555 timer

Discussion in 'The Projects Forum' started by scelesticsiva, Nov 29, 2013.

  1. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    I am working on my project with BOOST CONVERTER and i am thinking of using 555 timer for the gating pulses,i just wanna know by how many ways by which i can change the duty cycle of 555(in astable mode)?
     
  2. TheComet

    Member

    Mar 11, 2013
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    0%-50%, with the assumption the Threshold pin (5) remains within the voltage range 0-Vcc.

    You're probably better off with generating a triangle with with a schmitt-trigger and integrator, and use that to generate the PWM with a comparator. This can be done with a single quad opamp, such as the LT1802.

    Attached is an example of how to do that.
     
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  3. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    Yeah by connecting a diode across the resistor Rb,you can change the duty cycle below 50%,but can duty cycle be made voltage controlled?Thanks for the alternative btw,but i am specific about 555!
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Your question seems to be, "How many ways are there to use voltage to control the PWM duty cycle in a 555?" My question is, "How many ways do you already know?" :)
     
  5. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    i dont know any method regarding the voltage control over duty cycle,but i do know the resistances(ie,Ra and Rb) control of duty cycle and the variation of Vcc to 555 during the ON and OFF cycle (variation of capacitance charging and discharging periods)which will ultimately change the duty cycle of it!
     
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    If you don't mind the frequency changing you can easily vary the duty cycle from ~5% to 95% by applying a control voltage to the CV pin.
     
  7. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    I need the output to have constant frequency!anyway thanks:)
     
  8. iimagine

    Active Member

    Dec 20, 2010
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    Here is how I would do it: use the 555 to generate a linear ramp, add a couple transistors to use as a comparator and you are all set :)
     
  9. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    yeah that is a good one!but will not the efficiency be low?cause 555 continues to generate ramp signal no matter what the duty cycle is?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    You're wanting to use a 555 timer in a boost converter SPST without any other specifications, and suddenly you're worried about efficiency? Why?
     
  11. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    ok here it goes,i am planning to use the boost converter for the most compact(no microprocessors or anything of that sort), efficient and most importantly less costlier driver for solar street lights !
     
  12. iimagine

    Active Member

    Dec 20, 2010
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    Then why use 555? and why the need for adjustable duty cycle? If you want efficiency then just go fully discrete, build a 2 transistor abstable multivibrator and set its duty cycle to the most effective/efficient value.
     
  13. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    usually street lights will be on during the time from 6pm - 6am,but between the time 6-6:30 pm and 5:30-6 am there will be enough sunlight that street light can glow at its half brightness, thus saving energy!I want the duty cycle to toggle between these two levels to achieve this,now can any one give me a solution?BTW I am talking about summer season here.
     
  14. iimagine

    Active Member

    Dec 20, 2010
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    build 2 of them abstable multivibrators then find a way to switch power between them, less headache, cost effective and efficient! :D
     
  15. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    You can do voltage-controlled PWM at a fixed frequency with two 555s like this.
    The first 555 generates a sawtooth. R1 pulls the threshold down to improve linearity.
    The second 555 is used as a comparator:-
     
  16. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    This is not exactly what you asked, but have a look at the MC34063 IC, it is a simple little 8-pin IC similar to a 555 timer, but designed to make buck and or boost converters with.

    Here's the circuit for a boost converter;

    [​IMG]
     
  17. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    Thanks a lot for the replies,i am thinking of using two bjt astable multivibrator with current mirror circuit across the capacitor, thus generating a ramp signal which in turn can be using to generate variable duty cycle pwm signals using a comparator,but i need to know whether this will work!
     
  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A micro would probably be the cheapest solution for anyone doing this commercially.
     
  19. iimagine

    Active Member

    Dec 20, 2010
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    First you said generating a linear ramp is inefficient... now you want to use it! Stop contradicting yourself! :eek:

    Now if you want a discrete PWM, I have posted my discrete PWM version awhile ago, you can use that for reference or ignore it.
     
  20. scelesticsiva

    Thread Starter New Member

    Nov 29, 2013
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    IImagine,i think efficiency and compactness cant sail in the same boat,had to compromise any one in this case,since i am planning to commercialize the circuit,compromising efficiency would be the best choice coz people are not gonna look for the efficiency,rather they look for what a product does and how much does it cost(not in the intention of offending you in any way!)

    what does "micro" mean?
     
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