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RC TIMNG

Discussion in 'General Electronics Discussion' started by MAHABA, Apr 21, 2012.

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  1. MAHABA

    MAHABA

    14
    0
    Apr 13, 2012
    HI GUYS,,

    I have an RC circuit as shown below,,

    http://coppercavern.com/images/rctimer.gif

    Only, few changes is done, the resistor between the zener and the transistor is removed and the LED with its resistor replaced by a relay.

    When I choose small values for R and C, everything is Okay.

    When I choose higher vales like (R = 10M and C = 440u) it will not work any more!!

    Whey is that?? and what to do to solve this problem??

    (Additional Info : transistor-C1815, battery voltage- 9v, relay voltage- 9v,capacitor voltage- 16v, zener diode 5.6v)
     
    Last edited: Apr 21, 2012
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    The leakage of the capacitor means that the voltage across it never rises enough to reach the zener voltage.

    Even if it does, the resistor provides insufficient current to turn on the transistor.
     
  3. MAHABA

    MAHABA

    14
    0
    Apr 13, 2012
    Hi steve,,

    You clearly described your thought,,

    yah,, what to do in order to solve this problem??

    This circuit should give around 1 hour of delay!!

    Any heeeeelp??
     
  4. jackorocko

    jackorocko

    1,284
    1
    Apr 4, 2010
    An hour? That seems way to long for a RC circuit, A few seconds is appropriate, but not an hour.

    Maybe you should be looking at a 4060 or something similar.
     
  5. MAHABA

    MAHABA

    14
    0
    Apr 13, 2012
    Hi jack

    you said,,

    But,,

    Theoretically speaking,, it should go for more than one hour!! right??

    I am trying to find the simplest way to create a timer based on RC, suitable for school student to understand.

    thanx again
     
  6. gorgon

    gorgon

    603
    24
    Jun 6, 2011
    It all depends on the transistors base pulldown resistor, if it is too small related to the charge(timer) resistor you will get a resistor divider where the voltage is split between the resistors, never reaching the zener value(+LED ++)

    To get a really long time this way, could be to use a comparator with a low input current to measure the voltage over the capacitor. However, the best and most repeatable is to use an oscillator and a counter for very long intervals, as proposed before.

    TOK ;)
     
    Last edited: Apr 21, 2012
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,477
    2,820
    Jan 21, 2010
    Theoretically you should be able to choose appropriate values for any time up to eternity.

    However (quoting Einstein):
    There are limitations on what you can do in practice. Leakage, gain, etc, all have practical limits which can't be overcome.

    Google for "long duration timer schematic"

    The proper way to do this is with a circuit which counts time and does something when that time expires. Single RC events of this duration are prone to all sorts of practical limitations rendering them almost (if not completely) impossible.
     
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