Astable multivibrator doesnt work like i want to

Discussion in 'The Projects Forum' started by psoke0, Dec 6, 2017 at 2:09 PM.

  1. psoke0

    Thread Starter Member

    Mar 31, 2017
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    hi i made a astable multivibrator with npn transistors. its works just fine with leds. led1 lights up 5 second and after that led2 lights up for 5 seconds. but when im using the circuit for driving a transistor thats where problem shows up. external transistor is normally draws about 300mA and when i drive this transistor with multivibrator. the transistor doesnt stays on for 5 senconds instead its much more shorter then that. so im wondering when im drawing 300mA from the power source is it posible that because of the voltage drop couses multivibrator timing to getting lower or make such error ?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If the external transistor is drawing 300mA then its base current could be several mA, which comes from the astable circuit so may well affect the timing. We'd need a schematic of your complete circuit to see what could be adjusted to compensate.
     
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  3. Danko

    Member

    Nov 22, 2017
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    Drawing1.png
    Will it helpful ?
     
    Last edited: Dec 6, 2017 at 5:30 PM
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  4. psoke0

    Thread Starter Member

    Mar 31, 2017
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    thank you before i read the answer of yours i allready try it and its working better but still less timing but i rise the capacitance and its fixed it this way . also rising cap value doesnt work with before circuit but does work with this one :D so thank you but what that resistors job is ? what does it do
     
  5. psoke0

    Thread Starter Member

    Mar 31, 2017
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    i guess that resistor is acting as voltage divider with main transistor right ?
     
  6. Danko

    Member

    Nov 22, 2017
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    Role of resistor is to eliminate drop voltage on B-E junction of external transistor (bypass collector current of multivibrator ). It's value should be about 1 kOhm. Maybe it will good for timing.
    Edit: Not collector, emitter current.
     
    Last edited: Dec 6, 2017 at 6:23 PM
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  7. psoke0

    Thread Starter Member

    Mar 31, 2017
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    okay thank you :)
     
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