What do the two diodes do of the ne555?

Discussion in 'General Electronics Chat' started by JamesBond007, Oct 25, 2015.

  1. JamesBond007

    Thread Starter New Member

    Oct 25, 2015
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  2. MikeML

    AAC Fanatic!

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    Then don't post!
     
  3. JamesBond007

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    Oct 25, 2015
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    !OK!
     
  4. JamesBond007

    Thread Starter New Member

    Oct 25, 2015
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    MikeML
    Perhaps u misunderstood me.
    I am saying that I don't understand what role those two diodes play ?
    And therefore, I can't give a constructive answer.
    In other words, I won't give an answer that is useless.
    "Ditch the Darlington. Replace it with a modern NFET that has a Vth ~ 4V, Vds >50V, Id > 20A, Rds < 30mΩ, and mount the NFET on a suitable heatsink..."
    Do u no wot the diodes do, or do u not ?
    :cool:
     
  5. JamesBond007

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    Oct 25, 2015
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    NO ?
     
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    If you understand the operation of the 555 (charge and discharge of the .1Uf tied to Pins 2 and 6 and wiper of the pot) the diodes make perfectly good sense to give wide PWM control.
     
  7. JamesBond007

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    Oct 25, 2015
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    The 555 uses two comparitors, one that detects a voltage across the capacitor that is one third of the supply voltage, and one that detects a voltage across the capacitor that is two thirds of the supply voltage.
    I have shown the CCT diagram of the timer so that we can see how those two diodes affect the rise and fall profile of the output signal, which ultimately effects the inductive kickback of the motor.
     
  8. BillB3857

    Senior Member

    Feb 28, 2009
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    I'll toll along with you, JamesBond0! Explain how the two diodes effect the rise and fall of the output signal.
     
  9. JamesBond007

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    Oct 25, 2015
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    u should never answer a question with a question, but here goes. Back in II.
     
  10. MaxHeadRoom

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    Jul 18, 2013
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    I wonder what the reason was for marking the + sign on the diodes, it would not matter in this case as long as they were each connected back to back.??
    Max.
     
  11. JamesBond007

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    Oct 25, 2015
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    I think MaxHeadRoom is on the right track. Those diodes give a 0.6 V reference to the threshold pin 6 of the 555. Because the threshold goes below zero volts (capacitors do this) both diodes offer a stable reference during both half cycles. I need to build this CCt and CRO it, but I wont.
     
  12. BillB3857

    Senior Member

    Feb 28, 2009
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    Ok, enough is enough. The diodes provide two different paths for the charge and discharge current to the capacitor. By doing so, the frequency stays fairly constant and the duty cycle varies from near zero to near 100%, just as MikeML said.
    This link has a good write up explaining the circuit.
    http://pcbheaven.com/circuitpages/PWM_Fan_controller_using_a_555/
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    BillB3857 already given the answer for two diodes.
    MikeML is a good member here and he help a lots of members to solve their problem, so be nice to him, he could give you the knowledge you didn't know, if you want to ask for help here then you should be polite to other members who trying to help you and be careful about your attitude, thanks.
     
  14. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Yes, I think I know. And you will too, if you can follow this:
     
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