noticed that IB is still = 8mA and this is a great value, if we make R4 more bigger (I don’t know to any level) we can reduce IB.
    (2) please tell me about the importance of the presence of D5.
    (3) Is it true that IC = (12-0.2)/200 = 59mA?

  3. Hi guru

    Look at the datasheet for the old LM324 to see its darlington transistor at the output.

    yes i see it.

    1- what is the minimum value of  R4 to make T1 not smoking?
    2-If I remove this LED, put the correct value of R4 and we know that the relay resistance is 200 ohm,  what  the value of VC at T1?
    3- I have many relays, there is no resistance value printed on its case, how to know if this relay is 200 ohm or not, can i measure its resistance as if it a resistor?
    thank you guru
  4. Hi guru
    It is a good answer and excellent, but it opened the way to further questions.
    1-I believe that these calculations were in the case of D5 still in the circuit. If we remove this D5 entirely from the circuit, Are these calculations remain as they are?
    2-is there a darlengton pair connected to the o/p of op-amp N1 makes a voltage drop of 1.2V (2* 0.6), that is the Vcc= 12 in o/p it 12- 1.2=10.8V?
    3- I understand very well how IB to be equal 8mA,  But I do not assimilated  how VC = 10.2V, can you please explain it. If VCE = VC= 10.2, then T1 is nearly off.

    I noted the name under the above picture and laughed it. That name is very insulting telling story honestly

    Thank you guru.

  5. Hi

    I put the scheme to make the discussion easier.

    To guru

    Maybe the transistor will also blow up when it tries to saturate.


    To AN920
    The diode is added because the opamp is not a rail-to-rail opamp and may not switch low enough to completely turn off T1. The added drop over D5 gives more switch off margin.

    Guru said:
    D5 is not necessary because the output of the LM324 typically goes to +10mVDC when it doesn't sink much current.

    what u say?

    thank you very much
  6. Hi

    The circuit attached below is OVER-/UNDER-VOLTAGE PROTECTION.
    I have some questions:

    1- R4 & R8 are 1K why not 10K?
    2- Why D5?
    3- the voltage divider consisting of the resistors R1, VR1 and R2, is to monitor the voltage, i can use the same voltage division with smaller or bigger values of resistors. my question is Why these particular values?
    thank you very much

  7. Hi all my friends
    In the datasheets of  Lm308 attached below (at the end) I cut this part for discussion:


    and here are the notes:


    my questions:
    1- What is the differential i/p current, and how to ensure that it will not exceed    10mA?
    2- Please explain Note 1, I did not understand it?

    Thank you very much.

  • Create New...