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Posts posted by walid

  1. Hi
    The circuit shown below is the power supply section of a stairs light timer. I draw it from a real timer circuit. I have some questions.
    (1) The 18K-1/8w resistor. It waste (9V/18K=) 0.5mA current to ground all the time. What the purpose of this resistor?
    I check its connection many time and I'm sure it is connected in the place shown in the figure.
    (2) Why the designer uses two series 240 ohm - 1/2 watt resistors? Why not one 470 ohm 1/2 watt? What the difference? or what is his other choice?
    (3) The green led limiting the current to 20mA, i guess it i am not sure!



  2. Hi
    I analyze a timer circuit using transformerless power supply, that is no transformer, but a 330 nF in series with two 240 ohm 1/2 watt resistors.
    i noticed that the designer always use two-series 240 ohm 1/2 watt resistors. he do so three times in different places in the circuit.
    Also he connect the emitter of a small signal transistor (BC547B) to ground through three-series 1Kohm 1/2 watt.
    Why the designer do this?
    what the equivalant wattage of two 1/2 watt resistors connected in:
    (a) series
    (b) parallel
    thank u very much

  3. I read The following from: http://www.leminstruments.com/grounding_tutorial/html/soilresistivitytest.shtml
    Measuring Soil Resistivity 4 - Pole Method

    The calculation of this measurement can be simplified by converting distance in cm to distance in feet giving you the following equation:

          p = 191.5AR
    Where: p = the average soil resistivity to depth
              in ohm - cm
          A = the distance between electrodes in feet
          R = the measured resistance value in ohms
              from the test instrument
    For example, you have decided to install 10' ground rods as part of your grounding system. To measure the soil resistivity at a depth of 10' requires that the spacing between the test electrodes is 10'. The depth that the test electrodes is to be driven is A/20.

    Look at this"The depth that the test electrodes is to be driven is A/20."

    then he said: The ground stakes are positioned in a straight line equidistant form one another and at a distance between one another reflecting the depth to be measured. The ground stakes should be screwed in no deeper than 1/3 the distance from one another.

    he confuse me A/20 or no more than 1/3 A

    and in the figure he present (see below) he put a and in formulas he use A

    please help me to understand it


  4. Hi audioguru

    I don't know why you want to measure the resistance between two "grounded" chassis, 

    MP tell me this: "Set your multimeter to ohms and check the resistance between the ground of the two connected devices. For example, the resistance between the metal chassis of the two devices. If there is resistance, you have a ground loop problem. If you get a reading of zero ohms, both devices have the same ground potential. Next step is to check all internal connections to chassis. Make sure they are not loose, frayed, broken or corroded. If all is good, and all internal grounds are at the same potential as the case of both devices, you do not have a ground loop problem between these two devices.
  5. I read the following on:
    "The depth that the test electrodes is to be driven is A/20" then the writer said:
    "The ground stakes should be screwed in no deeper than 1/3 the distance from one another"
    please refer to the page link above to see the figur.
    i feel the two sentances above are in contrast

  6. any set (like TV) have a plug to connect it to electrical power
    each pluge has 3 terminals, phase neutral and ground (earth).
    Now, my question is: Is the metal chassis connected directly to the Ground (Earth) or not?

    And if i want to measure the resistance between the metal chassis of the two devices, i think the two devices must be plugged but OFF. Is this not affect the meter

  7. I read the following from the link above:
    A ground loop in the power or video signal occurs when some components in the same system are receiving its power from a different ground than other components, or the ground potential between two pieces of equipment is not identical.

    my questions are:
    (1) please draw an example for TV and PC and say amplifier, show me how exactly  some component in the system is receiving its power from a different ground than other components.
    (2) How to measure the ground potential between two pieces of equipment? If using AC voltmeter, across what points I should measure?
    thank you very much

  8. audioguru: "Without it then when the battery voltage reaches the threshold voltage, the circuit might oscillate."

    Walid: (1) What the threshold voltage if the charged batt is, say, 9v
              (2) is this op-amp oprate as a comparator. I never see a comparator with a feedback resistor.

  9. audioguru: "The actual value doesn't matter. It filters noise from the battery if it is loose in the connector and provides a small delay."
    Walid: it is good news that the cap value (4.7u) doesn't matter, but still i can't understand u, please explain it more for me, thanks.

  10. In the circuit shown below there is a Universal battery charger from:
    it is easy to understand and to implement, but i have some questions:

    (1) Why u think the designer use the 4u7 cap.
    (2) If LM358 is used as a comparator, why:
          (a) using 820K resistor
          (b) not using 741 single op-amp instead of this dual op-amps
    (3) If I want to use this charger to charge a 9-volt square batt, how can i tune this circuit to fully charge my batt
    (4) If I want to use a NPN trans type instead of BC557,what the modifications that must be done?
    (5) If I want to charge atwo 1.5V batts in series (3V) using a 5V power supply, what to do other than changing zener to 3.3V?

    thats all,


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