Not getting the ideal 555 output

Discussion in 'The Projects Forum' started by varundua, Jun 18, 2015.

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  1. varundua

    Thread Starter New Member

    Jun 15, 2015
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    Hey,
    I am providing a 230v ac supply to the attached circuit which gets rectified and the output that I get from the zener diode is a constant 12v dc supply.
    The requirement is that there should be just 1 negative trigger pulse that should be applied when the power supply is switched on and the output of the IC555 timer should be there for 500ms. I'm somehow not getting an output which specifies the above 2 criteria's. The output that I get is a positive trigger pulse and the output which remains low till infinity time..

    Please help..
     
    Last edited: Jun 18, 2015
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Your schematic is very difficult to read.
    Since everything has a common ground, why do you have an optoisolator in there?
    In you description you use the word "output" in two different ways. Which is it?
    Look at 555 pin 2. Are there continuous pulses there or just one edge at startup? The Monostable section of the 555 datasheet has a warning about changes to the trigger input while the output is in its timed state.
    This is a non-isolated power supply, and this thread will be shut down by the moderators.

    ak
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Have you computed the power dissipation in R1, R2, R3 and D5? Get a transformer.
     
  4. varundua

    Thread Starter New Member

    Jun 15, 2015
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    Alright I'll get the transformer but if considering the circuit ahead of it, I have used the optocoupler along with TL431 since I require just 1 single trigger at PIN 2 of IC555 when my power supply is switched ON and during the whole operation of the circuit no more triggers are produced. That I have somewhat achieved but getting a positive pulse instead of a negative one.

    What I'm more focussed on is the output I get from IC555 which should remain high for 500ms when the trigger pulse is applied.
     
  5. MikeML

    AAC Fanatic!

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    If the trigger pulse low duration is longer than the RC period of the timer, then the output of the timer stays high until the trigger pulse goes back high.

    Normally, the trigger pulse must be shorter than the timer duration. Usually, an RC differentiator is used to make the 555 trigger on the down-going edge of the trigger event.

    77s.gif 77l.gif

    I have no idea what you are trying to do with TL431???
     
    Last edited: Jun 18, 2015
  6. varundua

    Thread Starter New Member

    Jun 15, 2015
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    TL431 is actually being used to ensure that I get just a single trigger pulse as the power supply is switched ON and no further trigger pulses during the operation of the whole circuit. Is there any other way with which I can achieve this?
     
  7. Dodgydave

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    Jun 22, 2012
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  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    How about this?
    Start-up-pulse.gif
     
  9. MikeML

    AAC Fanatic!

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    This is the way I would do it:

    77t.gif
     
  10. MikeML

    AAC Fanatic!

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    Great minds think alike...
     
  11. varundua

    Thread Starter New Member

    Jun 15, 2015
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    But dont you require a negative going trigger pulse to activate the IC555?
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Yes. There is a short negative trigger at the start due to capacitor C2 momentarily holding the trigger input low for a few ms when the power is applied.
    Notice the V(trig) does not rise quite as fast as the supply voltage.

    But as an added note, I would increase the time constant of R1C2 to about 100ms from 10ms (say by changing C2 to 1uF). 10ms seems a little fast for a 50ms power supply rise-time.
     
    Last edited: Jun 18, 2015
  13. varundua

    Thread Starter New Member

    Jun 15, 2015
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    But in the above output I'm unable to see a negative trigger pulse..
     
  14. crutschow

    Expert

    Mar 14, 2008
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    It's not a negative voltage, just a negative going ( to 0V) pulse
     
  15. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    22 uF is excessive on pin 5, may screw up timing. Set up 555 as a 500ms one shot. hold pin 2 low with a cap., charge cap with R to Vcc; RC should be longer than rise of Vcc at turn on. Can add a diode anode to pin 3, cathode to pin 2 to rapidly charge C when 555 triggers so as not to interfear with 500 ms timing.
     
  16. WBahn

    Moderator

    Mar 31, 2012
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    Please show a revised schematic in which you are properly using a transformer to obtain your low voltage supply rail.
     
  17. Wendy

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    Mar 24, 2008
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    Basically a transformerless (non-isolated) power supply is a man trap, a killer. We don't allow those here for that reason. Wall warts are cheap, so are batteries. If WBahn had not responded first I would have just shut this thread down.

    The 555 Projects

    555 Monostable
     
  18. WBahn

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    And I will still shut it down if the TS doesn't respond back with a revised schematic in a reasonable amount of time.
     
  19. ScottWang

    Moderator

    Aug 23, 2012
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    Welcome to the AAC forum.

    The last thread that I already told you the rules and you knew it, but you still not to give up.

    The owners of All About Circuits has elected not to host discussions of transformerless threads.

    This thread is against the AAC forum rules, Terms of Service (ToS).

    Any kind of over-unity devices and systems
    Automotive modifications
    Devices designed to electrocute or shock another person
    LEDs to mains
    Phone jammers
    Rail guns and high-energy projectile devices
    Transformer-less power supplies
    Terms of Service (ToS).

    Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

    Please try to understand the reasons behind this action, and feel free to browse and use the forums.

    You might find answers to your questions in one of these forums:

    http://forum.allaboutcircuits.com/showthread.php?t=54400

    Another good forum that shares many of the same membership is http://www.electro-tech-online.com/ .

    Good luck.
     
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