Astable 555

Discussion in 'Homework Help' started by albertino, Nov 22, 2009.

  1. albertino

    Thread Starter New Member

    Apr 9, 2008
    5
    0
    Good morning everyone,
    I'm designing a 555 astable oscillator.
    The characteristics are:

    T(s)=33,33uS
    F(Hz)= 30Khz
    Duty(%)=60%
    Th=20uS
    Tl=13,33uS

    Using a software I got the values for the resistors and for the capacitor

    R1=9,62K = 10K
    R2=19,24K= 20K
    C=1nF

    My question is: where do I find the 1nF capacitor? Does it exist? What can I do?

    Thanks for your help

    Albertino
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
  3. kkazem

    Active Member

    Jul 23, 2009
    160
    26
    Hi All,
    You have to be a bit careful about the type of cap you install for the timing cap (1 nF). It will largely depend on the accuracy & stability that you need. Since I don't know what you are using it for, I cannot help you here unless you reply with more detailed specs for your astable. But, generally, an NPO dielectric is the most stable and one of the most expensive. IT is great for both innitial accuracy and stability over a wide temperature range and over time. If you only need +/-10% or +/-20% accuracy and stability, then go ahead and use a standard monolithic ceramic or even a cheap ceramic disk cap. The timing is largely independent of the power supply voltage since it works on the ratio of the supply voltage (about 1/3rd for low threahold and 2/3rds for the hi threshold if my memory is correct). There are two basic versions of the 555. The bipolar and the CMOS. I amost always prefer the CMOS as it uses far less bias current, can operate at higher frequencies, can operate over a wide range of power supply voltages, and a few other things I can't remember off the top of my head at present. Although the bipolar is a good unit too, it's just that it has more limitations. Please feel free to contact me or reply with more questions.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I use 1nF 5% metalized film capacitors that have good temperature stability.
     
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